seasonal affective disorder sorry no news is found... your search criteria may have been too narrow. you can quickly re - sort the news in different ways by clicking on the tabs at the top of this page. seasonal affective disorder ( sad ), also known as winter depression, winter blues, summer depression, summer blues, or seasonal depression, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or summer, spring or autumn year after year. in the diagnostic and statistical manual of mental disorders ( dsm - iv ), sad is not a unique mood disorder, but is " a specifier of major depression ". although experts were initially skeptical, this condition is now recognized as a common disorder, with its prevalence in the u. s. ranging from 1. 4 percent in florida to 9. 7 percent in new hampshire. the u. s. national library of medicine notes that " some people experience a serious mood change when the seasons change. they may sleep too much, have little energy, and may also feel depressed. though symptoms can be severe, they usually clear up. " the condition in the summer is often referred to as reverse seasonal affective disorder, and can also include heightened anxiety. sad was formally described and named in 1984 by norman e. rosenthal and colleagues at the national institute of mental health. there are many different treatments for classic hormones ( winter - based ) seasonal affective disorder, including light therapy with sunlight or bright lights, antidepressant medication, cognitive - behavioral therapy, ionized - air administration, and carefully timed supplementation of the hormone melatonin. this text uses material from wikipedia and is available under the gnu free documentation license. latest spotlight news you ' re standing near an airport luggage carousel and your bag emerges on the conveyor belt, prompting you to spring into action. how does your brain make the shift from passively waiting to taking action when... 26 minutes ago | not rated yet | 0 | widely available in pharmacies and health stores, phosphatidylserine is a natural food supplement produced from beef, oysters, and soy. proven to improve cognition and slow memory loss, it ' s a popular treatment for older... 50 minutes ago | 5 / 5 ( 1 ) | 0 | ( medical xpress ) — native peoples in regions where cameras are uncommon sometimes react with caution when their picture is taken. the fear that something must have been stolen from them to create the


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as the more accessible methods for micro energy ( microwatt - hour to watt - hour ) harvesting and storage become commercialized, the research community will focus on extremes in efficiency and cost breakthroughs and exploration of entirely new methods and applications. make no mistake ; there is plenty of exciting new ground to be covered! we will in this session profile the space of current and anticipated micro energy harvesting needs and approaches suitable for sensing, computing, and communications. the berkeley sensor & actuator center will conduct this afternoon - through - evening conference and reception at the leeds certified green david brower center on the edge of the beautiful uc berkeley campus on wednesday march 9th. applications drivers : energy consumption in communicating sensor networks will radical reductions in power requirements of wireless radios and integrated sensors and processors make energy “ harvesting ” ubiquitous, or unnecessary? application requirements, hardware trends, and protocol efficiency will be discussed in the context of energy harvesting solutions. advanced sensing technology for harsh environments present and future research directions for harsh environment telemetry will be discussed in this presentation. using the application of sensors in geothermal power plants as a focus, a comprehensive research program will be described for the development of sensors, telemetry, energy harvesting, materials, packaging and bonding. aluminum nitride and silicon carbide material technologies ( as well as other related material technologies ) will be described. energy harvesting and sensing for the power grid bsac work on energy harvesting and wireless sensors for the smart grid will be discussed. the current situation of sparse deployment of expensive sensors for even basic electrical parameters and without ability to quickly locate faults, and with no way of predicting future faults will be addressed by these technologies. as renewable but rapidly fluctuating wind and solar power sources are connected, the sensing and actuation situation will become critical. small, hermetically sealed, meso - scale and mems - based lightweight sensors and scavengers with the goal of increase in the granularity of sensing and the controllability of our power grids. some of these devices involve piezo - magnetic harvesting, mems - on - glass manufacture and packaging, and scavengers that draw the small amount of energy they need from the power lines themselves. they will include new methods for extending the usable life of piezoelectric members. cost effective and efficient microphotovoltaics solar energy presents a viable candidate for fossil fuel replacement. in this regard, developing technologies for $ 1 / w installed pv system is needed. the choice of materials and cell architecture, the cost


members. cost effective and efficient microphotovoltaics solar energy presents a viable candidate for fossil fuel replacement. in this regard, developing technologies for $ 1 / w installed pv system is needed. the choice of materials and cell architecture, the cost and abundance of the raw materials, the component manufacturing costs, the cell design and framework, the capital cost, and the installation costs are all important contributing factors to the final cost. although there is considerable current focus on the absorber materials and the required deposition processes, the rest of the cell, including transparent conductors, texturing, optical coatings, etc., and reliability are equally important. all of these are fundamentally materials related problems and progress requires an interdisciplinary approach drawing on materials science, chemistry, applied physics, mechanical and electrical engineering, however, significant challenges exist in developing efficient and low cost systems that are scalable to small formats and small capacities ( micropower ) as well as to commercial scale ( kilowatts to megawatts ). micro energy storage micro energy storage is indispensable for energy harvester systems. energy generation from various resources is required to be stored and released when the power is needed. two systems are commonly known in the macroscale systems : battery and supercapacitors. this talk will provide efforts in micro energy storage systems including high power density supercapacitors with high energy storage capability and nanostructure - based micro batteries. biologically - inspired scavenging technologies recent developments focusing on two very different bio - inspired energy harvesting microsystems will be discussed. the first, in collaboration with cfd corporation, is a miniaturized electrochemical system for harvesting energy from an insect ’ s reactions with the sugar trehalose. the system is being designed to produce 100 ′ s of uw ’ s in a compact footprint, suitable for powering instrumentation on a cyborg insect. a similar system is being studied for mammalian implants. the second topic will briefly discuss developments in the scavenging of power from evaporation and low pressure head flows, including our work on microscale boundary - layer turbines ( so - called tesla turbines ), a topic which has applications beyond scavenging. panel and mixing reception the topics are certain to generate interest if not some controversy among the audience and speakers. an extended reception and mixing event will allow continuation of the discussions and renewal of prior and establishment of new connections among practitioners, students, and investors of impacted applications. agenda and registration can be found here : http : / / bsac. berkeley


after participating in this activity, students will be able to : - describe the difference between herbivores, carnivores, and producers. - answer questions about the interdependence of herbivores, carnivores, and producers as members of a food chain. - answer questions about how pollution affects food chains. all living organisms depend on one another for food. by reviewing the relationships of organisms that feed on one another, students begin to see how all organisms — including humans — are linked. if students understand the relationships in a simple food chain, they will better understand the importance and sensitivity of these connections, and why changes to one part of the food chain almost always impact another. a food chain is a simplified way to show the relationship of organisms that feed on each other. it ’ s helpful to classify animals in a simple food chain by what they eat, or where they get their energy. green plants, called producers, form the basis of the aquatic food chain. they get their energy from the sun and make their own food through photosynthesis. in the great lakes, producers can be microscopic phytoplankton ( plant plankton ), algae, aquatic plants like elodea, or plants like cattails that emerge from the water ’ s surface. herbivores, such as ducks, small fish, and many species of zooplankton ( animal plankton ), eat plants. carnivores ( meat eaters ) eat other animals and can be small ( i. e, frog ) or large ( i. e, lake trout ). omnivores are animals ( including humans ) that eat both plants and animals. each is an important part of the food chain. in reality, food chains overlap at many points — because animals often feed on multiple species — forming complex food webs. food web diagrams depict all feeding interactions among species in real communities. these complex diagrams often appear as intricate spider webs connecting the species. see : unit 1, lesson 2 this lesson demonstrates that changes in one part of a food chain or web may affect other parts, resulting in impacts on carnivores, herbivores, and eventually on producers. an example of this might be the harmful effects of pollution. the point that should be made is that when something disrupts a food web, humans should try to understand and minimize the disturbance. students should also come to recognize that humans, too, are part of this complex web of life.


a few months ago i read about a very simple but fun probability puzzle. someone tells you : try to solve it yourself. john baez mentions that you would think or he would think that the information about tuesday is irrelevant because the days of the week are independent of the sex and we only care about the latter. “ i have two children. at least one is a boy born on a tuesday. [ and if it were not the case, i would have told you. ] what is the probability i have two boys? ” so you would think that there are 4 equally represented groups of 2 - kid families, namely boy - boy, boy - girl, girl - boy, and girl - girl families where the two hyphenated words refer to the younger and older kid, respectively. only the girl - girl families are eliminated, and 1 of the remaining 3 groups is a two - boy family, so the conditional probability is 1 / 3. however, that ' s a wrong result. the information about the tuesday actually does matter. here ' s why : in all families with exactly 2 children, one may label the children as the " younger " and " older " one, even if the difference is just in seconds. each kid may be born on any day and have any sex, so there are 14 equally likely possibilities for each child. the two children are independent ( forget that the phenomenon of twins tends to increase the same - day pairs ), so there are 14 x 14 possibilities for two kids. each of these 14 x 14 possibilities is equally likely. so 1 / 196 of the world ' s families with exactly 2 kids fits each condition. among the 196 types of the families, how many of them contain at least one tuesday son? well, in 14 of them, the younger kid is a tuesday son ( the older one may be anything chosen from the 14 possibilities ). in 14 other of them ( the younger can be anything ), the older one is a tuesday son. however, i have counted the families with two tuesday sons twice. so there are 14 + 14 - 1 = 27 possibilities among the 196 for which the condition " at least one kid is a tuesday son " is satisfied. this is the assumption which is a part of the calculation of the conditional probability. we need the other part, too. among these 27 / 196 of the families, 13 / 196 of all families have two boys, by pure counting, so the result is p = 13 / 27as the fraction of the families that satisfied the condition.


need the other part, too. among these 27 / 196 of the families, 13 / 196 of all families have two boys, by pure counting, so the result is p = 13 / 27as the fraction of the families that satisfied the condition. note that it is just slightly less than 1 / 2 = 13. 5 / 27 i. e. much more than 1 / 3. i had to highlight the result because almost no one reads the full article and almost no one notices that the right results is neither 1 / 3 nor 1 / 2. indeed, the large difference of the right result from 1 / 3 appears because one de facto identifies one of the sons by mentioning that it is the kid from tuesday. if you assumed there were infinitely many days in a week and you would take any family with at least one tuesday kid, the " tuesday " information would identify this kid completely ( two tuesday kids would be infinitesimally unlikely ), and the question what is the probability of 2 sons would be reduced to the question what is the probability that the other, equally specific kid - the non - tuesday kid - is male - which is of course 1 / 2. i will discuss this " identification " and reasons why the result is close to 1 / 2 at the very end. indistinguishable kids ' bound states with kids that would satisfy the bose or fermi statistics, the counting would be different but equally straightforward. instead of 14 x 14 = 196 possibilities, one has 14 x 15 / 2 = 105 for bosons ( the symmetric triangle ) and 14 x 13 / 2 = 91 ( the antisymmetric triangle ) for fermions. among the 105 or 91 options, how many of them contain at least one tuesday son? well, in these two cases, we can ' t say which of them is older and younger : they ' re identical. so if there is at least 1 tuesday son, the number of states with at least 1 tuesday son is 14 for the bosons - we can just create the other particle into the 1 - particle state - or 13 for the fermions - we can also add the second creation operator, but with another tuesday son, the state will vanish because of pauli ' s exclusion principle. among these 14 or 13 states respectively, for bosons and fermions, 7 or 6 are two - son states, respectively. so the odds are 7 / 14 = 1 / 2 for the bosons and 6 / 13 for the fermions. note that the


states respectively, for bosons and fermions, 7 or 6 are two - son states, respectively. so the odds are 7 / 14 = 1 / 2 for the bosons and 6 / 13 for the fermions. note that the bosons literally saturate the 1 / 2 bound while the fermions are just slightly below it. why not one third? finally, i want to comment on " why the information about tuesday matters ". if we sum up the probabilities for the problems where the son is born on sunday, monday... and up to saturday, shouldn ' t we get the same result? and by symmetry, the result must be equal for all 7 days, so doesn ' t each term have to be 1 / 3? the answer is that we can ' t add the probabilities in this way because the " at least one monday son " etc. are assumptions, not propositions conditioned by these assumptions, and they ' re not disjoint. at any rate, the calculation is nonlinear because the conditional probabilities have the probability of the assumption in the denominator rather than the numerator, so you can ' t simply add the possibilities in any way. the word term in the previous paragraph is therefore incorrect. how and why 1 / 3 gets enhanced to nearly 1 / 2 if you were only told that " one of the kids is a boy ", the mixed families would be overrepresented over the two - boy families by the 2 - to - 1 ratio because boy - girl and girl - boy families are as likely as boy - boy families ; again, the kids notation is younger - older. however, if you ' re told that " one of the kids is a tuesday boy ", this overrepresentation almost disappears. why? because 1 / 7 of the boy - girl and girl - boy families have a tuesday boy. but ( approximately ) 2 / 7 of the boy - boy families have at least one tuesday boy because each of these two boys has a chance to be born on tuesday. in this way, the boy - boy families ( nearly ) compensate the factor of two by which they were underrepresented relatively to the mixed families. bonus : this puzzle and crackpot sean carroll ' s misunderstanding of logic this logical puzzle is actually a very precise pedagogical example showing what ' s wrong with the thinking of various people about the arrow of time. some people - those who say that the information about tuesday doesn ' t


carroll ' s misunderstanding of logic this logical puzzle is actually a very precise pedagogical example showing what ' s wrong with the thinking of various people about the arrow of time. some people - those who say that the information about tuesday doesn ' t matter and who typically end up with the result 1 / 3 - think that prob ( cond, any _ day ) = prob ( cond, monday ) +... + prob ( cond, sunday ) where " cond " is an extra condition. so if we make a statement about a specific object and if this statement doesn ' t prefer any day of the week, then adding the information about " its " day of the week doesn ' t matter. it only reduces the probability by a factor of 7 if the probability is day - blind. that ' s right for " conclusions " or " outcomes ". however, the error that these people are making is that they think that this " additive " counting of the probabilities also holds for the probabilities of assumptions, i. e. probabilities of conditions in the conditional probability. but no such a linearity exists over there. conditions ( and initial states ) don ' t follow the same maths as the outcomes ( and final states )! there is no condition - outcome or past - future symmetry in mathematical logic! that ' s why it matters for the probabilities whether the information about tuesday is specified even though there is nothing special about tuesday.


potsdam – clarkson university ' s egon matijevic, the victor k. lamer professor of colloid and surface science, and evgeny katz, the milton kerker chair in colloid science, are listed in the h - index of living scientists. the h - index attempts to measure both the scientific productivity and the apparent scientific impact of a scientist. the index is based on the set of the scientist ' s most cited papers and the number of citations that they have received in other people ' s publications. at ranking # 218, matijevic, with an h of 69, has published 69 papers ( among total 575 papers ), which have each received at least 69 citations. at ranking # 380, katz, with an h of 60, has published 60 papers ( among more than 300 papers ), which have each received at least 60 citations. it should be noted that this listed of the most - cited chemists totals only 587 worldwide with the two clarkson ' s professors among them. the index was suggested by physicist jorge e. hirsch as a tool for determining quality of scientific publications of a person and is sometimes called the hirsch index or hirsch number. the purpose of this index is similar to the popular impact factor, which is used to rank the quality of scientific journals, but it is applied to a person. the definition of the h - index and the explanations of its importance can be found at http : / / en. wikipedia. org / wiki / h - index. the latest list, compiled by henry schaefer of the university of georgia, together with colleague amy peterson, was updated in november 2010 and can be found at http : / / www. rsc. org / chemistryworld / news / 2007 / april / 23040701. asp.


referendum, which allowed voters to repeal laws that they did not support, and recall, which allowed voters to remove elected officials from office. many progressives supported women ' s suffrage, helping women secure the right to vote through the adoption of the nineteenth amendment to the united states constitution in 1919. progressives also battled against city bosses, including cincinnati, ohio ' s george cox, by hiring city managers. while progressives enacted numerous positive reforms, some of their goals were questionable. they did seek to make the united states government more democratic and to protect american workers, but they also sought to force their social and political beliefs on others. progressives opposed immigration and enacted several immigration restrictions during the 1920s. progressives also tried to force immigrants to adopt progressive moral beliefs. one way they tried to accomplish this was through settlement houses. settlement houses existed in most major cities during the late nineteenth and the early twentieth centuries. they were places where immigrants could go to receive free food, clothing, job training, and educational classes. while all of these items greatly helped immigrants, progressives also used the settlement houses to convince immigrants to adopt " american " or progressive beliefs, causing the foreigners to forsake their own culture. during the 1920s, many progressives also joined the ku klux klan, a self - proclaimed religious group that was to enforce morality - - based on progressive beliefs - - on other people. due to such the progressives ' participation in prohibition, the ku klux klan, and immigration restrictions, many americans stopped supporting the progressive movement. while aspects of its beliefs remain today, as a functioning and clearly identifiable group, the progressive movement began to weaken by the late 1920s and the early 1930s.


using openmp - the book and examples use this forum to discuss the book : using openmp - portable shared memory parallel programming by barbara chapman, gabriele jost and ruud van der pashttp : / / mitpress. mit. edu / catalog / item / default. asp? ttype = 2 & tid = 11387 the sources are available as a free download under the bsd license. each source comes with a copy of the license. please do not remove this. you are encouraged to try out these examples and perhaps use them as a starting point to better understand and perhaps further explore openmp. each source file constitutes a full working program. other than a compiler and run time environment to support openmp, nothing else is needed. with the exception of one example, there are no source code comments. not only are these examples very straightforward, they are also discussed in the above mentioned book. as a courtesy, each source directory has a make file called " makefile ". this file can be used to build and run the examples in the specific directory. before you do so, you need to activate the appropriate include line in file makefile. there are include files for several compilers and unix based operating systems ( linux, solaris and mac os to precise ). these files have been put together on a best effort basis. the user ' s guide that is bundled with the examples explains this in more detail. please post your feedback about the book and / or these examples to this forum.


- proton transfer networks and the mechanism of long range proton transfer in proteins ( 2010 ) - the main energy providing reaction systems in living cells, for example the photosynthesis or the respiratory chain, are based on long range proton transfer ( lrpt ) reactions. even since these lrpt reactions have been heavily investigated in the last decades, the mechanism of these reactions is still not completely understood. the reaction kinetics of the lrpt are under heavy discussion and it is not clear, whether the reorientation of the hydrogen bond network ( hbn ) or the electrostatic barrier for the charge transfer is rate limiting. the main purpose of this work is to investigate the dynamics of chemical reactions inside of proteins, focused on long range proton transfer reactions. electron transfer reactions, rotations of water molecules or conformational changes of the protein are also considered. the developed sequential dynamical monte carlo ( sdmc ) method is applicable to almost all kinds of chemical reactions. for all proton transfer reactions, the hbn of a protein plays a major role. protons are transferred along such hydrogen bonds. therefore, knowledge about the hydrogen bond network of a protein is crucial for the simulation of lrpt systems. the hbn can be calculated from the protein structure and the rotational state of the amino acid side chains. the reaction rate can be calculated from the electrostatic energies of the participating proton donor and acceptor groups. these two criteria are combined for the decision if a proton transfer between two molecules is possible and how fast this transfer would happen. while the calculation of electrostatic energies of protonatable amino acid side chains or relevant cofactors in proteins ( among them also water molecules ) is already solved - implemented in various programs - the remaining tasks - calculating the hydrogen bond network followed by calculating the reaction rates - were solved during this work. before the hydrogen bond network and the electrostatic energies could be calculated, the lack of water positions in many available crystallographically resolved protein structures made it necessary to develop an algorithm to detect internal cavities in proteins and fill these cavities with water molecules. the derived water positions could be included in the electrostatic calculations as well as in the calculation of the hbn. the simulation of the lrpt in gramicidin a ( ga ) compared to experimental data of the proton transfer in this polypeptide showed the possibilities of the simulation of the lrpt by the sdmc algorithm. the promising results encouraged us to investigate the mechanism of the lrpt, especially, if the re


to experimental data of the proton transfer in this polypeptide showed the possibilities of the simulation of the lrpt by the sdmc algorithm. the promising results encouraged us to investigate the mechanism of the lrpt, especially, if the reorientation of the hbn or the electrostatic energy barrier of the charge transfer is rate limiting for the lrpt. the results indicate, that both effects influence the lrpt and none of them is exclusively responsible for the lrpt rate. further analysis of the hydrogen bond network topology showed that graph algorithms can be used to analyze these networks. hydrogen bond networks can be clustered into regions which are close connected to each other. on the other hand, residues connecting two or more of these densely connected regions might play an important role for proton transfer pathways since a loss of such residues cuts a proton transfer pathway. a comparison of an analysis of the hbn topology of the photosynthetic reaction center with mutation studies of the same system showed, that residues identified as important for proton transfer by the mutation studies are identified as connection points between clusters by the network analysis. the developed algorithms together with the introduction of a new method for the simulation of the lrpt process ( sdmc ) improved the picture of the proton transfer processes in proteins. starting from the protein structure, the developed algorithms cover all steps from the detection of protein cavities, the placement of water molecules in these cavities, the calculation and analysis of the hydrogen bond network, the simulation of the lrpt and the investigation of the reaction kinetics. the analysis of the hbn by graph theoretical methods gives further insight into the hbn topology and identifies residues important for proton transfer pathways and therefore important for the protein activity.


closing in on the planck constant sep 25, 1998 physicists in the us have made the most accurate measurements ever of the planck constant, h. edwin williams and colleagues at the national institute of standards and technology in gaithersburg, maryland, measured h by comparing the voltage needed to control the velocity of a coil moving vertically in an magnetic field, with the current that has to be passed through the coil to balance gravity in the same magnetic field. the measurement could lead to a new reference stand for the kilogram ( phys. rev. lett. 81 2404 ). the kilogram is currently defined by a platinum - iridium alloy maintained at the bureau international des poids et mesures ( bipm ) in paris and six official copies. however, the official mass of the standard kilogram has been known to vary with time, hence the interest in defining the kilogram is terms of fundamental constants like h. the highly stable magnetic field needed for the experiment is generated by a superconducting magnet that has been cooled to 4 kelvin. the experiment also uses two induction coils : the lower coil is fixed to the support structure of the experiment, while the upper coil can move. this upper coil is also attached to a wheel balance above the experiment. in the first stage of the measurement, the mass balance is empty and a small force is applied to the upper coil, forcing it to move at 2mm / s. the researchers found that this generated a voltage of 1. 018 ± 0. 001 v across the moving coil. in the second stage of the experiment, a 500 g countermass is balanced by a - 10. 18 ma current in the induction coil. both stages were repeated over many months to obtain a value of 6. 62606891 ( 58 ) x 10 - 34 joule seconds for the planck constant. this result - which corresponds to an accuracy of 9 parts in 108 - is a factor of 15 better than previous measurements. the team hope to improve on this result by another factor of 10 by modifying their experiment.


ultrafast electron microscope makes movies dec 8, 2006 physicists have created a new form of electron microscopy that can make " movies " of atoms as they undergo ultra - rapid chemical or structural transitions. ahmed zewail and colleagues at the california institute of technology in the us have used coincident electron and laser pulses to follow vanadium and oxygen atoms as they rearranged themselves on a vanadium oxide surface over the course of several picoseconds. the researchers say that the technique could also be used to study a wide range of ultrafast biological and physical phenomena. ( proc. natl. acad. sci. 103 18427 ) ). electron microscopes have better resolution than optical microscopes because high - energy electrons have a much shorter wavelength than light. the resolution can be further improved by using coherent electron wavepackets, which can contain as few as one electron. the wavelengths of these packets are much smaller than the space between individual atoms and can be brought to a very sharp focus, allowing objects to be imaged with atomic - scale resolution. the packets are of extremely short duration and this can be exploited to take “ snapshots ” of atoms as they undergo structural or chemical transitions. in 2005, zewail and colleagues used coherent electron packets to take single snapshots of a number of materials and biological samples. now the researchers have further refined their technique to take a time sequence of images that allowed them to watch vanadium and oxygen atoms rearrange themselves in a process that can take as little as 100 femtoseconds ( 10 - 13 seconds ). the timing sequence is generated by femtosecond laser pulses as illustrated in the figure " ultrafast microscope ". each pulse is split into two pulses – one is used by the microscope to create the electron pulse and the other is used to heat the sample. according to zewail, the crucial and most difficult part of the technique is coordinating the arrivals of the laser and electron pulses at the sample with an accuracy of just a few femtoseconds. this is particularly difficult because the laser pulse travels at the speed of light, while the electron pulse lags behind at about two thirds the speed of light. the coincident laser pulse is used to heat the sample and drive a transition from a low - temperature crystal structure to a high - temperature structure. by changing the delay between the laser and electron pulses in regular time steps, the researchers were able to take snapshots of the atoms at different sample temperatures. zewai


individual differences | methods | statistics | clinical | educational | industrial | professional items | world psychology | - for a non - technical introduction to the topic, please see introduction to genetics. genetics ( from ancient greek γενετικος genetikos, “ genitive ” and that from γενεσις genesis, “ origin ” ), a discipline of biology, is the science of heredity and variation in living organisms. the fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. however, the modern science of genetics, which seeks to understand the process of inheritance, only began with the work of gregor mendel in the mid - nineteenth century. although he did not know the physical basis for heredity, mendel observed that organisms inherit traits in a discrete manner — these basic units of inheritance are now called genes. genes correspond to regions within dna, a molecule composed of a chain of four different types of nucleotides — the sequence of these nucleotides is the genetic information organisms inherit. dna naturally occurs in a double stranded form, with nucleotides on each strand complementary to each other. each strand can act as a template for creating a new partner strand — this is the physical method for making copies of genes that can be inherited. the sequence of nucleotides in a gene is translated by cells to produce a chain of amino acids, creating proteins — the order of amino acids in a protein corresponds to the order of nucleotides in the gene. this is known as the genetic code. the amino acids in a protein determine how it folds into a three - dimensional shape ; this structure is, in turn, responsible for the protein ' s function. proteins carry out almost all the functions needed for cells to live. a change to the dna in a gene can change a protein ' s amino acids, changing its shape and function : this can have a dramatic effect in the cell and on the organism as a whole. although genetics plays a large role in the appearance and behavior of organisms, it is the combination of genetics with what an organism experiences that determines the ultimate outcome. for example, while genes play a role in determining a person ' s height, the nutrition and health that person experiences in childhood also have a large effect. - main article : history of genetics although the science of genetics began with the applied and theoretical work of gregor mendel in the mid - 1800s, other theories of inheritance preceded mendel. a


health that person experiences in childhood also have a large effect. - main article : history of genetics although the science of genetics began with the applied and theoretical work of gregor mendel in the mid - 1800s, other theories of inheritance preceded mendel. a popular theory during mendel ' s time was the concept of blending inheritance : the idea that individuals inherit a smooth blend of traits from their parents. mendel ' s work disproved this, showing that traits are composed of combinations of distinct genes rather than a continuous blend. another theory that had some support at that time was the inheritance of acquired characteristics : the belief that individuals inherit traits strengthened by their parents. this theory ( commonly associated with jean - baptiste lamarck ) is now known to be wrong — the experiences of individuals do not affect the genes they pass to their children. other theories included the pangenesis of charles darwin ( which had both acquired and inherited aspects ) and francis galton ' s reformulation of pangenesis as both particulate and inherited. mendelian and classical geneticsedit the modern science of genetics traces its roots to gregor johann mendel, a german - czech augustinian monk and scientist who studied the nature of inheritance in plants. in his paper " versuche uber pflanzenhybriden " ( " experiments on plant hybridization " ), presented in 1865 to the naturforschender verein ( society for research in nature ) in brunn, mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. although this pattern of inheritance could only be observed for a few traits, mendel ' s work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios. the importance of mendel ' s work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems re - discovered his research. william bateson, a proponent of mendel ' s work, coined the word genetics in 1905. ( the adjective genetic, derived from the greek word genesis - γενεσις, " origin " and that from the word genno - γεννω, " to give birth ", predates the noun and was first used in a biological sense in 1860. ) bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the third international conference on plant hybridization in london, england, in 1906. after the red


was first used in a biological sense in 1860. ) bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the third international conference on plant hybridization in london, england, in 1906. after the rediscovery of mendel ' s work, scientists tried to determine which molecules in the cell were responsible for inheritance. in 1910, thomas hunt morgan argued that genes are on chromosomes, based on observations of a sex - linked white eye mutation in fruit flies. in 1913, his student alfred sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome. < span id = " molecular " / > although genes were known to exist on chromosomes, chromosomes are composed of both protein and dna — scientists did not know which of these was responsible for inheritance. in 1928, frederick griffith discovered the phenomenon of transformation ( see griffith ' s experiment ) : dead bacteria could transfer genetic material to " transform " other still - living bacteria. sixteen years later, in 1944, oswald theodore avery, colin mcleod and maclyn mccarty identified the molecule responsible for transformation as dna. the hershey - chase experiment in 1952 also showed that dna ( rather than protein ) was the genetic material of the viruses that infect bacteria, providing further evidence that dna was the molecule responsible for inheritance. james d. watson and francis crick determined the structure of dna in 1953, using the x - ray crystallography work of rosalind franklin that indicated dna had a helical structure ( i. e., shaped like a corkscrew ). their double - helix model had two strands of dna with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder. this structure showed that genetic information exists in the sequence of nucleotides on each strand of dna. the structure also suggested a simple method for duplication : if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. although the structure of dna showed how inheritance worked, it was still not known how dna influenced the behavior of cells. in the following years, scientists tried to understand how dna controls the process of protein production. it was discovered that the cell uses dna as a template to create matching messenger rna ( a molecule with nucleotides, very similar to dna ). the nucleotide sequence of a messenger rna is used to create an amino acid sequence in protein ;


was discovered that the cell uses dna as a template to create matching messenger rna ( a molecule with nucleotides, very similar to dna ). the nucleotide sequence of a messenger rna is used to create an amino acid sequence in protein ; this translation between nucleotide and amino acid sequences is known as the genetic code. with this molecular understanding of inheritance, an explosion of research became possible. one important development was chain - termination dna sequencing in 1977 by frederick sanger : this technology allows scientists to read the nucleotide sequence of a dna molecule. in 1983, kary banks mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of a dna from a mixture. through the pooled efforts of the human genome project and the parallel private effort by celera genomics, these and other techniques culminated in the sequencing of the human genome in 2003. features of inheritanceedit discrete inheritance and mendel ' s lawsedit - main article : mendelian inheritance at its most fundamental level, inheritance in organisms occurs by means of discrete traits, called genes. this property was first observed by gregor mendel, who studied the segregation of heritable traits in pea plants. in his experiments studying the trait for flower color, mendel observed that the flowers of each pea plant were either purple or white - and never an intermediate between the two colors. these different, discrete versions of the same gene are called alleles. in the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent. many organisms, including humans, have this pattern of inheritance. organisms with two copies of the same allele are called homozygous, while organisms with two different alleles are heterozygous. the set of alleles for a given organism is called its genotype, while the observable trait the organism has is called its phenotype. when organisms are heterozygous, often one allele is called dominant as its qualities dominate the phenotype of the organism, while the other allele is called recessive as its qualities recede and are not observed. some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once. when a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. these observations of discrete inheritance and the segregation of


an intermediate phenotype, or codominance by expressing both alleles at once. when a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. these observations of discrete inheritance and the segregation of alleles are collectively known as mendel ' s first law or the law of segregation. notation and diagramsedit geneticists use diagrams and symbols to describe inheritance. a gene is represented by a letter ( or letters ) — the capitalized letter represents the dominant allele and the recessive is represented by lowercase. often a " + " symbol is used to mark the usual, non - mutant allele for a gene. in fertilization and breeding experiments ( and especially when discussing mendel ' s laws ) the parents are referred to as the " p " generation and the offspring as the " f1 " ( first filial ) generation. when the f1 offspring mate with each other, the offspring are called the " f2 " ( second filial ) generation. one of the common diagrams used to predict the result of cross - breeding is the punnett square. interactions of multiple genesedit organisms have thousands of genes, and in sexually reproducing organisms assortment of these genes are generally independent of each other. this means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. this phenomenon, known as " mendel ' s second law " or the " law of independent assortment ", means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. ( some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article. ) often different genes can interact in a way that influences the same trait. in the blue - eyed mary ( omphalodes verna ), for example, there exists a gene with alleles that determine the color of flowers : blue or magenta. another gene, however, controls whether the flowers have color at all : color or white. when a plant has two copies of this white allele, its flowers are white - regardless of whether the first gene has blue or magenta alleles. this interaction between genes is called epistasis, with the second gene epistatic to the first. many traits are not discrete features ( eg. purple or white flowers ) but are instead continuous features ( eg. human height and skin color ). these complex traits are the product of many


with the second gene epistatic to the first. many traits are not discrete features ( eg. purple or white flowers ) but are instead continuous features ( eg. human height and skin color ). these complex traits are the product of many genes. the influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. the degree to which an organism ' s genes contribute to a complex trait is called heritability. measurement of the heritability of a trait is relative - in a more variable environment, the environment has a bigger influence on the total variation of the trait. for example, human height is a complex trait with a heritability of 89 % in the united states. in nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62 %. molecular basis for inheritanceedit dna and chromosomesedit - main article : dna the molecular basis for genes is deoxyribonucleic acid ( dna ). dna is composed of a chain of nucleotides, of which there are four types : adenine ( a ), cytosine ( c ), guanine ( g ), and thymine ( t ). genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the dna chain. viruses are the only exception to this rule — sometimes viruses use the very similar molecule rna instead of dna as their genetic material. dna normally exists as a double - stranded molecule, coiled into the shape of a double - helix. each nucleotide in dna preferentially pairs with its partner nucleotide on the opposite strand : a pairs with t, and c pairs with g. thus, in its two - stranded form, each strand effectively contains all necessary information, redundant with its partner strand. this structure of dna is the physical basis for inheritance : dna replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand. genes are arranged linearly along long chains of dna sequence, called chromosomes. in bacteria, each cell has a single circular chromosome, while eukaryotic organisms ( which includes plants and animals ) have their dna arranged in multiple linear chromosomes. these dna strands are often extremely long ; the largest human chromosome, for example, is about 247 million base pairs in length. the dna of a chromosome is associated with structural proteins that organize, compact, and control access to the


in multiple linear chromosomes. these dna strands are often extremely long ; the largest human chromosome, for example, is about 247 million base pairs in length. the dna of a chromosome is associated with structural proteins that organize, compact, and control access to the dna, forming a material called chromatin ; in eukaryotes, chromatin is usually composed of nucleosomes, repeating units of dna wound around a core of histone proteins. the full set of hereditary material in an organism ( usually the combined dna sequences of all chromosomes ) is called the genome. while haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two of each chromosome and thus two copies of every gene. the two alleles for a gene are located on identical loci of sister chromatids, each allele inherited from a different parent. an exception exists in the sex chromosomes, specialized chromosomes many animals have evolved that play a role in determining the sex of an organism. in humans and other mammals, the y chromosome has very few genes and triggers the development of male sexual characteristics, while the x chromosome is similar to the other chromosomes and contains many genes unrelated to sex determination. females have two copies of the x chromosome, but males have one y and only one x chromosome - this difference in x chromosome copy numbers leads to the unusual inheritance patterns of sex - linked disorders. - main article : asexual reproduction when cells divide, their full genome is copied and each daughter cell inherits one copy. this process, called mitosis, is the simplest form of reproduction and is the basis for asexual reproduction. asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. offspring that are genetically identical to their parents are called clones. eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. the process of sexual reproduction alternates between forms that contain single copies of the genome ( haploid ) and double copies ( diploid ). haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. diploid organisms form haploids by dividing, without replicating their dna, to create daughter cells that randomly inherit one of each pair of chromosomes. most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes. although they do not use the haploid / diploid method


that randomly inherit one of each pair of chromosomes. most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes. although they do not use the haploid / diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. some bacteria can undergo conjugation, transferring a small circular piece of dna to another bacterium. bacteria can also take up raw dna fragments found in the environment and integrate them into their genome, a phenomenon known as transformation. these processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated. - main article : chromosomal crossover the diploid nature of chromosomes allows for genes on different chromosomes to assort independently during sexual reproduction, recombining to form new combinations of genes. genes on the same chromosome would theoretically never recombine, however, were it not for the process of chromosomal crossover. during crossover, chromosomes exchange stretches of dna, effectively shuffling the gene alleles between the chromosomes. this process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells. the probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between them. for an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. for genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage - alleles for the two genes tend to be inherited together. the amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome. - main article : genetic code genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell. proteins are chains of amino acids, and the dna sequence of a gene ( through rna intermediate ) is used to produce a specific protein sequence. this process begins with the production of an rna molecule with a sequence matching the gene ' s dna sequence, a process called transcription. this messenger rna molecule is then used to produce a corresponding amino acid sequence through a process called translation. each group of three nucleotides in the sequence, called a codon, corresponds to one of the twenty possible amino acids in protein - this correspondence is called the genetic code. the flow of information is unidirectional : information is transferred


group of three nucleotides in the sequence, called a codon, corresponds to one of the twenty possible amino acids in protein - this correspondence is called the genetic code. the flow of information is unidirectional : information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of dna — a phenomenon francis crick called the central dogma of molecular biology. the specific sequence of amino acids results in a unique three - dimensional structure for that protein, and the three - dimensional structures of protein are related to their function. some are simple structural molecules, like the fibers formed by the protein collagen. proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules ( without changing the structure of the protein itself ). protein structure is dynamic ; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. a single nucleotide difference within dna can cause a single change in the amino acid sequence of a protein. because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. for example, sickle - cell anemia is a human genetic disease that results from a single base difference within the coding region for the β - globin section of hemoglobin, causing a single amino acid change that changes hemoglobin ' s physical properties. sickle - cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. these sickle - shaped cells no longer flow smoothly through blood vessels, having a tendency to clog or degrade, causing the medical problems associated with this disease. some genes are transcribed into rna but are not translated into protein products - these are called non - coding rna molecules. in some cases, these products fold into structures which are involved in critical cell functions ( eg. ribosomal rna and transfer rna ). rna can also have regulatory effect through hybridization interactions with other rna molecules ( eg. microrna ). nature versus nurtureedit although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotype — a dichotomy often referred to as


molecules ( eg. microrna ). nature versus nurtureedit although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotype — a dichotomy often referred to as " nature vs. nurture. " the phenotype of an organism depends on the interaction of genetics with the environment. one example of this is the case of temperature - sensitive mutations. often, a single amino acid change within the sequence of a protein does not change its behavior and interactions with other molecules, but it does destabilize the structure. in a high temperature environment, where molecules are moving more quickly and hitting each other, this results in the protein losing its structure and failing to function. in a low temperature environment, however, the protein ' s structure is stable and functions normally. this type of mutation is visible in the coat coloration of siamese cats, where a mutation in an enzyme responsible for pigment production causes it to destabilize and lose function at high temperatures. the protein remains functional in areas of skin that are colder — legs, ears, tail, and face — and so the cat has dark fur at its extremities. environment also plays a dramatic role in effects of the human genetic disease phenylketonuria. the mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build - up of an intermediate molecule that, in turn, causes severe symptoms of progressive mental retardation and seizures. if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, however, they remain normal and healthy. - main article : regulation of gene expression the genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. a gene is expressed when it is being transcribed into mrna ( and translated into protein ), and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. transcription factors are regulatory proteins that bind to the start of genes, either promoting or inhibiting the transcription of the gene. within the genome of escherichia coli bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. however, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. the presence of


, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. however, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. the presence of tryptophan directly affects the activity of the genes — tryptophan molecules bind to the tryptophan repressor ( a transcription factor ), changing the repressor ' s structure such that the repressor binds to the genes. the tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process. differences in gene expression are especially clear within multicellular organisms, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. all the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. as no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells. within eukaryotes there exist structural features of chromatin that influence the transcription of genes, often in the form of modifications to dna and chromatin that are stably inherited by daughter cells. these features are called " epigenetic " because they exist " on top " of the dna sequence and retain inheritance from one cell generation to the next. because of epigenetic features, different cell types grown within the same medium can retain very different properties. although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of dna as the basis for inheritance. - main article : mutation during the process of dna replication, errors occasionally occur in the polymerization of the second strand. these errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. error rates are usually very low — 1 error in every 10 – 100 million bases — due to the " proofreading " ability of dna polymerases. ( without proofreading error rates are a thousand - fold higher ; because many viruses rely on dna and rna polymerases that lack proofreading ability, they experience higher mutation rates. ) processes that increase the rate of changes in


beneficial mutations can cause a species to evolve into forms that better survive their environment, a process called adaptation. new species are formed through the process of speciation, a process often caused by geographical separations that allow different populations to genetically diverge. the application of genetic principles to the study of population biology and evolution is referred to as the modern synthesis. as sequences diverge and change during the process of evolution, these differences between sequences can be used as a molecular clock to calculate the evolutionary distance between them. genetic comparisons are generally considered the most accurate method of characterizing the relatedness between species, an improvement over the sometimes deceptive comparison of phenotypic characteristics. the evolutionary distances between species can be combined to form evolutionary trees - these trees represent the common descent and divergence of species over time, although they cannot represent the transfer of genetic material between unrelated species ( known as horizontal gene transfer and most common in bacteria ). research and technologyedit model organisms and geneticsedit although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. the fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few model organisms became the basis for most genetics research. common research topics in model organism genetics include the study of gene regulation and the involvement of genes in development and cancer. organisms were chosen, in part, for convenience — short generation times and easy genetic manipulation made some organisms popular genetics research tools. widely used model organisms include the gut bacterium escherichia coli, the plant arabidopsis thaliana, baker ' s yeast ( saccharomyces cerevisiae ), the nematode caenorhabditis elegans, the common fruit fly ( drosophila melanogaster ), and the common house mouse ( mus musculus ). medical genetics researchedit medical genetics seeks to understand how genetic variation relates to human health and disease. when searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic pedigree charts to find the location on the genome associated with the disease. at the population level, researchers take advantage of mendelian randomization to look for locations in the genome that are associated with diseases, a technique especially useful for multigenic traits not clearly defined by a single gene. once a candidate gene is found, further research is often done on the same gene ( called an ortholo


look for locations in the genome that are associated with diseases, a technique especially useful for multigenic traits not clearly defined by a single gene. once a candidate gene is found, further research is often done on the same gene ( called an orthologous gene ) in model organisms. in addition to studying genetic diseases, the increased availability of genotyping techniques has led to the field of pharmacogenetics — studying how genotype can affect drug responses. although it is not an inherited disease, cancer is also considered a genetic disease. the process of cancer development in the body is a combination of events. mutations occasionally occur within cells in the body as they divide. while these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. there are biological mechanisms that attempt to stop this process ; signals are given to inappropriately dividing cells that should trigger cell death, but sometimes additional mutations occur that cause cells to ignore these messages. an internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body. dna can be manipulated in the laboratory. restriction enzymes are a commonly used enzyme that cuts dna at specific sequences, producing predictable fragments of dna. the use of ligation enzymes allows these fragments to be reconnected, and by ligating fragments of dna together from different sources, researchers can create recombinant dna. often associated with genetically modified organisms, recombinant dna is commonly used in the context of plasmids - short circular dna fragments with a few genes on them. by inserting plasmids into bacteria and growing those bacteria on plates of agar ( to isolate clones of bacteria cells ), researchers can clonally amplify the inserted fragment of dna ( a process known as molecular cloning ). ( cloning can also refer to the creation of clonal organisms, through various techniques. ) dna can also be amplified using a procedure called the polymerase chain reaction ( pcr ). by using specific short sequences of dna, pcr can isolate and exponentially amplify a targeted region of dna. because it can amplify from extremely small amounts of dna, pcr is also often used to detect the presence of specific dna sequences. dna sequencing and genomicsedit one of the most fundamental technologies developed to study genetics, dna sequencing allows researchers to determine the sequence of nucleotide


small amounts of dna, pcr is also often used to detect the presence of specific dna sequences. dna sequencing and genomicsedit one of the most fundamental technologies developed to study genetics, dna sequencing allows researchers to determine the sequence of nucleotides in dna fragments. developed in 1977 by frederick sanger and coworkers, chain - termination sequencing is now routinely used to sequence dna fragments. with this technology, researchers have been able to study the molecular sequences associated with many human diseases. as sequencing has become less expensive and with the aid of computational tools, researchers have sequenced the genomes of many organisms by stitching together the sequences of many different fragments ( a process called genome assembly ). these technologies were used to sequence the human genome, leading to the completion of the human genome project in 2003. new high - throughput sequencing technologies are dramatically lowering the cost of dna sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars. the large amount of sequences available has created the field of genomics, research that uses computational tools to search for and analyze patterns in the full genomes of organisms. genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze large sets of biological data. - animal breeding - animal mate selection - animal strain differences - assortive mating - behavioural genetics - blood groups - family resemblance - genetic counselling - population genetics - genetic disorders - genetic engineering - human accelerated regions - hybrids ( biology ) - instinctive behaviour - reproductive technology - selective breeding - sexual reproduction - species differences - translocation ( chromosomes ) - ↑ genetikos, henry george liddell, robert scott, " a greek - english lexicon ", at perseus - ↑ genesis, henry george liddell, robert scott, " a greek - english lexicon ", at perseus - ↑ online etymology dictionary - ↑ griffiths et al. ( 2000 ), chapter 1 ( genetics and the organism ) : introduction - ↑ hartl d, jones e ( 2005 ) - ↑ weiling f ( 1991 ). historical study : johann gregor mendel 1822 – 1884. american journal of medical genetics 40 ( 1 ) : 1 – 25 ; discussion 26. - ↑ lamarck, j - b ( 2008 ). in encyclopædia britannica. retrieved from encyclopædia britannica online on 2008 - 03 - 16. - ↑ peter j. bowler, the mendelian revolution


, j - b ( 2008 ). in encyclopædia britannica. retrieved from encyclopædia britannica online on 2008 - 03 - 16. - ↑ peter j. bowler, the mendelian revolution : the emergency of hereditarian concepts in modern science and society ( baltimore : johns hopkins university press, 1989 ) : chapters 2 & 3. - ↑ 9. 0 9. 1 mendel, gj ( 1866 ). versuche uber pflanzen - hybriden. verhandlungen des naturforschenden vereins brunn 4 : 3 – 47. ( in english in 1901, j. r. hortic. soc. 26 : 1 – 32 ) english translation available online - ↑ genetics, n., oxford english dictionary, 3rd ed. - ↑ bateson w. letter from william bateson to alan sedgwick in 1905. the john innes centre. url accessed on 2008 - 03 - 15.. note that the letter was to an adam sedgwick, a zoologist at trinity college, cambridge, not " alan ", and not to be confused with the renown british geologist, adam sedgwick, who lived some time earlier. - ↑ genetic, adj., oxford english dictionary, 3rd ed. - ↑ bateson, w ( 1907 ). " the progress of genetic research ". wilks, w ( editor ) report of the third 1906 international conference on genetics : hybridization ( the cross - breeding of genera or species ), the cross - breeding of varieties, and general plant breeding, london : royal horticultural society. - initially titled the " international conference on hybridisation and plant breeding ", wilks changed the title for publication as a result of bateson ' s speech. - ↑ moore ja ( 1983 ). thomas hunt morgan — the geneticist. american zoologist 23 ( 4 ) : 855 – 865. - ↑ sturtevant ah ( 1913 ). the linear arrangement of six sex - linked factors in drosophila, as shown by their mode of association. journal of experimental biology 14 : 43 – 59. pdf from electronic scholarly publishing - ↑ avery ot, macleod cm, and mccarty m ( 1944 ). studies on the chemical nature of the substance inducing transformation of pneumococcal types : induction of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type iii. journal of experimental


1944 ). studies on the chemical nature of the substance inducing transformation of pneumococcal types : induction of transformation by a desoxyribonucleic acid fraction isolated from pneumococcus type iii. journal of experimental medicine 79 ( 1 ) : 137 – 158. 35th anniversary reprint available - ↑ hershey ad, chase m ( 1952 ). independent functions of viral protein and nucleic acid in growth of bacteriophage. the journal of general physiology 36 : 39 – 56. - ↑ judson, horace ( 1979 ). the eighth day of creation : makers of the revolution in biology, 51 – 169, cold spring harbor laboratory press. - ↑ watson jd, crick fhc ( 1953 ). molecular structure of nucleic acids : a structure for deoxyribose nucleic acid. nature 171 ( 4356 ) : 737 – 738. - ↑ watson jd, crick fhc ( 1953 ). genetical implications of the structure of deoxyribonucleic acid. nature 171 ( 4361 ) : 964 – 967. - ↑ sanger f, nicklen s, and coulson ar ( 1977 ). dna sequencing with chain - terminating inhibitors. nature 74 ( 12 ) : 5463 – 5467. - ↑ saiki rk, scharf s, faloona f, mullis kb, horn gt, erlich ha, arnheim n ( 1985 ). enzymatic amplification of β - globin genomic sequences and restriction site analysis for diagnosis of sickle cell anemia. science 230 ( 4732 ) : 1350 – 1354. - ↑ 23. 0 23. 1 human genome project information. human genome project. url accessed on 2008 - 03 - 15. - ↑ griffiths et al. ( 2000 ), chapter 2 ( patterns of inheritance ) : introduction - ↑ griffiths et al. ( 2000 ), chapter 2 ( patterns of inheritance ) : mendel ' s experiments - ↑ 26. 0 26. 1 26. 2 griffiths et al. ( 2000 ), chapter 3 ( chromosomal basis of heredity ) : mendelian genetics in eukaryotic life cycles - ↑ griffiths et al. ( 2000 ), chapter 4 ( gene interaction ) : interactions between the alleles of one gene - ↑ richard w. cheney. genetic notation. url accessed on 2008 - 03 - 18. - ↑ griffiths et al. ( 2000 ), chapter 2


2000 ), chapter 4 ( gene interaction ) : interactions between the alleles of one gene - ↑ richard w. cheney. genetic notation. url accessed on 2008 - 03 - 18. - ↑ griffiths et al. ( 2000 ), chapter 2 ( patterns of inheritance ) : human genetics - ↑ griffiths et al. ( 2000 ), chapter 4 ( gene interaction ) : gene interaction and modified dihybrid ratios - ↑ mayeux r ( 2005 ). mapping the new frontier : complex genetic disorders. the journal of clinical investigation 115 ( 6 ) : 1404 – 1407. - ↑ griffiths et al. ( 2000 ), chapter 25 ( quantitative genetics ) : quantifying heritability - ↑ luke a, guo x, adeyemo aa, wilks r, forrester t, lowe w jr, comuzzie ag, martin lj, zhu x, rotimi cn, cooper rs ( 2001 ). heritability of obesity - related traits among nigerians, jamaicans and us black people. int j obes relat metab disord 25 ( 7 ) : 1034 – 1041. abstract from ncbi - ↑ pearson h ( 2006 ). genetics : what is a gene?. nature 441 ( 7092 ) : 398 – 401. - ↑ prescott, l ( 1993 ). microbiology, wm. c. brown publishers. - ↑ griffiths et al. ( 2000 ), chapter 8 ( the structure and replication of dna ) : mechanism of dna replication - ↑ gregory sg et al. ( 2006 ). the dna sequence and biological annotation of human chromosome 1. nature 441 : 315 – 321. free full text available - ↑ alberts et al. ( 2002 ), ii. 4. dna and chromosomes : chromosomal dna and its packaging in the chromatin fiber - ↑ griffiths et al. ( 2000 ), chapter 2 ( patterns of inheritance ) : sex chromosomes and sex - linked inheritance - ↑ griffiths et al. ( 2000 ), chapter 7 ( gene transfer in bacteria and their viruses ) : bacterial conjugation - ↑ griffiths et al. ( 2000 ), chapter 7 ( gene transfer in bacteria and their viruses ) : bacterial transformation - ↑ griffiths et al. ( 2000 ), chapter 5 ( basic eukaryotic chromosome mapping ) : nature of crossing - over - ↑ griffiths et al. ( 2000 ), chapter 5 ( basic eukaryotic chromosome mapping ) : linkage maps - ↑ berg jm, tymoczko j


basic eukaryotic chromosome mapping ) : nature of crossing - over - ↑ griffiths et al. ( 2000 ), chapter 5 ( basic eukaryotic chromosome mapping ) : linkage maps - ↑ berg jm, tymoczko jl, stryer l, clarke nd ( 2002 ). biochemistry, 5th edition, new york : w. h. freeman and company. i. 5. dna, rna, and the flow of genetic information : amino acids are encoded by groups of three bases starting from a fixed point - ↑ crick, f ( 1970 ) : central dogma of molecular biology ( pdf ). nature 227, 561 – 563. pmid 4913914 - ↑ alberts et al. ( 2002 ), i. 3. proteins : the shape and structure of proteins - ↑ alberts et al. ( 2002 ), i. 3. proteins : protein function - ↑ how does sickle cell cause disease?. brigham and women ' s hospital : information center for sickle cell and thalassemic disorders. url accessed on 2007 - 07 - 23. - ↑ imes dl, geary la, grahn ra, lyons la ( 2006 ). albinism in the domestic cat ( felis catus ) is associated with a tyrosinase ( tyr ) mutation. animal genetics 37 ( 2 ) : 175. - ↑ medlineplus : phenylketonuria. nih : national library of medicine. url accessed on 2008 - 03 - 15. - ↑ brivanlou ah, darnell je jr ( 2002 ). signal transduction and the control of gene expression. science 295 ( 5556 ) : 813 – 818. - ↑ alberts et al. ( 2002 ), ii. 3. control of gene expression – the tryptophan repressor is a simple switch that turns genes on and off in bacteria - ↑ jaenisch r, bird a. epigenetic regulation of gene expression : how the genome integrates intrinsic and environmental signals. nature genetics 33 ( 3s ) : 245 – 254. - ↑ chandler vl ( 2007 ). paramutation : from maize to mice. cell 128 : 641 – 645. - ↑ griffiths et al. ( 2000 ), chapter 16 ( mechanisms of gene mutation ) : spontaneous mutations - ↑ kunkel ta ( 2004 ). dna replication fidelity. journal of biological chemistry 279 ( 17 ) : 16895 – 16898. - ↑


↑ griffiths et al. ( 2000 ), chapter 16 ( mechanisms of gene mutation ) : spontaneous mutations - ↑ kunkel ta ( 2004 ). dna replication fidelity. journal of biological chemistry 279 ( 17 ) : 16895 – 16898. - ↑ griffiths et al. ( 2000 ), chapter 16 ( mechanisms of gene mutation ) : induced mutations - ↑ griffiths et al. ( 2000 ), chapter 17 ( chromosome mutation i : changes in chromosome structure ) : introduction - ↑ sawyer sa, parsch j, zhang z, hartl dl ( 2007 ). prevalence of positive selection among nearly neutral amino acid replacements in drosophila. proc. natl. acad. sci. u. s. a. 104 ( 16 ) : 6504 – 10. - ↑ griffiths et al. ( 2000 ), chapter 24 ( population genetics ) : variation and its modulation - ↑ griffiths et al. ( 2000 ), chapter 24 ( population genetics ) : selection - ↑ griffiths et al. ( 2000 ), chapter 24 ( population genetics ) : random events - ↑ darwin, charles ( 1859 ). on the origin of species, 1st, 1, john murray.. related earlier ideas were acknowledged in darwin, charles ( 1861 ). on the origin of species, 3rd, xiii, john murray. - ↑ gavrilets s ( 2003 ). perspective : models of speciation : what have we learned in 40 years?. evolution 57 ( 10 ) : 2197 – 2215. - ↑ wolf yi, rogozin ib, grishin nv, koonin ev ( 2002 ). genome trees and the tree of life. trends genet. 18 ( 9 ) : 472 – 479. - ↑ the use of model organisms in instruction. university of wisconsin : wisconsin outreach research modules. url accessed on 2008 - 03 - 15. - ↑ ncbi : genes and disease. nih : national center for biotechnology information. url accessed on 2008 - 03 - 15. - ↑ davey smith, g, ebrahim, s ( 2003 ). ‘ mendelian randomization ’ : can genetic epidemiology contribute to understanding environmental determinants of disease?. international journal of epidemiology 32 : 1 – 22. - ↑ pharmacogenetics fact sheet. nih : national institute of general medical sciences. url accessed on 2008 - 03 - 15. - ↑ strachan t, read ap ( 1999 ). human molecular genetics 2, second edition, john wiley


diagnosing fasd in canada and harmonized most differences between the iom and university of washington ' s systems. each diagnostic system requires that a complete fasd evaluation include assessment of the four key features of fasd, described below. a positive finding on all four features is required for a diagnosis of fas, the first diagnosable condition of fasd that was discovered. however, prenatal alcohol exposure and central nervous system damage are the critical elements of the spectrum of fasd, and a positive finding in these two features is sufficient for an fasd diagnosis that is not " full - blown fas. " diagnoses are described in a following section. key features of fasdedit each of the key features of fasd can vary widely within one individual exposed to prenatal alcohol. while consensus exists for the definition and diagnosis of fas across diagnostic systems, minor variations among the systems lead to differences in definitions and diagnostic cut - off criteria for other disgnoses across the fasd continuum. ( the central nervous system ( cns ) damage criteria particularly lack clear consensus. ) a working knowledge of the key features is helpful in understanding fasd diagnoses and conditions, and each are reviewed with attention to similarities and differences across the four diagnostic systems. in terms of fasd, growth deficiency is defined as significantly below average height, weight or both due to prenatal alcohol exposure, and can be assessed at any point in the lifespan. growth measurements must be adjusted for parental height, gestational age ( for a premature infant ), and other postnatal insults ( e. g., poor nutrition ), although birth height and weight are the preferred measurements. deficiencies are documented when height or weight falls at or below the 10th percentile of standardized growth charts appropriate to the patient ' s population. criteria for fasd are least specific in the iom diagnostic system ( " low birth weight..., decelerating weight not due to nutrition..., [ or ] disproportional low weight to height " p. 4 of executive summary ), while the cdc and canadian guidelines use the 10th percentile as a cut - off to determine growth deficiency. the " 4 - digit diagnostic code " allows for mid - range gradations in growth deficiency ( between the 3rd and 10th percentiles ) and severe growth deficiency at or below the 3rd percentile. growth deficiency ( at severe, moderate, or mild levels ) contributes to diagnoses of fas and pfas


damage to the brain or brain structures caused by prenatal alcohol exposure. structural impairments may include microcephaly ( small head size ) of two or more standard deviations below the average, or other abnormalities in brain structure ( e. g., agenesis of the corpus callosum, cerebellar hypoplasia ). microcephaly is determined by comparing head circumference ( often called occipitofrontal circumference, or ofc ) to appropriate ofc growth charts. other structural impairments must be observed through medical imaging techniques by a trained physician. because imaging procedures are expensive and relatively inaccessible to most patients, diagnosis of fasd is not frequently made via structural impairments except for microcephaly. when structural impairments are not observable or do not exist, neurological impairments are assessed. in the context of fasd, neurological impairments are caused by prenatal alcohol exposure which causes general neurological damage to the central nervous system ( cns ), the peripheral nervous system, or the autonomic nervous system. a determination of a neurological problem must be made by a trained physician, and must not be due to a postnatal insult, such as a high fever, concussion, traumatic brain injury, etc. all four diagnostic systems show virtual agreement on their criteria for cns damage at the neurological level, and evidence of a cns neurological impairment due to prenatal alcohol exposure will result in a diagnosis of fas or pfas, and functional impairments are highly likely. neurological problems are expressed as either hard signs, or diagnosable disorders, such as epilepsy or other seizure disorders, or soft signs. soft signs are broader, nonspecific neurological impairments, or symptoms, such as impaired fine motor skills, neurosensory hearing loss, poor gait, clumsiness, poor eye - hand coordination, or sensory integration dysfunction. many soft signs have norm - referenced criteria, while others are determined through clinical judgment. when structural or neurological impairments are not observed, all four diagnostic systems allow cns damage due to prenatal alcohol exposure to be assessed in terms of functional impairments. functional impairments are deficits, problems, delays, or abnormalities due to prenatal alcohol exposure ( rather than hereditary causes or postnatal insults ) in observable and measurable domains related to daily functioning, often referred to as developmental disabilities. there is no consensus on a specific pattern of functional impairment


abnormalities due to prenatal alcohol exposure ( rather than hereditary causes or postnatal insults ) in observable and measurable domains related to daily functioning, often referred to as developmental disabilities. there is no consensus on a specific pattern of functional impairments due to prenatal alcohol exposure and only cdc guidelines label developmental delays as such, so criteria ( and fasd diagnoses ) vary somewhat across diagnostic systems. the four diagnostic systems list various cns domains that can qualify for functional impairment that can determine an fasd diagnosis : - evidence of a complex pattern of behavior or cognitive abnormalities inconsistent with developmental level in the following cns domains - sufficient for a pfas or arnd diagnosis using iom guidelines - performance at two or more standard deviations on standardized testing in three or more of the following cns domains - sufficient for an fas, pfas or static encephalopathy diagnosis using 4 - digit diagnostic code - general cognitive deficits ( e. g., iq ) at or below the 3rd percentile on standardized testing - sufficient for an fas diagnosis using cdc guidelines - performance at or below the 16th percentile on standardized testing in three or more of the following cns domains - sufficient for an fas diagnosis using cdc guidelines - performance at two or more standard deviations on standardized testing in three or more of the following cns domains - sufficient for an fas diagnosis using canadian guidelines ten brain domainsedit a recent effort to standardize assessment of functional cns damage has been suggested by an experienced fasd diagnostic team in minnesota. the proposed framework attempts to harmonize iom, 4 - digit diagnostic code, cdc, and canadian guidelines for measuring cns damage viz - a - viz fasd evaluations and diagnosis. the standardized approach is referred to as the ten brain domains and encompasses aspects of all four diagnostic systems ' recommendations for assessing cns damage due to prenatal alcohol exposure. the framework provides clear definitions of brain dysfunction, specifies empirical data needed for accurate diagnosis, and defines intervention considerations that address the complex nature of fasd with the intention to avoid common secondary disabilities. the proposed ten brain domains include : - achievement, adaptive behavior, attention, cognition, executive functioning, language, memory, motor skills, sensory integration or soft neurological problems, social communication the fetal alcohol diagnostic program ( fadp ) uses unpublished minnesota state criteria of performance at 1. 5 or more standard deviations on standardized testing in three or more of the ten brain domains to determine cns damage. however, the ten brain domains are


the fetal alcohol diagnostic program ( fadp ) uses unpublished minnesota state criteria of performance at 1. 5 or more standard deviations on standardized testing in three or more of the ten brain domains to determine cns damage. however, the ten brain domains are easily incorporated into any of the four diagnostic systems ' cns damage criteria, as the framework only proposes the domains, rather than the cut - off criteria for fasd. prenatal alcohol exposureedit prenatal alcohol exposure is determined by interview of the biological mother or other family members knowledgeable of the mother ' s alcohol use during the pregnancy ( if available ), prenatal health records ( if available ), and review of available birth records, court records ( if applicable ), chemical dependency treatment records ( if applicable ), or other reliable sources. exposure level is assessed as confirmed exposure, unknown exposure, and confirmed absence of exposure by the iom, cdc and canadian diagnostic systems. the " 4 - digit diagnostic code " further distinguishes confirmed exposure as high risk and some risk : - high risk - confirmed use of alcohol during pregnancy known to be at high blood alcohol levels ( 100mg / dl or greater ) delivered at least weekly in early pregnancy. - some risk - confirmed use of alcohol during pregnancy with use less than high risk or unknown usage patterns. - unknown risk - unknown use of alcohol during pregnancy. - no risk - confirmed absence of prenatal alcohol exposure. amount, frequency, and timing of prenatal alcohol use can dramatically impact the other three key features of fasd. while consensus exists that alcohol is a teratogen, there is no clear consensus as to what level of exposure is toxic. the cdc guidelines are silent on these elements diagnostically. the iom and canadian guidelines explore this further, acknowledging the importance of significant alcohol exposure from regular or heavy episodic alcohol consumption in determining, but offer no standard for diagnosis. canadian guidelines discuss this lack of clarity and parenthetically point out that " heavy alcohol use " is defined by the national institute on alcohol abuse and alcoholism as five or more drinks per episode on five or more days during a 30 day period. " the 4 - digit diagnostic code " ranking system distinguishes between levels of prenatal alcohol exposure as high risk and some risk. it operationalizes high risk exposure as a blood alcohol concentration ( bac ) greater than 100mg / dl delivered at least weekly in early pregnancy. this bac level is typically reached by a 55kg female drinking six to eight beers in one sitting. for many adopted


susan. backside of lip - philtrum guides ( 2004 ) ( pdf ). university of washington, fetal alcohol syndrome diagnostic and prevention network. retrieved on [ [ 2007 - 04 - 11 ] - ↑ west, j. r. ( ed. ) ( 1986 ). alcohol and brain development. new york : oxford university press. - ↑ 17. 0 17. 1 fadp - fetal alcohol diagnostic program - ↑ 18. 0 18. 1 18. 2 18. 3 18. 4 18. 5 lang, j. ( 2006 ). ten brain domains : a proposal for functional central nervous system parameters for fetal alcohol spectrum disorder diagnosis and follow - up. journal of the fas institute, 4, 1 - 11. can be downloaded at http : / / www. motherisk. org / jfas _ documents / jfas _ 5012 _ final _ e12 _ 6. 28. 6. pdf - ↑ u. s. department of health and human services. ( 2000 ). national institute on alcohol abuse and alcoholism. tenth special report to the u. s > congress on alcohol and health : highlights frfom current research. washington, dc : the institute. - ↑ 20. 0 20. 1 20. 2 streissguth, a. ( 1997 ). fetal alcohol syndrome : a guide for families and communities. baltimore : brookes publishing. isbn 1 - 55766 - 283 - 5. - ↑ malbin, d. ( 2002 ). fetal alcohol spectrum disorders : trying differently rather than harder. portland, or : fascets, inc. isbn 0 - 9729532 - 0 - 5. - ↑ smith, d. w. ( 1981 ). fetal alcohol syndrome and fetal alcohol effects. neurobehavioral toxicology and teratology, 3, 127. - ↑ aase, j. m., jones, k. l., & clarren, s. k. ( 1995 ). do we need the term fae? pediatrics, 95 ( 3 ), 428 - 430. - ↑ sokol, r. j., & clarren, s. k. ( 1989 ). guidelines for use of terminology describing the impact of prenatal alcohol on the offspring. alcoholism : clinical and experimental research, 13 ( 4 ), 597 - 598. - samhsa fetal alcohol spectrum disorders center of excellence - u. s. congressional caucus on fetal alcohol spectrum disorders - nofas - uk | this page


that you ’ re a religious person. " and i realized that being a person of faith and having a community, that having a base in faith, is a source of support that others don ’ t have. now, some people said to me that my reaction was courageous. but i ’ ve always felt anger at the suffering of others. for me, anger is energy, it ’ s a force. you channel energy positively or negatively. being sensitive to situations of injustice and the necessity of confronting difficult situations like those we see every day, we have to get angry to provoke energy and react. if an act of injustice doesn ’ t provoke anger in me, it could be seen as indifference, passivity. it ’ s injustice that motivates us to do something, to take risks, knowing that if we don ’ t, things will remain the same. anger has made us confront police and soldiers. something that i discovered is that the police and soldiers are used to their superiors shouting at them, and they ’ re used to being mistreated. so when they run into a woman, otherwise insignificant to them, who demands things of them and shouts at them in an authoritarian way, they are paralyzed. and we get results. i consider myself an aggressive person, and it has been difficult for me to manage that within the context of my religious education. but it does disarm authorities. i normally dress this way, in a way that my friends call monklike. that ’ s fine. it keeps people off guard. i give a certain mild image, but then i can, more efficiently, demand things, shout. for example, one time there was a guy who disappeared for twenty days. we knew he was in the military hospital, and we filed habeas corpus petitions on his behalf. but the authorities simply denied having him in custody. one night we were informed that he was being held at a particular state hospital. we went the next day. they denied us access. i spent the whole morning studying the comings and goings at the hospital to see how i could get in. during a change in shifts, i slipped by the guards. when i got to the room where this person was, the nurse at the door told me i could not go in. " we are not even allowed in, " she said. i told her that i would take care of myself ; all i asked of her was that she take note of what i was going to do and that if they did something to me, she should call


for one aes is built for three key sizes 128, 192 or 256 bits. currently, brute - forcing 128 bits is not even close to feasible. hypothetically, if an aes key had 129 bits, it would take twice as long to brute - force a 129 bit key than a 128 bit key. this means larger keys of 192 bits and 256 bits would take much much much longer to attack. it would take so incredibly long to brute - force one of these keys that the sun would stop burning before the key was realized. that ' s a big freaking number. that ' s how many possibly keys there are. assuming the key is random, if you divide that by 2 then you have how many keys it will take on average to brute - force aes - 256 in a sense we do have the really big cipher keys you are talking of. the whole point of a symmetric key is to make it unfeasible to brute - force. in the future, if attacking a 256bit key becomes possible then keysizes will surely increase, but that is quite a ways down the road. the reason rsa keys are much larger than aes keys is because they are two completely different types of encryption. this means a person would not attack a rsa key the same as they would attack an aes key. attacking symmetric keys is easy. - start with a bitstring - decrypt ciphertext with that bitstring. - if you can read it, you succeeded. - if you cannot read it then increment the bitstring attacking an rsa key is different... because rsa encryption / decryption works with big semi - prime numbers... the process is mathy. with rsa, you don ' t have to try every possible bit string. you try far fewer than 2 ^ 2048 bitstrings... but it ' s still not possible to bruteforce. this is why rsa and aes keys differ in size. to sum up everything and answer your question in 1 sentence. we don ' t need ridiculously big symmetric keys because we already have ridiculously big symmetric keys. 256 bit encryption sounds wimpy compared to something like a 2048 bit rsa key, but the algorithms are different and can ' t really be compared ' bit to bit ' like that. in the future if there is a need to longer keys then there will be new algorithms developed to handle larger keys. and if we ever wanted to go bigger on current hardware,