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shiny metal spheres standing near each other on tall, slender rods. When a very high static electrical charge was built up on one sphere, sparks would jump across to the other sphere. 430 nm 400 nm After constructing this device, Hertz set out to investigate whether the sparking would create invisible electromagnetic ... |
long-wavelength light. X rays, which contain a great deal of energy, have very short wavelengths—much shorter than visible light, making them ideal for high-resolution microscopes. Hertz had noted that the strength of the photoelectric effect depends on the wavelength of light; short wavelengths are much more effectiv... |
distance. A specific atom can, therefore, absorb only certain photons of light—namely, those that correspond to the atom’s available electron energy levels. As a result, each molecule has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. Molecules that are good absor... |
lorophylls and Carotenoids Chlorophylls absorb photons by means of an excitation process analogous to the photoelectric effect. These pigments contain a complex ring structure, called a porphyrin ring, with alternating single and double bonds. At the center of the ring is a magnesium atom. Photons absorbed by the pigme... |
. Although retinal absorbs a broad range of wavelengths, it does so with relatively low efficiency. Chlorophyll, in contrast, absorbs in only two narrow bands, but does so with high efficiency. Therefore, plants and most other photosynthetic organisms achieve far higher overall photon capture rates with chlorophyll tha... |
in summer Oak leaf in autumn FIGURE 10.8 Fall colors are produced by carotenoids and other accessory pigments. During the spring and summer, chlorophyll in leaves masks the presence of carotenoids and other accessory pigments. When cool fall temperatures cause leaves to cease manufacturing chlorophyll, the chlorophyll... |
ons across the membrane, generating a gradient of proton concentration. Its arrival at the pump induces the transport of a proton across the membrane. The electron is then passed to an acceptor. 4. Chemiosmosis. The protons that accumulate on one side of the membrane now flow back across the membrane through specific p... |
. Light is absorbed by any one of the hundreds of pigment molecules in a photosystem, which transfer their excitation energy to one with a lower energy level than the others. This reaction center of the photosystem acts as an energy sink, trapping the excitation energy. It was the saturation of these reaction centers, ... |
trap for photon energy, passing an excited electron to an acceptor precisely positioned as its neighbor. Note that here the excited electron itself is transferred, not just the energy as we saw in pigment-pigment transfers. This allows the photon excitation to move away from the chlorophylls and is the key conversion ... |
not visible to the human eye) by the photosystem results in the transmission of an energetic electron along an electron transport chain, eventually combining with a proton to form a hydrogen atom. In the sulfur bacteria, the proton is extracted from hydrogen sulfide, leaving elemental sulfur as a by-product. In bacter... |
way: a second, more powerful photosystem using another arrangement of chlorophyll a was combined with the original. In this second photosystem, called photosystem II, molecules of chlorophyll a are arranged with a different geometry, so that more shorter wavelength, higher energy photons are absorbed than in the ances... |
320 mV). Far-red light on Off Red light on Off Both lights on Off Time In sulfur bacteria, excited electrons ejected from the reaction center travel a circular path, driving a proton pump and then returning to their original photosystem. Plants employ two photosystems in series, which generates power to reduce NADP+ to... |
production of NADPH. For every pair of electrons obtained from water, one molecule of NADPH and slightly more than one molecule of ATP are produced. Photosystem II Stroma Photon Thylakoid membrane Photon Antenna complex Q H+ + NADP+ NADPH Fd H2O pC Proton gradient Plastoquinone Plastocyanin Ferredoxin Water-splitting ... |
Chemiosmosis The primary electron acceptor for the light-energized electrons leaving photosystem II is a quinone molecule, as it was in the bacterial photosystem described earlier. The reduced quinone which results (plastoquinone, symbolized Q) is a strong electron donor; it passes the excited electron to a proton pum... |
in a chloroplast. The b6-f complex embedded in the thylakoid membrane pumps protons into the interior of the thylakoid. ATP is produced on the outside surface of the membrane (stroma side), as protons diffuse back out of the thylakoid through ATP synthase channels. Photosystem I passes electrons to ferredoxin on the s... |
, cells use raw materials provided by the light reactions: 1. Energy. ATP (provided by cyclic and noncyclic photophosphorylation) drives the endergonic reactions. 2. Reducing power. NADPH (provided by photosystem I) provides a source of hydrogens and the energetic electrons needed to bind them to carbon atoms. Much of ... |
Calvin cycle. Melvin Calvin and his coworkers at the University of California worked out the first step of what later became known as the Calvin cycle. They exposed photosynthesizing algae to radioactive carbon dioxide (14CO2). By following the fate of a radioactive carbon atom, they found that it first binds to a mol... |
ATP and NADPH to work building carbon-based molecules, a process that essentially reverses the breakdown of such molecules that occurs in mitochondria. Taken together, chloroplasts and mitochondria carry out a cycle in which energy enters from the sun and leaves as heat and work. Chapter 10 Photosynthesis 199 THE CALV... |
Calvin cycle enzymes—including rubisco—are light activated; that is, they become functional or operate more efficiently in the presence of light. Light also promotes transport of three-carbon intermediates across chloroplast membranes that are required for Calvin cycle reactions. And finally, light promotes the influx... |
-phosphate in the chloroplast is converted to glucose 1-phosphate, in an analogous set of reactions to those done in the cytoplasm, by reversing several reactions similar to those of glycolysis. The glucose 1-phosphate is then combined into an insoluble polymer, forming long chains of starch stored as bulky starch grai... |
PEP carboxylase has a much greater affinity for CO2 than does rubisco. In the C4 pathway, the four-carbon compound undergoes further modification, only to be decarboxylated. The CO2 which is released is then captured by rubisco and drawn into the Calvin cycle. Because an organic compound is donating the CO2, the effec... |
mesophyll cell, where two of the high-energy bonds in an ATP molecule are split to convert the pyruvate back into phosphoenolpyruvate, thus completing the cycle. The enzymes that carry out the Calvin cycle in a C4 plant are located within the bundle-sheath cells, where the increased CO2 concentration decreases photore... |
4 pathway. These organic compounds accumulate throughout the night and are decarboxylated during the day to yield high levels of CO2. In the day, these high levels of CO2 drive the Calvin cycle and minimize photorespiration. Like C4 plants, CAM plants use both C4 and C3 pathways. They differ from C4 plants in that they... |
another photon of light is absorbed, and energized electrons are channeled to a primary electron acceptor, which reduces NADP+ to NADPH. Use of NADPH rather than NADH allows plants and algae to keep the processes of photosynthesis and oxidative respiration separate from each other. 3. How is the energy of light captur... |
in packaging DNA in chromosomes. 11.3 Mitosis is a key phase of the cell cycle. In interphase, the Phases of the Cell Cycle. The cell cycle consists of three growth phases, a nuclear division phase, and a cytoplasmic division stage. Interphase: Preparing for Mitosis. cell grows, replicates its DNA, and prepares for ce... |
DNA circle into the bacterial cell is a remarkable feat of packaging—fully stretched out, the DNA of a bacterium like Escherichia coli is about 500 times longer than the cell itself. The DNA circle is attached at one point to the cytoplasmic surface of the bacterial cell’s plasma membrane. At a specific site on the DN... |
, replication is complete. 208 Part IV Reproduction and Heredity 11.2 Chromosomes are highly ordered structures. Discovery of Chromosomes Chromosomes were first observed by the German embryologist Walther Fleming in 1882, while he was examining the rapidly dividing cells of salamander larvae. When Fleming looked at the... |
yeast) INSECTS Mosquito Drosophila Honeybee Silkworm Total Number of Chromosomes 7 16 6 8 32 56 Group PLANTS Haplopappus gracilis Garden pea Corn Bread wheat Sugarcane Horsetail Adder’s tongue fern Total Number of Chromosomes Group Total Number of Chromosomes 2 14 20 42 80 216 1262 VERTEBRATES Opossum Frog Mouse Human... |
within chromosome Coiling within supercoil Chromatin Chromatin fiber DNA Central histone Nucleosome DNA DNA double helix (duplex) FIGURE 11.5 Levels of eukaryotic chromosomal organization. Nucleotides assemble into long double strands of DNA molecules. These strands require further packaging to fit into the cell nucle... |
of the chromosomes are cut out of the photograph and arranged in order (see figure 11.6). How Many Chromosomes Are in a Cell? With the exception of the gametes (eggs or sperm) and a few specialized tissues, every cell in a human body is diploid (2n). This means that the cell contains two nearly identical copies of eac... |
cycle. Phases of the Cell Cycle The increased size and more complex organization of eukaryotic genomes over those of bacteria required radical changes in the process by which the two replicas of the genome are partitioned into the daughter cells during cell division. This division process is diagrammed as a cell cycle... |
cell cycle in about 212 Part IV Reproduction and Heredity M Metaphase Anaphase Prophase Telophase C G2 S Interphase (G1, S, G2 phases) Mitosis (M) Cytokinesis (C) G1 FIGURE 11.8 The cell cycle. Each wedge represents one hour of the 22-hour cell cycle in human cells growing in culture. G1 represents the primary growth ... |
1 and G2 segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cell’s DNA replicates only during the S phase of the cell cycle. After the chromosomes have replicated in S phase, they remain fully extended and uncoiled. This makes them invisible under the li... |
RNA is transcribed. heterochromatin The portion of a chromosome that remains permanently condensed and, therefore, is not transcribed into RNA. Most centromere regions are heterochromatic. homologues Homologous chromosomes; in diploid cells, one of a pair of chromosomes that carry equivalent genes. kinetochore A disk ... |
indle fibers extend completely across the cell, from one pole to the other. Their orientation determines the plane in which the cell will subsequently divide, through the center of the cell at right angles to the spindle apparatus. In animal cell mitosis, the centrioles extend a radial array of microtubules toward the ... |
axis of cell division. Positioned by the microtubules attached to the kinetochores of their centromeres, all of the chromosomes line up on the metaphase plate (figure 11.11). At this point, which marks the end of metaphase, their centromeres are neatly arrayed in a circle, equidistant from the two poles of the cell, w... |
chromosomes separate. In these electron micrographs of dividing diatoms, the overlap of the microtubules lessens markedly during spindle elongation as the cell passes from metaphase to anaphase. Anaphase and Telophase: Separation of the Chromatids and Reformation of the Nuclei Of all the stages of mitosis, anaphase is... |
During prophase, microtubules attach the centromeres joining pairs of sister chromatids to opposite poles of the spindle apparatus. During metaphase, each chromosome is drawn to a ring along the inner circumference of the cell by the microtubules extending from the centromere to the two poles of the spindle apparatus.... |
fuses with it, effectively dividing the cell in two. Cellulose is then laid down on the new membranes, creating two new cell walls. The space between the daughter cells becomes impregnated with pectins and is called a middle lamella. Cytokinesis in Fungi and Protists In fungi and some groups of protists, the nuclear m... |
events to occur. In principle, a variety of methods can achieve this goal. For example, an internal “clock” can be employed to allow adequate time for each phase of the cycle to be completed. This is how many organisms control their daily activity cycles. The disadvantage of using such a clock to control the cell cycl... |
points” in the cell cycle. proceed to S? pause? withdraw to Go? FIGURE 11.16 The G1 checkpoint. Feedback from the cell determines whether the cell cycle will proceed to the S phase, pause, or withdraw into G0 for an extended rest period. initiating S phase. The G1 checkpoint is where the more complex eukaryotes typical... |
2 checkpoint. But eventually, other cellular enzymes phosphorylate and so activate a few molecules of MPF. These activated MPFs in turn increase the activity of the enzymes that phosphorylate MPF, setting up a positive feedback that leads to a very rapid increase in the cellular concentration of activated MPF. When the... |
. Chapter 11 How Cells Divide 219 Controlling the Cell Cycle in Multicellular Eukaryotes The cells of multicellular eukaryotes are not free to make individual decisions about cell division, as yeast cells are. The body’s organization cannot be maintained without severely limiting cell proliferation, so that only certai... |
shape fitting that growth factor precisely. When the growth factor binds with its receptor, the receptor reacts by triggering events within the cell (figure 11.19). The cellular selectivity of a particular growth factor depends upon which target cells bear its unique receptor. Some growth Growth Factor Epidermal growt... |
precursors. Most animal cells need a combination of several different growth factors to overcome the various controls that inhibit cell division. The G0 Phase. If cells are deprived of appropriate growth factors, they stop at the G1 checkpoint of the cell cycle. With their growth and division arrested, they remain in ... |
examined! It is precisely because p53 is nonfunctional that these cancer cells are able to repeatedly undergo cell division without being halted at the G1 checkpoint (figure 11.20). To test this, scientists administered healthy p53 protein to rapidly dividing cancer cells in a petri dish: the cells soon ceased dividin... |
are myc, fos, and jun, all of which cause unrestrained cell growth and division when they are overexpressed. In a normal cell, the myc proto-oncogene appears to be important in regulating the G1 checkpoint. Cells in which myc expression is prevented will not divide, even in the presence of growth factors. A critical a... |
Growth factor receptor Ras protein More per cell in many breast cancers Activated by mutations of ras in 20–30% of all cancers Src kinase Activated by mutations in 2–5% of all cancers Rb protein p53 protein Mutated in 40% of all cancers Mutated in 50% of all cancers FIGURE 11.22 Mutations cause cancer. Mutations in ge... |
which the mitotic spindle apparatus forms. • In the second stage of mitosis, metaphase, the chromosomes are arranged in a circle around the periphery of the cell. • At the beginning of the third stage of mitosis, anaphase, the centromeres joining each pair of sister chromatids separate, freeing the sister chromatids f... |
osis • Mitosis • Mitosis • Student Research: Nuclear Division in Drosophila • Exploration: Regulating the cell cycle 12 Sexual Reproduction and Meiosis Concept Outline 12.1 Meiosis produces haploid cells from diploid cells. Discovery of Reduction Division. Sexual reproduction does not increase chromosome number because... |
es (eggs and sperm) each contained two chromosomes, while the somatic (nonreproductive) cells of embryos and mature individuals each contained four. Fertilization From his observations, van Beneden proposed in 1887 that an egg and a sperm, each containing half the complement of chromosomes found in other cells, fuse to... |
by the haploid sperm of the father. 226 Part IV Reproduction and Heredity Haploid (n) multicellular organism Gamete formation Mitosis Haploid (n) cells Meiosis Haploid (n) Diploid (2n) germ-line cells Diploid (2n) Gametes Sperm (n) Egg (n) Fertilization Diploid (2n) zygote FIGURE 12.3 Alternation of generations. In se... |
diploid and haploid generations. Diploid (2n) Haploid (n) Grows into adult male or adult female Female (diploid) 2n Male (diploid) 2n Meiosis Meiosis Sperm (haploid) n Egg (haploid) n Fertilization Zygote (diploid) 2n FIGURE 12.4 The sexual life cycle. In animals, the completion of meiosis is followed soon by fertiliz... |
most respects, the second meiotic division is identical to a normal mitotic division. However, because of the crossing over that occurred during the first division, the sister chromatids in meiosis II are not identical to each other. Meiosis is a continuous process, but it is most easily studied when we divide it into... |
threads actually consists of two sister chromatids joined at their centromeres. In prophase I, homologous chromosomes become closely associated in synapsis, exchange segments by crossing over, and then separate. An Overview Prophase I is traditionally divided into five sequential stages: leptotene, zygotene, pachytene... |
plotene. At the beginning of diplotene, the protein lattice of the synaptonemal complex disassembles. Diplotene is a period of intense cell growth. During this period the chromosomes decondense and become very active in transcription. Diakinesis. At the beginning of diakinesis, the transition into metaphase, transcript... |
segments. Chiasma Formation Evidence of crossing over can often be seen under the light microscope as an X-shaped structure known as a chiasma (Greek, “cross”; plural, chiasmata; figure 12.9). The presence of a chiasma indicates that two chromatids (one from each homologue) have exchanged parts (figure 12.10). Like sm... |
ata hold the homologous chromosomes together in metaphase I, so that only one side of each centromere faces outward from the complex; the other side is turned inward toward the other homologue (figure 12.11). Consequently, spindle microtubules are able to attach to kinetochore proteins only on the outside of each centr... |
form two daughter centromeres, as they are in mitosis. Instead, the entire centromere moves to one pole, taking both sister chromatids with it. When the spindle fibers have fully contracted, each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair. Because of the random orie... |
figure 12.15). No two are alike, because of the crossing over in prophase I. Nuclear envelopes then form around each haploid set of chromosomes. The cells that contain these haploid nuclei may develop directly into gametes, as they do in animals. Alternatively, they may themselves divide mitotically, as they do in plan... |
males. Parthenogenesis even occurs among the vertebrates. Some lizards, fishes, and amphibians are capable of reproducing in this way; their unfertilized eggs undergo a mitotic nuclear division without cell cleavage to produce a diploid cell, which then develops into an adult. Recombination Can Be Destructive If repro... |
within cells can induce such breaks. As organisms became larger and longer-lived, it must have become increasingly important for them to be able to repair such damage. The synaptonemal complex, which in early stages of meiosis precisely aligns pairs of homologous chromosomes, may well have evolved originally as a mech... |
, most of the time, thus manages to keep pace with ever-changing physical and biological constraints. This “treadmill evolution” is sometimes called the “Red Queen hypothesis,” after the Queen of Hearts in Lewis Carroll’s Through the Looking Glass, who tells Alice, “Now, here, you see, it takes all the running you can ... |
especially demanding habitats. In vertebrates, on the other hand, the evolutionary premium appears to have been on versatility, and sexual reproduction is the predominant mode of reproduction by an overwhelming margin. The close association between homologous chromosomes that occurs during meiosis may have evolved as ... |
is thought to have evolved initially as a mechanism to repair double-strand breaks in DNA, in which the broken chromosome is paired with its homologue while it is being repaired. • The evolutionary significance of meiosis is that it generates large amounts of recombination, rapidly reshuffling gene combinations, produ... |
Inheritance Can Be Deduced from Pedigrees. Hemophilia is sex-linked. Gene Disorders Can Be Due to Simple Alterations of Proteins. Sickle cell anemia is caused by a single amino acid change. Some Defects May Soon Be Curable. Cystic fibrosis may soon be cured by gene replacement therapy. 13.3 Genes are on chromosomes. C... |
of the thinking about heredity before the twentieth century. The first is that heredity occurs within species. For a very long time people believed that it was possible to obtain bizarre composite animals by breeding (crossing) widely different species. The minotaur of Cretan mythology, a creature with the body of a b... |
passed to offspring, guiding the growth of the corresponding part in the developing embryo. Most similar theories of the direct transmission of hereditary material assumed that the male and female contributions blend in the offspring. Thus, parents with red and brown hair would produce children with reddish brown hair... |
from the same mating exhibited a different alternative (smooth leaves). This segregation of alternative forms of a character, or traits, provided the clue that led Gregor Mendel to his understanding of the nature of heredity. Knight Studies Heredity in Peas Over the next hundred years, other investigators elaborated o... |
(figure 13.6), Mendel initiated a series of experiments on plant hybridization. The results of these experiments would ultimately change our views of heredity irrevocably. Why Mendel Chose the Garden Pea For his experiments, Mendel chose the garden pea, the same plant Knight and many others had studied earlier. The ch... |
flower (longitudinal section). In a pea plant flower, the petals enclose the male anther (containing pollen grains, which give rise to haploid sperm) and the female carpel (containing ovules, which give rise to haploid eggs). This ensures that self-fertilization will take place unless the flower is disturbed. Pollen t... |
s. He then placed that pollen onto the stigma (part of the carpel) of a purple flower whose anthers had been removed, causing crossfertilization to take place. All the seeds in the pod that resulted from this pollination were hybrids of the white-flowered male parent and the purple-flowered female parent. After plantin... |
1 plants as recessive. For each of the seven pairs of contrasting traits that Mendel examined, one of the pair proved to be dominant and the other recessive. The F2 Generation After allowing individual F1 plants to mature and selfpollinate, Mendel collected and planted the seeds from each plant to see what the offsprin... |
truebreeding dominant, two not-true-breeding dominant, and one true-breeding recessive. A Disguised 1:2:1 Ratio Mendel went on to examine how the F2 plants passed traits on to subsequent generations. He found that the recessive 1⁄4 were always true-breeding. In the cross of white-flowered with purple-flowered plants, ... |
kaptonuria Red-green color blindness Cystic fibrosis Lack of melanin pigmentation Middigital hair Inability to metabolize homogenistic acid Inability to distinguish red or green wavelengths of light Brachydactyly Huntington’s disease Abnormal gland secretion, leading to liver degeneration and lung failure Phenylthiocar... |
zygote is said to be homozygous; when the two haploid gametes contain different alleles, the individual offspring is heterozygous. In modern terminology, Mendel’s factors are called genes. We now know that each gene is composed of a particular DNA nucleotide sequence (see chapter 3). The particular location of a gene ... |
test his model, Mendel first expressed it in terms of a simple set of symbols, and then used the symbols to interpret his results. It is very instructive to do the same. Consider again Mendel’s cross of purple-flowered with white-flowered plants. We will assign the symbol P to the dominant allele, associated with the ... |
after its originator, the English geneticist Reginald Crundall Punnett (figure 13.13). Mendel’s model, ana- 248 Part IV Reproduction and Heredity Gametes P p Gametes P p (a) P p P p (b) P p P p P p pp Pp pp P p P p Pp P PP Pp Pp pp p Pp pp FIGURE 13.13 A Punnett square. (a) To make a Punnett square, place the differen... |
outcomes of crosses. If both F1 parents are Pp (heterozygotes), the probability that a particular F2 individual will be pp (homozygous recessive) is the probability of receiving a p gamete from the male (1⁄2) times the probability of receiving a p gamete from the female (1⁄2), or 1⁄4. This is the same operation we per... |
is also difficult (but not impossible) to distinguish between the two possible test plant genotypes by crossing with a heterozygous individual. However, if you cross the test plant with a homozygous recessive individual, the two possible test plant genotypes will give totally different results (figure 13.15): Alternat... |
recessive (white) FIGURE 13.15 A testcross. To determine whether an individual exhibiting a dominant phenotype, such as purple flowers, is homozygous or heterozygous for the dominant allele, Mendel crossed the individual in question with a plant that he knew to be homozygous recessive, in this case a plant with white ... |
Therefore, the probability that there will be one albino child among the three children is: 3p2q = 3 × (3⁄4)2 × (1⁄4) = 27⁄64, or 42% This means that the chance of having one albino child in the three is 42%. Table 13.A Binomial Distribution of the Sexes of Children in Human Families Composition of Family 3 boys 2 boy... |
masked in heterozygotes by the presence of a dominant allele. Chapter 13 Patterns of Inheritance 251 Mendel’s Second Law of Heredity: Independent Assortment After Mendel had demonstrated that different traits of a given character (alleles of a given gene) segregate independently of each other in crosses, he asked whet... |
Y allele but are homozygous recessive for shape (rrY__); and 1 combination among the 16 is homozygous recessive for both genes (rryy). The hypothesis that color and shape genes assort independently thus predicts that the F2 generation will display a 9:3:3:1 phenotypic ratio: nine individuals with round, yellow seeds, ... |
as Mendel’s Second Law of Heredity, or the Law of Independent Assortment. Genes that assort independently of one another, like the seven genes Mendel studied, usually do so because they are located on different chromosomes, which segregate independently during the meiotic process of gamete formation. A modern restatem... |
in this case, by measuring the heights of the individuals in inches, rounding fractions of an inch to the nearest whole number. Each height, in inches, is a separate phenotypic category. Plotting the numbers in each height category produces a histogram, such as that in figure 13.17. The histogram approximates an ideali... |
emia, heart failure, increased susceptibility to pneumonia, kidney failure, enlargement of the spleen, and many other symptoms. It is usually difficult to deduce the nature of the primary defect from the range of a gene’s pleiotropic effects. Chapter 13 Patterns of Inheritance 253 Lack of Complete Dominance Not all alt... |
is above 33°C and the tyrosinase enzyme is inactive, while it is more active at body extremities such as the tips of the ears and tail, where the temperature is below 33°C. The dark melanin pigment this enzyme produces causes the ears, snout, feet, and tail of Himalayan rabbits and Siamese cats to be black. FIGURE 13.... |
by 16 to obtain 7. Thus, Emerson had a modified ratio of 9:7 instead of the usual 9:3:3:1 ratio. Why Was Emerson’s Ratio Modified? When genes act sequentially, as in a biochemical pathway, an allele expressed as a defective enzyme early in the pathway blocks the flow of material through the rest of the pathway. This m... |
eye rims Yellow fur, black nose, lips, eye rims Brown fur, nose, lips, eye rims Black fur, nose, lips, eye rims FIGURE 13.21 The effect of epistatic interactions on coat color in dogs. The coat color seen in Labrador retrievers is an example of the interaction of two genes, each with two alleles. The E gene determines... |
ive to other alleles. When two seemingly normal individuals who are heterozygous for such an allele produce offspring homozygous for the allele, the offspring suffer the detrimental effects of the mutant allele. When a detrimental allele occurs at a significant frequency in a population, the harmful effect it produces ... |
ive) Carrier (heterozygous) Normal (homozygous dominant) FIGURE 13.22 Tay-Sachs disease. Homozygous individuals (left bar) typically have less than 10% of the normal level of hexosaminidase A (right bar), while heterozygous individuals (middle bar) have about 50% of the normal level—enough to prevent deterioration of t... |
i allele because both IA or IB alleles lead to sugar addition and the i allele does not. The different combinations of the three alleles produce four different phenotypes (figure 13.24): 1. Type A individuals add only galactosamine. They are either IAIA homozygotes or IAi heterozygotes. 2. Type B individuals add only ... |
the rhesus monkey in which they were first described. About 85% of adult humans have the Rh cell surface marker on their red blood cells, and are called Rh-positive. Rh-negative persons lack this cell surface marker because they are homozygous for the recessive gene encoding it. If an Rh-negative person is exposed to ... |
01; figure 13.25). In the five generations since Queen Victoria, 10 of her male descendants have had hemophilia. The present British royal family has escaped the disorder because Queen Victoria’s son, King Edward VII, did not inherit the defective allele, and all the subsequent rulers of England are his descendants. Th... |
a third line of descent. Half-shaded symbols represent carriers with one normal allele and one defective allele; fully shaded symbols represent affected individuals. Chapter 13 Patterns of Inheritance 259 Gene Disorders Can Be Due to Simple Alterations of Proteins Sickle cell anemia is a heritable disorder first noted... |
ous for the sickle cell allele are generally indistinguishable from normal persons. However, some of their red blood cells show the sickling characteristic when they are exposed to low levels of oxygen. The allele responsible for sickle cell anemia is particularly common among people of African descent; about 9% of Afr... |
s May Soon Be Curable Some of the most common and serious gene defects result from single recessive mutations, including many of the defects listed in table 13.2. Recent developments in gene technology have raised the hope that this class of disorders may be curable. Perhaps the best example is cystic fibrosis (CF), th... |
working cf gene was successfully transferred via adenovirus into human lung cells growing in tissue culture. The defective cells were “cured,” becoming able to transport chloride ions across their plasma membranes. Then in 1991, a team of researchers successfully transferred a normal human cf gene into the lung cells ... |
individuals have two copies of each heritable gene and gametes have one. Finally, chromosomes segregate during meiosis, and each pair of homologues orients on the metaphase plate independently of every other pair. Segregation and independent assortment were two characteristics of the genes in Mendel’s model. A Problem... |
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