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Chapter 12 : From Genotype to Phenotype

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From Genotype to Phenotype, Page 1 of 2

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Abstract:

Mendel studied traits that show simple inheritance patterns, now known as Mendelian one-gene traits with distinct, observable phenotypic differences and clear dominance relationships. Very few visible phenotypic traits have this type of genetic basis. The primary misconception is that one gene leads to one trait and that any one trait can be traced to a single gene. However, a gene usually affects many visible phenotypic characteristics, which is known as pleiotropy. On the other hand, a visible phenotypic trait almost always results from the activities of many genes interacting in different ways, such as additive or epistatic interactions. In addition, the same visible phenotypic trait may be traced to mutations in completely different genes, which is known as genetic heterogeneity. The other misconception is related to the power of genes to determine a trait. Many people assume that someone who has a gene for a trait will definitely have that trait and that someone lacking that gene will not have the trait. However, a gene never acts alone; interactions among many genes, as well as environmental factors, create phenotypic traits. As a result, a person with a gene associated with a trait may not exhibit that trait, while someone without that gene may have the phenotype. Many types of environmental factors influence the phenotypic expression of genetic information. Studies of human twins can shed a little light on the relative contributions of genes and environments to human traits, but in general, they tend to overestimate the amount of genetic contribution.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Figures

Image of Figure 12.1
Figure 12.1

Continuous and discontinuous variation. Most of the variation observed in nature is continuous. In order to determine if a trait exhibits continuous or discontinuous variation, scientists count the number of individuals having a certain trait and create a graph of the results. Frequency histograms represent the number of individuals (frequency) with a certain trait as a bar. In this example of continuous variation, the phenotype being studied is the number of seeds per plant, and the measurement categories range from 0 to 10 seeds/plant. The number of plants with each phenotype determines the height of each bar. For example, one plant had no seeds, one plant had nine seeds, and eight plants had four seeds. A frequency histogram showing discontinuous variation, such as the results Mendel obtained when he crossed pea plants that were heterozygous for flower colors, exhibits no gradation of phenotypes. Offspring occurred in a 3:1 ratio of the two parental types, either purple or white flowers. None of the flowers of the offspring had the lighter or darker shades of purple shown in panel .

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.2
Figure 12.2

Relationships among genes and observable traits. A single gene affects many traits, and any one trait results from the actions of many genes. Although this diagram may seem complex, it omits many intermediate products and feedback loops. In addition, it is limited to genetic interactions and ignores the influence of environmental factors on observable traits.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.3
Figure 12.3

Pedigree analysis for hemophilia. By diagramming the individuals in a family with and without the hemophilia phenotype, medical geneticists determined that its inheritance was associated with the sex of the individual. Therefore, hemophilia is a sex-linked trait, and the gene is located on the X chromosome.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.4
Figure 12.4

From gene mutation to clinical symptoms. Scientists have detailed most of the steps in the chain from the specific nature of the genetic mutation to the encoded protein, the cellular malfunction, and, ultimately, the phenotype for the disease cystic fibrosis. In 75% of individuals with cystic fibrosis, the mutation is a three-nucleotide deletion that leads to the loss of a single amino acid, phenylalanine, from the membrane protein that controls chloride movement in and out of epithelial cells. The channel protein remains closed, and chloride ions build up within the epithelial cells. This leads to osmotic imbalances that cells try to rectify by taking up more water and preventing the influx of sodium. As a result, thick mucus accumulates in the air passageways in the lungs (bronchioles) and in secretory ducts of organs, such as the pancreas and liver, all of which are lined with epithelial cells. The mucus in the lungs encourages the growth of microorganisms; people with cystic fibrosis have difficulty breathing and are susceptible to frequent respiratory infections. Mucus clogging the secretory ducts prevents the release of digestive juices, so people with cystic fibrosis cannot digest their food and their bodies are deprived of nutrients. They have a difficult time gaining weight, irrespective of their food intake.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.5
Figure 12.5

Cumulative pace of gene discovery. New technologies are contributing to an exponential increase in the number of genes identified every year. Minimum estimated values are shown.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.6
Figure 12.6

Pedigree analysis for and . Mutations in the tumor suppressor genes and significantly increase the risk of developing hereditary breast or ovarian cancer. Women with a mutation in either gene are approximately three to seven times more likely to develop hereditary breast cancer than those lacking a or mutation. Not every woman with a mutation in or will develop breast or ovarian cancer. Only 5 to 10% of breast cancers are hereditary breast cancers associated with these two gene loci.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.7
Figure 12.7

Cystic fibrosis pattern of inheritance. Cystic fibrosis is an autosomal recessive trait. One in 2,500 people of European descent has cystic fibrosis, and 1 in 25 is a carrier. Cystic fibrosis pedigree analysis. Cystic fibrosis Punnett square.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.8
Figure 12.8

Penetrance and expressivity. Different mutations in the gene encoding the chloride channel protein have different phenotypic effects.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.9
Figure 12.9

Polydactyly penetrance and expressivity. Polydactylism is an autosomal dominant disorder. At least one member of the highlighted couples has the polydactyly allele but does not express it. Of those that express it, the degree of expression varies, as evidenced by the variable number of digits among individuals with the same allele. In addition, expressivity is variable within a single individual. Polydactyly can occur as an isolated phenotypic effect or as one of many pleiotropic effects of genetic disorders of the skeletal system. The child in the photograph suffers from both dwarfism and polydactylism. A mutation in a single gene is responsible for this autosomal recessive disorder. (Photograph courtesy of Victor McKusick.)

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.10
Figure 12.10

Incomplete dominance. When homozygous red (RR) and white (R'R') snapdragons are crossed, all of the first-generation offspring have pink flowers. When these offspring self-pollinate, all three phenotypes appear. These results reaffirm Mendel's theory of discrete particle inheritance in spite of the “blended” appearance of the first-generation flowers.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.11
Figure 12.11

ABO typing. The laboratory technique of blood typing is based on an immunological reaction of blood to molecules it sees as foreign, or nonself. To determine a person's blood type, a small sample of the blood is divided into four parts, and each part is exposed to one of the other blood types. Blood having one type of carbohydrate (A or B) forms clumps with blood having a different carbohydrate, unless that carbohydrate is the H substance (type O). Synthesis of the H substance is common to all blood types (with the rare exception of the Bombay phenotype), so the H substance is viewed as self by all blood types. As a result, people with type O blood are called universal donors. However, type O blood reacts to all other blood types, because both the A and B carbohydrates are foreign. Type AB blood is the universal recipient, because all three carbohydrates, A, B, and H(O), are self to blood with both the A and B alleles. Type AB blood added to any other blood type causes clumping, because at least one of its carbohydrates, A or B, is foreign to all other types.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.12
Figure 12.12

Molecular basis of ABO blood types. Three different carbohydrate molecules are responsible for the ABO blood groups. All three can be traced to the activity of a single gene that occurs in three allelic forms.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.13
Figure 12.13

Molecular basis of coat color. The three coat colors of Labrador retrievers result from an epistatic interaction between two nonhomologous genes, each having two homologous alleles, one of which is completely dominant to the other. (Photograph of black Lab courtesy of the American Kennel Club, photograph of brown [chocolate] Lab courtesy of Thomas A. Martin, photograph of yellow Lab with black nose courtesy of Mary Lynn D' Aubin, and photograph of yellow Lab with brown nose courtesy of Donna Morgan.)

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.14
Figure 12.14

Pigment synthesis. In mammalian pigment-producing cells (melanocytes),the enzyme tyrosinase converts the amino acid tyrosine to dopaquinone. The pigment synthesis pathway then splits; some dopaquinone is converted to red and yellow pigments (phaeomelanin), and some is converted to black and brown pigments (eumelanin). The enzyme TRP-2 converts dopaquinone to brown pigment molecules, and a second enzyme, TRP-1, converts brown pigment molecules to black pigment molecules. Brown (chocolate) Labrador retrievers cannot convert brown pigments to black pigments because they have a mutant form of TRP-1.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.15
Figure 12.15

Epistatic interaction and coat color. Epistasis, which means “standing on,” is a type of interaction in which one gene prevents the phenotypic expression of a nonhomologous gene. In this case, a mutation in a gene encoding a receptor “stands on” the gene encoding enzymes responsible for brown and black pigment synthesis. Because of a mutation that changes the shape of the MSH-R protein, MSH cannot bind to the cells that synthesize pigments, the melanocytes. The signaling molecule that activates the expression of TRP-1 and TRP-2 is not released, and all of the dopaquinone is converted to phaelomelanin, the precursor for yellow and red pigments. Normal MSH-R protein permits expression of black or brown coat color genotype. Mutant MSH-R protein inhibits expression of black or brown coat color genotype.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.16
Figure 12.16

Polygenic inheritance. Humans express a wide range of variation in hair and eye color, and that variation is continuous. Traits exhibiting continuous phenotypic variation are usually polygenic traits. (Photograph in top left corner courtesy of Hannah Vaughan; all others courtesy of Thomas A. Martin.)

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.17
Figure 12.17

Polygenic inheritance of kernel color. Wheat kernels occur in five different colors that range gradually from white to dark red. Two genes are responsible for this trait, and each has both an additive allele (R and R) and a nonadditive allele (r and r). Each of the additive alleles contributes a small amount of red coloration to the kernel.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.18
Figure 12.18

Polygenic inheritance and continuous variation. If a trait is polygenic, offspring of a cross of two individuals that are heterozygous at all contributing loci occur as a range of different types that gradually change along a continuum. Because the wheat kernel trait is governed by two genes that have one additive and one nonadditive allele, there are five distinct phenotypes that occur in a 1:4:6:4:1 ratio. If three gene loci, each having one additive and one nonadditive allele, contribute to a trait, the offspring of a heterozygous cross would occur as seven phenotypes in a 1:6:15:20:15:6:1 ratio. In this example, the polygenic trait is plant height.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.19
Figure 12.19

Mendelian or polygenic inheritance. Geneticists determine the genetic basis of traits by crossing individuals that are homozygous for the trait of interest, recording the numbers of offspring with certain phenotypes, and then crossing those offspring with each other. If wheat kernel color had been a one-gene, two-allele trait and one allele was completely dominant to the other, all of the first-generation offspring would have been the same color as one of the parents. Instead, all were an intermediate color, and this result is consistent with both a polygenic trait and a Mendelian trait governed by alleles with incomplete dominance. Crossing the first-generation offspring allows geneticists to distinguish between these two possibilities.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.20
Figure 12.20

Resistance to insecticides. Insects can evolve resistance to insecticides that are used frequently. The genetic basis of this resistance is often additive. Insects with the greatest number of additive alleles are most likely to survive when exposed to the insecticide. Note, however, that each allele does not contribute equally to the resistance trait.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.21
Figure 12.21

A temperature-dependent phenotype. As spring arrives, the arctic fox replaces its white winter coat with a brown one. Both coats help the fox blend into its surroundings so it can approach its prey unnoticed. (Photographs by Keith Morehouse [white fox] and Brian Anderson [brown fox], courtesy of the U.S. Fish and Wildlife Service.)

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.22
Figure 12.22

Environmental effects on phenotypes. The water buttercup lives at the edges of ponds where the water level can change frequently. The submerged parts of the plants produce finely divided leaves, and plant stems above water produce large, undivided leaves.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.23
Figure 12.23

Environmental and genetic effects on phenotypes. Fruit flies have compound eyes made up of a number of individual units, or facets. Temperature during development affects the eye size of fruit flies, which is determined by counting the facets. For each temperature, there is variation in the number of facets (represented by dotted vertical lines); therefore, the solid line represents the average number of facets at a given temperature. (Photograph of Mediterranean fruit fly by Scott Bauer, courtesy of Agricultural Research Service, U. S. Department of Agriculture. Scanning electron micrograph courtesy of David Waddell, University of Queensland Center for Microscopy and Microanalysis.)

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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Image of Figure 12.24
Figure 12.24

Genetic and environmental effects on phenotypes. Heights of seven different lines of yarrow plants grown at three elevations. All of the plants within a single line are genetically identical. Therefore, any differences within one of the seven plant lines are due to environmental factors related to elevation. When the heights of the seven lines at three elevations are graphed, it is clear that no generalizations can be made about the effect of elevation on the height of yarrow plants.

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
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References

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Tables

Generic image for table
Table 12.1

Risk of developing breast or ovarian cancer

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
Generic image for table
Table 12.2

Relationship of genotype to phenotype

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
Generic image for table
Untitled

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12
Generic image for table
Table 12.3

Assessing genetic and environmental contributions

Citation: Kreuzer H, Massey A. 2005. From Genotype to Phenotype, p 257-286. In Biology and Biotechnology. ASM Press, Washington, DC. doi: 10.1128/9781555816094.ch12

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