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Ch. 4 - Gene Interaction
Chapter 4, Problem 5

Describe the difference between continuous phenotypic variation and discontinuous variation. Explain how polygenic inheritance could be the basis of a trait showing continuous phenotypic variation. Explain how polygenic inheritance can be the basis of a threshold trait.

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Continuous phenotypic variation refers to traits that show a range of phenotypes, such as height or skin color, where the trait can take any value within a range. Discontinuous variation, on the other hand, refers to traits that have distinct categories, such as blood type or flower color in Mendel's peas, where the trait can only take specific values.
Polygenic inheritance involves multiple genes contributing to a single trait. Each gene may have a small additive effect, and the combination of these effects results in a continuous range of phenotypes. For example, human height is influenced by many genes, each contributing to the overall height, leading to a continuous distribution of heights in the population.
In polygenic inheritance, the more genes involved, the greater the potential for variation, as each gene adds to the overall phenotype. Environmental factors can also influence these traits, further contributing to the continuous variation observed.
A threshold trait is a type of polygenic trait that is expressed only when a certain threshold of genetic and/or environmental factors is exceeded. For example, a disease might only manifest if an individual inherits a certain number of risk alleles and is exposed to specific environmental conditions.
In threshold traits, individuals below the threshold appear normal, while those above it express the trait. This can make the trait appear discontinuous, even though it is influenced by multiple genes, because the expression is an all-or-nothing response once the threshold is crossed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Continuous vs. Discontinuous Variation

Continuous variation refers to traits that show a range of phenotypes, such as height or skin color, where individuals can have any value within a spectrum. In contrast, discontinuous variation involves distinct categories or classes, such as blood type or flower color, where traits are clearly defined and do not blend. Understanding these differences is crucial for analyzing how traits are inherited and expressed.
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Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait, resulting in a continuous range of phenotypes. Each gene may have a small additive effect, leading to variations that can be measured on a continuum. This concept is essential for explaining traits like height, where many genes interact to produce a wide array of possible outcomes.
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Threshold Traits

Threshold traits are characteristics that are influenced by multiple genes but only manifest when a certain genetic or environmental threshold is crossed. For example, a person may have a genetic predisposition for a condition, but it only appears if environmental factors trigger it. This concept illustrates how polygenic inheritance can lead to traits that are not continuously variable but instead appear in a binary or categorical manner.
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Related Practice
Textbook Question

In a test of his chromosome theory of heredity, Morgan crossed a female Drosophila with red eyes to a male with white eyes. The females were produced from Cross A shown in Figure 3.19. Predict the offspring Morgan would have expected under his hypothesis that the gene for eye color is on the X chromosome in fruit flies.

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Textbook Question

Compare and contrast broad sense heritability and narrow sense heritability, giving an example of each measurement and identifying how the measurement is used.

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Textbook Question

In a cross of two pure-breeding lines of tomatoes producing different fruit sizes, the variance in grams (g) of fruit weight in the F₁ is 2.25 g and the variance among the F₂ is 5.40 g. Determine the genetic and environmental variance (VG and VE) for the trait and the broad sense heritability of the trait.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.

In a cross involving these three X-linked genes, do you expect any gene pair(s) to show genetic linkage? Explain your reasoning.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.

Do you expect any of these gene pair(s) to assort independently? Explain your reasoning.

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Textbook Question

In Drosophila, the map positions of genes are given in map units numbering from one end of a chromosome to the other. The X chromosome of Drosophila is 66 m.u. long. The X-linked gene for body color—with two alleles, y⁺ for gray body and y for yellow body—resides at one end of the chromosome at map position 0.0. A nearby locus for eye color, with alleles w⁺ for red eye and w for white eye, is located at map position 1.5. A third X-linked gene, controlling bristle form, with f⁺ for normal bristles and f for forked bristles, is located at map position 56.7. At each locus the wild-type allele is dominant over the mutant allele.

A wild-type female fruit fly with the genotype y⁺w⁺f/ywf⁺ is crossed to a male fruit fly that has yellow body, white eye, and forked bristles. Predict the frequency of each progeny phenotype class produced by this mating.

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