Multiple Choice
Heritability calculations were calculated for a variety of different traits. Which of the following traits would respond best to selection?
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20. Quantitative Genetics
Heritability
2:36 minutesProblem 7a
Textbook Question
Erma and Harvey were a compatible barnyard pair, but a curious sight. Harvey's tail was only 6 cm long, while Erma's was 30 cm. Their F₁ piglet offspring all grew tails that were 18 cm. When inbred, an F₂ generation resulted in many piglets (Erma and Harvey's grandpigs), whose tails ranged in 4-cm intervals from 6 to 30 cm (6, 10, 14, 18, 22, 26, and 30). Most had 18-cm tails, while 1/64 had 6-cm tails and 1/64 had 30-cm tails.
Explain how these tail lengths were inherited by describing the mode of inheritance, indicating how many gene pairs were at work, and designating the genotypes of Harvey, Erma, and their 18-cm-tail offspring.
Verified step by step guidance1
Step 1: Recognize that the inheritance pattern described in the problem suggests polygenic inheritance, where multiple gene pairs contribute to the trait (tail length). The continuous variation in tail lengths and the 4-cm intervals between phenotypes indicate quantitative traits influenced by additive alleles.
Step 2: Determine the number of gene pairs involved. The phenotypic ratio in the F₂ generation (1/64 for the extreme phenotypes) suggests that 6 gene pairs are at work. This is because the formula for determining the number of gene pairs is 1/(2^n), where n is the number of gene pairs. Solving 1/(2^n) = 1/64 gives n = 6.
Step 3: Assign genotypes to Harvey and Erma. Harvey's tail length (6 cm) represents the shortest phenotype, which corresponds to a genotype with all recessive alleles (e.g., aabbccdd...). Erma's tail length (30 cm) represents the longest phenotype, which corresponds to a genotype with all dominant alleles (e.g., AABBCCDD...).
Step 4: Explain the F₁ generation. The F₁ piglets all have 18-cm tails, which is the intermediate phenotype. This occurs because the F₁ generation inherits one dominant allele and one recessive allele for each gene pair (e.g., AaBbCcDd...), resulting in an equal contribution of dominant and recessive alleles.
Step 5: Describe the F₂ generation. When the F₁ piglets are inbred, the offspring display a range of tail lengths due to the random assortment and recombination of alleles. The phenotypes range from 6 cm (all recessive alleles) to 30 cm (all dominant alleles), with intermediate phenotypes being more common due to the higher probability of mixed allele combinations.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Mode of Inheritance
The mode of inheritance refers to the way traits are passed from parents to offspring, which can be influenced by dominant and recessive alleles. In this case, the tail length appears to follow a quantitative trait inheritance pattern, where multiple genes contribute to the phenotype, resulting in a range of tail lengths in the offspring.
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Polygenic Inheritance
Polygenic inheritance involves multiple gene pairs influencing a single trait, leading to continuous variation in phenotypes. The tail lengths of the piglets suggest that at least two gene pairs are involved, as indicated by the range of tail lengths observed in the F₂ generation, which includes various intermediate lengths.
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Genotype Designation
Genotype designation involves identifying the specific alleles present in an organism. In this scenario, Harvey likely has a homozygous recessive genotype for short tails (e.g., 'aa'), while Erma has a homozygous dominant genotype for long tails (e.g., 'AA'). The F₁ offspring, all with 18-cm tails, would have a heterozygous genotype (e.g., 'Aa'), reflecting the intermediate phenotype resulting from the combination of both parents' alleles.
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