Ch. 25 - Quantitative Genetics and Multifactorial Traits

Problem 24

Chapter 24, Problem 24

In a series of crosses between two true-breeding strains of peaches, the F₁ generation was uniform, producing 30-g peaches. The F₂ fruit mass ranges from 38 to 22 g at intervals of 2 g. Using gene symbols of your choice, give the genotypes of the parents and the F₂.

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Step 1: Define the gene symbols. Let's assume two genes, A and B, each with two alleles: A/a and B/b.

Step 2: Determine the genotypes of the true-breeding parent strains. Since they are true-breeding, one parent could be AABB and the other aabb.

Step 3: Determine the genotype of the F₁ generation. The F₁ generation is uniform, indicating they are all heterozygous, so the genotype is AaBb.

Step 4: Consider the F₂ generation. The F₂ generation results from crossing AaBb x AaBb, which will produce a variety of genotypes.

Step 5: Calculate the possible genotypes in the F₂ generation. Use a Punnett square or the forked-line method to determine the genotypic ratios of the F₂ generation, which will include combinations like AABB, AaBb, aabb, etc.

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Key Concepts

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

True-breeding Strains

True-breeding strains are organisms that, when mated, produce offspring that are identical to themselves for specific traits. In genetics, this means that the parents are homozygous for the traits being studied, ensuring that the F₁ generation will display a uniform phenotype, as seen in the 30-g peaches.

F₁ and F₂ Generations

The F₁ generation is the first filial generation resulting from a cross between two true-breeding parents. In this case, the F₁ peaches are uniform at 30 g. The F₂ generation arises from self-fertilizing the F₁ individuals, leading to a phenotypic range, as observed with fruit masses ranging from 22 g to 38 g.

Phenotypic Variation and Gene Symbols

Phenotypic variation refers to the observable differences in traits among individuals, which can be attributed to genetic and environmental factors. In this scenario, the range of peach weights in the F₂ generation suggests multiple alleles at play. Gene symbols can be assigned to represent these alleles, helping to illustrate the inheritance patterns and genotypes of the parents and offspring.

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Textbook Question

In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F₁ are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F₂ individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

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Textbook Question

Type A1B brachydactyly (short middle phalanges) is a genetically determined trait that maps to the short arm of chromosome 5 in humans. If you classify individuals as either having or not having brachydactyly, the trait appears to follow a single-locus, incompletely dominant pattern of inheritance. However, if one examines the fingers and toes of affected individuals, one sees a range of expression from extremely short to only slightly short. What might cause such variation in the expression of brachydactyly?

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Textbook Question

In a series of crosses between two true-breeding strains of peaches, the F₁ generation was uniform, producing 30-g peaches. The F₂ fruit mass ranges from 38 to 22 g at intervals of 2 g. Using these data, determine the number of polygenic loci involved in the inheritance of peach mass.

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Textbook Question

Students in a genetics laboratory began an experiment in an attempt to increase heat tolerance in two strains of Drosophila melanogaster. One strain was trapped from the wild six weeks before the experiment was to begin; the other was obtained from a Drosophila repository at a university laboratory. In which strain would you expect to see the most rapid and extensive response to heat-tolerance selection, and why?

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Textbook Question

Consider a true-breeding plant, AABBCC, crossed with another true-breeding plant, aabbcc, whose resulting offspring are AaBbCc. If you cross the F₁ generation, and independent assortment is operational, the expected fraction of offspring in each phenotypic class is given by the expression N!/M!(N−M)! where N is the total number of alleles (six in this example) and M is the number of uppercase alleles. In a cross of AaBbCc×AaBbCc, what proportion of the offspring would be expected to contain two uppercase alleles?

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Textbook Question

Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.

Considering that differences in control values represent year-to-year differences in greenhouse conditions, calculate (in mm) the average response to selection over the three-year period. <>

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