Quantitative Genetics Questions and Answers

Answer: A

The allele frequencies are F = 0.69 and S = 0.31.

Count alleles: each of the 65 birds carries two, 130 in all. FF birds carry two F alleles each and FS birds one.

\[ \begin{aligned} p(F) &= \frac{2(35) + 20}{130} = \frac{90}{130} \approx 0.69 \\ q(S) &= \frac{2(10) + 20}{130} = \frac{40}{130} \approx 0.31 \end{aligned} \]

The same result follows from the genotype frequencies: \(p = 0.54 + \frac{0.30}{2} = 0.69\), and \(p + q = 1\) gives \(q = 0.31\).

The option with F = 0.31 and S = 0.69 swaps the two alleles. The 0.59 and 0.50 options do not come from any correct count; 0.50 would require equal numbers of FF and SS birds.

Answer: D

The ratio shows a polygenic trait controlled by two genes.

The ratio sums to 16, the number of F2 combinations from two gene pairs (4 × 4). The five classes, 1 : 4 : 6 : 4 : 1, correspond to plants carrying 4, 3, 2, 1 and 0 'red' alleles, each allele adding equally to colour intensity. This is Nilsson-Ehle's classic result for kernel colour in wheat.

Three genes would give 64 combinations and seven classes (1 : 6 : 15 : 20 : 15 : 6 : 1). Duplicate genes give 15 : 1 and epistatic genes give ratios such as 9 : 3 : 4 or 12 : 3 : 1, not five graded classes.

Answer: A

Sampling occurs because not all gametes form zygotes.

Each parent produces a huge number of gametes, but only a few of them meet and form the next generation. The alleles carried by those few are a sample of the parents' gene pool, and by chance their frequencies can differ from the parental frequencies; this sampling error is the basis of genetic drift.

Every allele a parent carries does reach some of its gametes, through Mendelian segregation. Imperfect copying in meiosis and recombination shuffle alleles into new combinations, but they do not by themselves change the allele frequencies of the pool.

Answer: D

The best parents for hybrid varieties are Inbred lines.

Hybrid varieties, as in maize, are the F₁ of a cross between two inbred lines. Repeated selfing makes each line homozygous and uniform, so their F₁ is uniform too, and lines with good combining ability are chosen to give the greatest heterosis. The same cross can be repeated each season to produce seed.

Isogenic and near-isogenic lines differ from each other at only one or a few genes, so crossing them gives almost no heterosis. Pure lines of self-pollinated crops are generally released as varieties in their own right rather than used as hybrid parents.