Two genes, A and B, are located on the same chromosome. In a testcross between an individual with the genotype AaBb and one with aabb, the following offspring were produced:

380 AaBb

370 aabb

120 Aabb

130 aaBb

Based on these results, what can be concluded about the relationship between genes A and B?

Question

Two genes, A and B, are located on the same chromosome. In a testcross between an individual with the genotype AaBb and one with aabb, the following offspring were produced:

380 AaBb

370 aabb

120 Aabb

130 aaBb

Based on these results, what can be concluded about the relationship between genes A and B?

BlackTom AI · Accepted Answer

The problem presents a testcross between AaBb and aabb, with the following offspring counts: 380 AaBb, 370 aabb, 120 Aabb, and 130 aaBb.

First, examine the parental (non-recombinant) phenotypes. In a testcross, the parental phenotypes are the most frequent classes, since they result from no or very little recombination between linked genes. Here, the two most frequent classes are AaBb (380) and aabb (370). These align with the parental chromosome combinations AaB- and a-b- in the coupling phase.

Next, look at the recombinant phenotypes. The less frequent classes are Aabb (120) and aaBb (130). These arise from a crossover event between the two genes during meiosis, producing new allele combinations not present in the parental genotype.

To quantify linkage, compute the recombination frequency (RF):
- Total offspring = 380 + 370 + 120 + 130 = 1000
- Recombinants = 120 + 130 = 250
- RF = (recombinants / total) × 100 = (250 / 1000) × 100 = 25%

An RF of 25% is significantly less than 50%, which indicates that the genes are not assorting independently. In other words, A and B are on the same chromosome and are linked, with crossing over between them occurring at a measurable frequency.

Interpreting the data, you can infer that there is linkage between A and B (they are on the same chromosome), and the observed recombinant classes confirm that crossing over takes place between them, albeit at a moderate rate given the 25% RF. If the genes were unlinked, you would expect roughly equal frequencies among all four classes, around 25% each, which is not what the data show here.

In summary, the pattern of parental dominance (AaBb and aabb being most frequent) together with the presence of recombinants (Aabb and aaBb) demonstrates that A and B are linked but that crossing over between them occurs, producing the recombinant offspring observed.

AGRI10051_2025_SM2 Revision Questions Weeks 1-4 UNMARKED

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