A geneticist is studying two genes, A and B, located on the same chromosome. The distance between these genes is very small. When examining the offspring from a cross between a parent with genotype AaBb and another with aabb, the geneticist observes that alleles A and B appear together more frequently than expected by random assortment. What does this observation suggest about genes A and B?

Question

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We start by restating what's being asked: a geneticist observes that alleles A and B appear together more frequently than would be expected if the two genes assorted independently, in a cross between AaBb and aabb. The question asks what this observation suggests about the relationship between genes A and B.

Option review: In this case, there are no explicit answer options provided to evaluate individually. However, we can reason about the concept that fits the observation.

- When two genes are on the same chromosome and are very close to each other, they are said to be tightly linked. Tight linkage means that recombination between them is rare, so their alleles tend to be inherited together as a unit more often than if they were independently assorted. This leads to parental allele combinations (such as AB and ab) appearing at higher frequencies in offspring than recombinant combinations (such as Ab and aB).

- If A and B were unlinked and acted independently (as would be the case if they were far apart on the chromosome or on different chromosomes), we would expect the four possible allele combinations to appear in roughly equal frequencies, once enough offspring are observed. The observed skew toward AB and ab indicates reduced recombination between the loci, consistent with tight linkage.

- The observation can be explained by the concept of linkage disequilibrium in the offspring, driven by the physical proximity of A and B on the chromosome. The closer two genes are, the lower the probability that a crossover occurs between them during meiosis, which preserves the parental haplotype.

- It’s also helpful to connect this to recombination frequency: a small distance between genes corresponds to a low recombination frequency (e.g., well under 50%). A recombination frequency significantly less than 50% supports the interpretation of tight linkage and inheritance of alleles A and B together more often than not.

- Why other interpretations would be unlikely: if A and B were unlinked or far apart, recombination would randomize allele associations and AB would not be observed more frequently than the other combinations. If the observation were due to a sampling artifact or selection for certain genotypes, we would need additional data to attribute the pattern to that cause, not to linkage.

In summary, the key point is that the increased co-occurrence of A and B alleles in offspring points to the two genes being tightly linked on the same chromosome, so they are inherited together more frequently than expected by independent assortment.

AGRI10051_2025_SM2 Revision Questions Weeks 1-4 UNMARKED

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