Three‑point testcrosses are often used to map genes.

The two least frequent classes from such crosses usually represent which types of progeny?

Question

Three‑point testcrosses are often used to map genes.

The two least frequent classes from such crosses usually represent which types of progeny?

BlackTom AI · Accepted Answer

In three‑point testcrosses, the distribution of progeny classes reflects the frequency of recombination events among the three loci. 
Option 1: 'single‑crossover progeny' — These classes arise from a single crossover event between adjacent loci. They occur more frequently than double crossovers because a single exchange is more common than two exchanges, so this option is not among the two least frequent.
Option 2: 'nonrecombinant progeny' — Also called parental types, these progeny result from no crossover events. They are typically the most frequent, not among the least frequent, so this statement is incorrect as the two least frequent classes.
Option 3: 'half the nonrecombinant progeny' — This is a numerical claim about the count, but the standard expectation is not framed as exactly half of nonrecombinants; the nonrecombinants are usually the most frequent, while the least frequent are specific double‑crossovers. Hence this option mischaracterizes the distribution.
Option 4: 'double‑crossover progeny' — In a three‑point cross, double crossovers involve two recombination events and are the rarest classes. There are two such double‑crossover configurations, and they typically occupy the smallest percentages, making this the correct understanding of the least frequent classes.
Option 5: 'one nonrecombinant class and one single‑crossover class of progeny' — This suggests one parental type and one single‑crossover type as the two least frequent, which contradicts the usual pattern where nonrecombinant types are the most frequent and double crossovers are the two least frequent. Therefore, this description is inaccurate.
Thus, the reasoning points to double‑crossover progeny as the classes that are typically the two least frequent in a three‑point testcross, since they require two separate recombination events and occur least often relative to nonrecombinant and single‑crossover classes.

01:447:380:17 GENETICS Week 10- In Person problem set_Fa25

Three‑point testcrosses are often used to map genes. The two least frequent classes from such crosses usually represent which types of progeny?

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