A testcross is performed on an individual who is heterozygous for three linked traits. What phenotypes concerning the three genes will be found most often among the progeny?

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Question restatement: In a testcross involving an individual heterozygous for three linked traits, which progeny phenotypes occur most frequently?

Option 1: The three dominant phenotypes. While parental phenotypes are often the most common, this choice assumes the dominant traits align together in the parental haplotype. If the heterozygote’s arrangement is the opposite (recessive alleles in the parental chromosome), the most frequent phenotypes would be the parental combination of those alleles, not necessarily all three dominants. Therefore, this blanket statement is not reliably correct for all coupling configurations.

Option 2: Combinations that indicate double crossovers within the test individual's genotype. Double crossovers involve two recombination events and are statistically rare relative to single crossovers and nonrecombinant (no recombination) classes. In a 3-gene testcross, double crossovers occur far less frequently, so they would not be the most frequent progenyPhenotypes.

Option 3: Combinations that indicate single crossovers within the test individual's genotype. Single crossovers do occur, creating recombinant phenotypes, but they are more frequent than double crossovers yet still far less frequent than nonrecombinant (no crossover) progeny. Therefore, while important for mapping, they are not the most common class.

Option 4: Combinations that indicate no recombination within the test individual’s genotype. In a testcross with linked genes, the largest class of offspring is typically the parental (nonrecombinant) type, produced when no crossover occurs in the region containing the three genes. This class is most frequent because recombination events are relatively rare, especially across three genes, making parental combinations the predominant outcome.

Summary of reasoning: The key concept is that linked genes in a heterozygote produce a parental (nonrecombinant) class most often in a testcross; recombination events (single or double crossovers) create rarer recombinant classes, with double crossovers being especially uncommon. Therefore, the most frequent progeny phenotypes correspond to no recombination within the genotype, matching option 4.

BIOL_2004_81774_202509 Genetics Final Exam- Requires Respondus LockDown Browser

A testcross is performed on an individual who is heterozygous for three linked traits. What phenotypes concerning the three genes will be found most often among the progeny?

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