When mapping the relative positions of three linked genes by examining traits of progeny in a test cross, changes in phenotype due to movement of the allele in the middle of the three, relative to the parental alleles, will occur ____________ movement of the flanking alleles.

Question

When mapping the relative positions of three linked genes by examining traits of progeny in a test cross, changes in phenotype due to movement of the allele in the middle of the three, relative to the parental alleles, will occur ____________  movement of the flanking alleles.

BlackTom AI · Accepted Answer

When mapping the relative positions of three linked genes by examining progeny in a test cross, we consider the types of recombination events and their frequencies.

Option 1: 'with approximately the same frequency as' would imply that movements of the middle allele relative to the parental alleles occur as often as movements of the flanking alleles. This is not correct because recombination events involving the middle gene typically require different crossover patterns (single or double crossovers) with distinct frequencies. In general, single crossovers between a pair of genes are more frequent than double crossovers that involve both flanks and the middle gene, so the middle gene’s movement does not occur at the same rate as the flanking movements.

Option 2: 'less frequently than' suggests that the movement of the middle allele relative to the parental alleles happens less often than movements of the flanking alleles. This aligns with the standard expectation in three-point mapping: double crossovers, which can rearrange the middle gene relative to both flanks, are rarer than single crossovers that rearrange only one flank relative to the middle. Therefore, changes in the middle gene’s position relative to the parental set occur less often than changes involving the flanks alone.

Option 3: 'more frequently than' would imply that the middle gene moves more often than the flanking genes, which contradicts the typical crossover frequency pattern. Double crossovers are less common than single crossovers, so it would be unusual for the middle gene to show a higher rearrangement frequency than the flanks.

In summary, the standard expectation in three-point coupling mapping is that recombination events affecting the middle gene relative to the parental arrangement are rarer than those affecting the flanking genes, making option 2 the correct characterization. The other two options misstate the relative frequencies of these crossover types and do not fit the observed recombination patterns in three-point crosses.

BIOL_2004_81774_202509 Homework 2

When mapping the relative positions of three linked genes by examining traits of progeny in a test cross, changes in phenotype due to movement of the allele in the middle of the three, relative to the parental alleles, will occur ____________ movement of the flanking alleles.

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