Question

In most two-factor crosses involving linked genes, we cannot tell if a double crossover between the two genes has occurred because the offspring will inherit the nonrecombinant pattern of alleles. How does the inability to detect double crossovers affect the calculation of map distance? Is map distance underestimated or overestimated because of our inability to detect double crossovers? Explain your answer.

Short answer

The inability to detect double crossovers leads to an underestimation of the map distance between two genes. This is because not all recombination events (which contribute to map distance) are being accounted for.

Step-by-step solution

Understanding Genetic Linkage and Map Distance

Genetic linkage refers to the tendency of genes that are close together on a chromosome to be inherited together. Map distance between two genes is a measure of the frequency of recombination between them; it is measured in centimorgans (cM). One cM is equivalent to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. When two genes are far apart on the same chromosome, the chance of a crossover between them is higher.

Understanding Double Crossovers

A double crossover involves two events of genetic recombination that occur in the same chromosome during meiosis. Although these events are rare, they can and do occur. These double crossovers will not be detectable if they occur between the same two genes because they will result in the original genetic configuration.

Impact of Undetected Double Crossovers on Map Distance

When double crossovers cannot be detected, the frequency of recombination (and thus the map distance) between two genes is underestimated because not all actual recombination events are being counted. If double crossovers were able to be detected and taken into account, this would indicate a higher recombination frequency between the two genes; hence, a greater map distance.