gene mapping

Meaning: Gene mapping places genes in order along a chromosome and estimates the distances between them. Genes on the same chromosome are linked, but crossing over in meiosis I can separate them and produce recombinant gametes. The farther apart two genes are, the more likely a crossover falls between them, so recombination frequency serves as a measure of distance.

Equation: Map distance (cM) = recombination frequency × 100 = (recombinant offspring ÷ total offspring) × 100. Offspring counts are unitless; 1 map unit = 1 centimorgan = 1% recombination.

On the MCAT: A passage gives offspring counts from a test cross of a heterozygote with a homozygous recessive. Identify the recombinants (the least common classes for linked genes), calculate the percentage, and use three pairwise distances to find gene order: the two genes with the largest distance lie on the outside. A frequency near 50% means the genes assort as if they were on separate chromosomes.

Common confusion: Students treat the most common offspring as recombinants; for linked genes the parental types are the majority. Map units are also relative genetic distances, not base pairs. Because double crossovers go undetected, long distances can be underestimated, and recombination frequency is capped near 50%.