Detection of Linkage

When genes at two different loci are linked, the probability of two genes occuring in the same gamete is no longer independent. We cannot multiply to get the joint probability of two events when the events are not independent. When two genes are closer than 50 cross-over units, the genes are linked.

The relative frequency of recombinant types between two linked genes is r. The relative frequency of non-recombinant types is 1-r.

r + (1-r) = 1

Each crossover event results in two parental and two non-parental combinations of genes.

The result of a single crossover event is 1AB:1Ab:1aB:1ab. Each crossover event results in 50% recombinant progeny. Therefore we double the percent of recombinant progeny to determine the relative frequency of crossover events.