Chromosome Mapping

The further apart two genes are on a chromosome, the more likely it is that a chiasma forms between them and separates their alleles. So the recombination frequency between two linked genes can be used as a measure of the distance between them. This allows the relative positions of genes to be worked out, called chromosome mapping (or linkage mapping).The idea was first used by Alfred Sturtevant, a student of Morgan, with Drosophila in 1913. Distances are given in map units:1% recombination frequency = 1 map unit (1 centimorgan)A map shows the order of genes and their relative distances, not exact physical distances in bases.Worked example: three linked genes have these recombination frequencies: A and B 12%, B and C 5%, A and C 17%. The largest distance (A to C, 17) must be between the two outer genes. Since 12 + 5 = 17, gene B lies between A and C:A ← 12 → B ← 5 → CThe recombination frequency can never be more than 50%, because at 50% all gamete types are equally common. Genes that are very far apart on the same chromosome therefore behave as though they are unlinked, and their distance must be worked out by adding smaller intervals.Maps are only approximate, because more than one crossover can happen between genes that are far apart, which can hide recombination. Short distances give the most reliable estimates.