Linkage, crossing over and recombination

Humans have about 20,000 to 25,000 protein-coding genes on only 23 pairs of chromosomes, so each chromosome must carry many genes. Genes on the same chromosome tend to be inherited together: this is linkage. All the genes on one chromosome form a linkage group.

Bateson and Punnett crossed sweet peas differing in two traits (red flowers and long pollen × white flowers and short pollen). In the F2 they got far more of the parental combinations than the 9:3:3:1 ratio predicts, but they could not explain why.

Thomas Hunt Morgan (1910) explained it: genes lie in a line along chromosomes. In the fruit fly Drosophila, he crossed grey-bodied, vestigial-winged flies with black-bodied, long-winged ones. The F1 (BbVv), crossed with bbvv flies, gave about 83% offspring like the parents (parental types) and 17% with new combinations (recombinants).

Where do the new combinations come from? From crossing over: during meiosis, non-sister chromatids of a chromosome pair exchange segments, swapping genes between them. Two genes far apart on a chromosome are more likely to be separated by a crossover than two genes close together.

So the frequency of recombinants measures the distance between two genes: 1% recombination = 1 map unit = 1 centimorgan (cM). In this way scientists built maps of the order of genes along chromosomes.

Creighton and McClintock (1931) proved that crossing over is a real physical exchange of chromosome pieces. In maize, they used a chromosome 9 with visible markers (including a knob at one end). Every recombinant kernel had a chromosome with the parts swapped.