Chromosomal theory, linkage and recombination

Better microscopes let scientists follow chromosome behaviour in division. Chromosomes, like Mendel's factors, occur in pairs, separate at gamete formation and assort independently.

Walter Sutton and Theodore Boveri noticed that chromosome movement in meiosis parallels the behaviour of Mendel's factors. Sutton combined chromosomal segregation with Mendel's principles into the chromosomal theory of inheritance.

Thomas Hunt Morgan and colleagues tested this experimentally with the fruit fly Drosophila melanogaster, which grows on a simple synthetic medium, finishes its life cycle in about two weeks, gives many progeny from one mating, has easily distinguishable males and females and shows many heritable variations visible under a low-power microscope.

Morgan crossed flies differing in X-linked genes. In the F₂, the parental gene combinations appeared much more often than the new (non-parental) ones, so these genes did not assort independently.

Morgan explained this with linkage, the physical association of genes on one chromosome, and used recombination for the production of non-parental gene combinations.

Tightly linked genes show little recombination, while loosely linked genes show more. In Morgan's data, yellow body and white eye genes gave 1.3 per cent recombination, whereas white eye and miniature wing genes gave 37.2 per cent.

Morgan's student Alfred Sturtevant took the recombination frequency between two genes on one chromosome as a measure of how far apart they are, and used it to map gene positions. Genetic maps of this kind now serve as a starting point for sequencing whole genomes.