Chromosomal theory and linkage

The essentials

In 1900, Mendel's work was rediscovered. By 1902, chromosome movement during meiosis had been worked out, and Walter Sutton and Theodore Boveri noted that chromosomes behave like Mendel's factors: they occur in pairs, segregate at gamete formation and assort independently. Sutton united chromosome segregation with Mendelian principles in the chromosomal theory of inheritance.

Thomas Hunt Morgan provided experimental proof using the fruit fly Drosophila melanogaster, which is easy to breed, completes its life cycle in about two weeks, produces many progeny, shows clear sex differences and has many visible hereditary variations. Morgan carried out dihybrid crosses involving sex-linked genes (for example body colour with eye colour, and eye colour with wing size). He found that the F₂ ratios deviated strongly from 9:3:3:1 because the genes were on the X chromosome and did not assort independently.

Linkage is the physical association of genes on a chromosome. Recombination is the generation of non-parental gene combinations, which happens by crossing over during meiosis. Morgan found that some linked genes were tightly linked (low recombination, e.g. about 1.3%) while others were loosely linked (higher recombination, e.g. about 37.2%). His student Alfred Sturtevant used recombination frequency between gene pairs to map their positions on the chromosome.

Key points

• Tightly linked genes: more parental-type offspring, less recombination.
• Recombination frequency is roughly proportional to the distance between genes.
• Genes on different chromosomes assort independently; linked genes tend to be inherited together.

Common mistakes

• Linkage reduces recombination; it does not produce it. Crossing over is what breaks linkage.