Linked inheritance and sex genetics

Genes lie on chromosomes and there are many more genes than chromosomes, so genes on one chromosome usually pass on together — they are linked genes, and the number of linkage groups equals the haploid number of chromosomes (7 in pea, 4 in Drosophila). T. Morgan and his students studied this in the fruit fly Drosophila (it breeds fast, has few chromosomes and many traits). When a dihybrid heterozygous female (grey body, normal wings) was test-crossed with a black, vestigial-winged male, instead of Mendel’s 1:1:1:1 mostly two classes — grey-normal and black-vestigial — appeared: Morgan’s law (linked inheritance of genes on one chromosome). When homologous chromosomes undergo crossing over (exchange of segments) in meiosis, in some gametes the genes are combined in a new way; the crossing-over percentage is proportional to the distance between genes: the closer the genes, the less often they separate, 1% ≈ 1 morganide. On this basis genetic maps are built — the order and distances of genes on a chromosome. Crossing over does not occur in male Drosophila, so a female is used for the test cross. Today we know the Drosophila genome has thousands of genes (about 14 000 protein-coding); the textbook figure of about 500 genes is outdated.