Chromosome theory. Genetics of sex

The chromosome theory of heredity (T. Morgan and his students, early 20th century) proved that genes lie on chromosomes. The experiments used the fruit fly Drosophila melanogaster: it breeds fast, has few chromosomes (2n = 8) and many traits. A gene occupies a definite place on a chromosome – a locus – and genes are arranged in a line. Genes on one chromosome are inherited together as linked genes and form a linkage group; the number of groups equals the haploid number of chromosomes (4 in a female Drosophila, 23 in a woman; in males X and Y count as separate groups, so 24 in a man). Linkage is not complete: in prophase of meiosis homologous chromosomes exchange segments – crossing over – producing gametes with new combinations (crossover gametes). The farther apart two genes are, the higher the chance of crossing over. If crossover gametes make up 1 %, the distance between the genes is taken as 1 morganide; genetic maps of chromosomes are built on this basis.