Thomas Hunt Morgan carried out several dihybrid cross experiments with Drosophila to study sex-linked genes. He crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males to produce the F1 generation.
Morgan then intercrossed the F1 generation to obtain the F2 generation, and observed that the gene did not separate independently, which was an exception to Mendel’s Law of Independent Assortment. Morgan found that this was due to physical association or linkage, which describes the tendency of certain genes to be inherited together and their ability to retain their physical combination in progeny. Genes which are located on the same chromosome and that are inherited together are known as linked genes. If an individual inherits one of these traits, he or she is most likely to inherit the other trait too. This phenomenon of linkage can be better understood through a process known as crossing-over, which occurs during meiosis. During the pachytene stage of meiosis one, sections of a chromosome usually intertwine and exchange genetic material between non-sister chromatids of homologous chromosomes. This process, where chromatids of a homologous pair exchange segments between them is known as crossing-over. Crossing-over leads to recombination or the creation of different combinations of alleles that do not exist in either parents, and hence it is one of the most important events that lead to variations in offspring. Another fact discovered by Morgan and his colleagues was that the likelihood of two genes separating during crossing-over was proportional to the distance between them. In other words, the closer the genes, the more likely that they will stay together, and the further they are, the more likely that they will separate.
The yellow and white genes showed a recombination of just one point three per cent as these genes were tightly linked, while the white and miniature genes showed a recombination of thirty seven point two per cent as these genes were loosely linked. This linkage and the frequency of recombination of genes on the same chromosome were used by Morgan’s student Alfred Sturtevant as a measure of the distance between genes to map their position on the chromosome.
Today, genetic maps are being extensively used for linkage studies and analysis and are a key tool for genome sequencing or discovering the location and identity of genes. Thus, Morgan’s experiments on linkage ruled out Mendel's Law of Independent Assortment, when linked genes are considered.