Chromosomal theory of heredity. Linked genes
In 1902, Sutton and Boveri, in the United States and Germany respectively, saw that there was a relationship between the inheritance of hereditary factors and the behavior of chromosomes during meiosis. They deduced that Mendel's "hereditary factors", genes, were located on chromosomes.
Later, in 1933, the American geneticist TH Morgan demonstrated this with his experiments with the fruit or vinegar fly (Drosophila melanogaster), which earned him the Nobel Prize in Physiology and Medicine.
Morgan found that fruit flies had three pairs of homologous or autosomal chromosomes and another pair of chromosomes that were different according to sex. The females had these two identical chromosomes (XX) and the males had them different (XY), which he called heterochromosomes or sex chromosomes.
When crossing individuals of Drosophila, he discovered that there were four groups of characters that, in most cases, were transmitted together. That is, if one of them appeared, so did the rest of the group. Realizing that there were the same number of genes that were inherited together, as the number of chromosomes in the fly, also four, he called them linked genes.
Another contribution of Morgan was the discovery that, sometimes, linked genes are transmitted independently. This is due to the genetic recombination or swapping of chromosome fragments that occurs during meiosis.