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.

Morgan also discovered that genes are arranged in a linear fashion on chromosomes, always located in the same places on the chromosome called loci (singular locus).

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.

Genes y herencia: Thomas Hunt Morgan por raulespert

With the results of his research, Morgan formulated the so-called chromosomal theory of inheritance, which is summarized in the following postulates: 

• The hereditary factors (genes) that determine the characters are located in the chromosomes, being arranged in a linear way along the chromosomes. 
• Each gene occupies a specific place or locus (plural "loci") within a specific chromosome . 
• In diploid organisms, homologous chromosomes contain alleles (or "antagonistic factors") for the same inherited traits located at the same loci, so each trait is governed by a pair of allele genes.
• Genes located on the same chromosome are linked to each other and are passed on together (except in a small proportion of occasions when they are recombinantly separated during meiosis that gives rise to gametes).