What is the reason that many traits in organisms are inherited jointly?

A person has a huge number of genes, which is tens and even thousands of times greater than the number of chromosomes of a species. In order to manifest themselves as hereditary material, these genes will be reproduced (inherited) together, that is, linked, thus forming linkage groups. Thus, genes responsible for the development of many traits – eye color, hair color, the presence or absence of diseases, for example, cataracts – can be located in the same chromosomal region. This phenomenon was first studied by Thomas Morgan.