Is Mendel's 2nd Law always obeyed?

The discovery that genes are located on chromosomes has created a stalemate in understanding Mendel's 2nd Law.

As we have seen, under this law, two or more non-allele genes segregate independently as long as they are located on different chromosomes. However, a problem arises. Mendel claimed that genes related to two or more traits always had independent segregation. If this premise were true, then there would be a chromosome for each gene.

If we consider that there is a multitude of genes, then there would be an astonishing amount of chromosomes within a cell, which is not true. Therefore, as there are relatively few chromosomes in the nucleus of cells and numerous genes, it is intuitive to conclude that in each chromosome there is a multitude of genes, responsible for the numerous characteristics typical of each species. We say that these genes present on the same chromosome are linked or in linkage and go together to form gametes.

So Mendel's 2nd Law not always obeyedsimply for the genes to be located on the same chromosome, that is, they are in linkage.


Genes joined on the same chromosome

T. H. Morgan and his collaborators worked with the fruit fly, Drosophila melanogaster, and performed crosses where they studied two or more gene pairs, verifying that, in fact, Mendel's 2nd Law was not always obeyed. They concluded that these genes were not on different chromosomes but were on the same chromosome (in linkage).

One of Morgan's crosses

In one of his experiments, Morgan crossed wild gray-body flies with long wings with black-body mutants and short wings (called vestigial wings). All descendants of F1 had a gray body and long wings, attesting that the gene that conditions gray body (P) dominates what determines black body (P)just like the long-wing gene (V) is dominant over the (v) which conditions emergence of vestigial wings.

Morgan then crossed offspring of F1 with double-recessive (ie, performed test crosses). For Morgan, the results of test crosses would reveal whether the genes were located on different chromosomes (segregation-independent) or on the same chromosome (linkage).

Surprisingly, however, none of the expected results were obtained. The separation and counting of the descendants of F2 revealed the following result:

  • 41.5% of flies with gray body and long wings;
  • 41.5% of flies with black body and vestigial wings;
  • 8.5% flies with black body and long wings;
  • 8.5% of flies with gray body and vestigial wings.

In analyzing this result, Morgan became convinced that the P and V genes were located on the same chromosome. If they were located on different chromosomes, the expected ratio would be different (1: 1: 1: 1). However, the question remained: how to explain the occurrence of gray body / vestigial wings and black body / long wings phenotypes?

The answer was not difficult to obtain. By this time the meiosis process was already fairly clear. In 1909, cytologist F. A. Janssens (1863-1964) described the chromosomal phenomenon known as permutation or crossing over, which occurs during meiosis prophase I and consists of the exchange of fragments between homologous chromosomes.

In 1911 Morgan used this observation to conclude that the gray body / vestigial wings and black body / long wings phenotypes were recombinants and due to the occurrence of crossing over.