In this article, we will be discussing two terms that are of the utmost importance in genetics. The terms are Linkage and Crossing Over. It is one of the bases of inheritance and variation in any living organism. The concept of linkage and crossing over has been used in many applications such as genetic mapping, Human Genome Sequencing Project, etc. So let us learn about these two terms in details along with examples for the better understanding.
Linkage and Crossing Over
According to Mendel’s law of independent assortment, the gene controlling different characters get assorted independent to each other. It is correct if the genes are present on two different chromosomes, but if these genes are present on the same chromosome they may or may not show independent assortment. If crossing over takes place between these two genes then the genes get segregated and they will assort independently to each other. But if there is no crossing over between these two genes there is no segregation, hence only parental combination will be found in gametes.
Morgan carried out several dihybrid crosses in Drosophila to study genes that were X-linked. The crosses were similar to Mendel’s dihybrid crosses carried out in peas.
Dihybrid crosses conducted by Morgan:
At first (cross A) he crossed yellow-bodied (y) and white-eyed (w) female with brown-bodied (y+) red-eyed (w+) male and got F1 generation in the form of brown–bodied red-eyed female and yellow–bodied white–eyed male. In the F2 generation, obtained by intercrossing of F1 hybrids, the ration deviated significantly from expected. He found 98.7% to be parental and 1.3% as recombinants. In a second cross (B) between white-eyed and miniature-winged female (wwmm) with wild red-eyed (w+) normal-winged male (m+) the F1 generation included red-eyed normal-winged female and white-eyed miniature-winged male. After intercrossing the F1 progeny was found to be 62.8% parental and 37.2% recombinant type.
Observations of Morgan Dihybrid Crosses
In both of the crosses (A and B), he observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio. Phenotypic ratio as 9:3:3:1 in F2 generation is obtained in the dihybrid cross if both genes are showing independent assortment. Morgan and his group knew that the genes in both crosses were located on the X-chromosome (i.e., same chromosome).
In both crosses, Morgan found out that proportion of parental gene combination was much higher than the non-parental gene combinations.
Morgan attributed this because of the physical association of the two genes and coined the term linkage to describe this physical association of genes on the same chromosome and the term recombination to describe the generation of non-parental gene combinations.
Morgan observed that recombinant types were low (1.3%) in cross A as compared to cross B, it means genes for white eye and yellow body were very lightly linked. Genes for white eye and miniature wing were loosely linked as they showed comparatively higher recombination (37.2%). Now it is clear that when genes are grouped on the same chromosome, some genes are tightly linked while some are loosely linked.
Alfred Sturtevant (student of Morgan) used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. Two genes show a higher frequency of crossing over if the distance between them is higher and lower frequency if the distance is small. Today genetic maps are extensively used as a starting point in the sequencing of whole genomes as was done in case of the Human Genome Sequencing Project.