In this article we will discuss about the linkage and recombination of genes.
Morgan and his group observed in Drosophila that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type or new combinations (also called recombination) of genes.
They also found that the proportion of recombinants varies, even if the two genes are present on the same chromosome. They attributed this due to physical association of the two genes and coined the term ‘linkage’.
Linkage is the physical association of genes on a chromosome and the term recombination describes the generation of non-parental gene combinations. Morgan and his group found that even when genes were grouped on the same chromosome, some genes are tightly linked, i.e., linkage is stronger between two genes, if the frequency of recombination is low.
Whereas, the frequency of recombination’s is higher, if genes are loosely linked, i.e., linkage is weak between two genes, e.g., They hybridised yellow-bodied and white eyed females with brown-bodied and red eyed males (wild type) (cross I) and inter-crossed their F1-progeny.
In F2-generation, parental combinations were 98.7% and the recombinants were 1,3%. In another cross (cross II) between white-bodied female fly with miniature wing and a male fly with yellow body and normal-wing, parental combinations were 62.8% and recombinants were 37.2% in F2-generation. Thus, it was proved from the crosses that the linkage between genes for yellow-body and white-eyes is stronger than the linkage between for white body and miniature wing.
Alfred Sturtevant (Morgan’s student) 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. Genetic map are now used as a starting point in the sequencing of whole genomes as done in case of human genome sequencing project.