Chromosomal theory of inheritance came into existence long after Mendelian genetics. The lack of advanced technology and physical proof were two reasons for the rejection of Mendel’s concept of genetics. However, later experiments confirmed Mendel’s results. Biologists developed the chromosomal theory of inheritance. The work was further carried forward and proved by T.H.Morgan. This led to the concept of linkage and recombination of the genes.
Thomas Hunt Morgan used Drosophila melanogaster(fruit flies)to show how sexual reproduction gave rise to variations. He chose to work on Drosophila melanogaster for the following reasons:
- They could be easily cultivated in the laboratory on synthetic medium.
- They have a short lifespan (two weeks).
- They could produce numerous progeny in a single mating.
- The male and female sexes are differentiable.
- They are different types of hereditary variations.
As Mendel’s dihybrid cross in peas, Morgan conducted dihybrid cross between yellow-bodied, white-eyed females and brown-bodied, red-eyed males. Shockingly, the self-crossing of F1 generation gave an F2 generation but not in the ratio of 9:3:3:1. The result showed a deviation from Mendel’s dihybrid cross.
Linkage and Recombination
Morgan observed that while crossing a set of characteristics, two genes did not segregate as per Mendel’s law. If two genes were present on the same chromosome, the probability of getting a parental combination was much higher in the next generation as compared to the non-parental combination. This physical association of genes was termed as linkage.
The term genetic recombination described the non-parental gene combinations in a dihybrid cross.
In addition, they noted that the probability recombination is dependent on how strong the linkage is. In other words, though there is a link between two genes on chromosomes, genes may or may not be tightly linked. Some genes have strong linkage giving less chance of recombination while another linkage of genes is weak (loosely linked) giving a higher chance of recombination.
Once the linked genes were discovered, the frequency of linked genes also influenced the appearance of traits in the next generation. A student of Morgan, Sturtevant discovered the position of linked genes on a chromosome by calculating their frequency of genetic recombination by the process of gene mapping. This method of generating link map was extensively used during Human Genome Project.
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