Question
- Explain linkage and recombination as put forth by T.H. Morgan based on his observations with Drosophila melanogaster crossing experiment.
- Write the basis on which Alfred Sturtevant explained gene mapping.
Thomas Hunt Morgan and his colleagues used fruitfly, or Drosophila melanogaster to study linkage. They showed how sexual reproduction gave rise to variations. Similar to 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 did not give a ratio of 9 : 3 : 3 : 1 in the F2 generation. The result showed a deviation from Mendel’s dihybrid cross.
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.
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.
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| S. No. | Component-I | Component-II | Chemical linkage bonding the two components | Product |
| (i) | A | B | C | Nucleotide |
| (ii) | Nucleotide | D | E | Nucleotide |
| (iii) | Nucleotide | Nucleotide | F | Dinucleotide |