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Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation3 Marks
Question
  1. Explain linkage and recombination as put forth by T.H. Morgan based on his observations with Drosophila melanogaster crossing experiment.
  2. Write the basis on which Alfred Sturtevant explained gene mapping.
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Answer

  1. The linkage is a term that describes the tendency of certain loci or alleles to be inherited together. Genetic loci on the same chromosome are physically close to one another and tend to stay together during meiosis, thus are genetically linked. Genetic recombination is the formation of new combinations of alleles in offspring as a result of the exchange of DNA sequences between chromosomes. It occurs naturally, as in sexual reproduction during meiosis or artificially, as a result of experiments on genetic engineering.
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 $F_1$ generation did not give a ratio of 9 : 3 : 3 : 1 in the $F_2$ 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.
  1. Alfred Sturtevant was a student of Morgan. He 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 linkage map was extensively used during Human Genome Project.

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