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  • (a) Why did T.H. Morgan select Drosophila melanogaster as a specimen for his experiment? (b) Morgan, in his dihybrid crosses with Drosophila observed deviations in the phenotypc ratio of 

(a) Why did T.H. Morgan select Drosophila melanogaster as a specimen for his experiment?

(b) Morgan, in his dihybrid crosses with Drosophila observed deviations in the phenotypc ratio of F_{2} progeny in comparison to that of Mendel. With the help of a suitable example, explain how his results deviated from that of Mendel.

 

 

 

 
 
 
 
 

Answers (1)

a) Morgan worked with the tiny fruit files, Drosophila melanogaster, which were found very suitable for his experiments because of the following reasons

  1.  They could be grown on simple synthetic medium in the laboratory.
  2.  They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies.
  3.  There was a clear differentiation of the sexes – the male and female flies are easily distinguisable.
  4.  It has many types of hereditary variations that can be seen with low power microscopes.

b) Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. 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 (expected when the two genes are independent).

Morgan and his group knew that the genes were located on the X chromosome and saw quickly 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. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations .

Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) (Figure , Cross A) while others were loosely linked (showed higher recombination) (Figure, Cross B). For example he found that the genes white and yellow were very tightly linked and showed only 1.3 per cent recombination while white and miniature wing showed 37.2 per cent recombination.

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Priyanka Kumari

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