Read the following statements about Morgan’s work on Drosophila:
A. Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny.
B. When the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much lower than the non-parental type.
C. Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes.
D. Morgan coined the term linkage to describe the generation of non-parental gene combinations.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A and C only
A. Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. — True. This was one of Morgan’s dihybrid crosses in Drosophila with sex-linked genes. The two genes did not segregate independently, and the F2 ratio was far from 9:3:3:1.
B. When the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much lower than the non-parental type. — False. Morgan found the parental combinations much higher, not lower, than the non-parental type. Genes on the same chromosome tend to stay together, which is how linkage was detected.
C. Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes. — True. Sturtevant, Morgan’s student, used this measure to map the positions of genes on the chromosome. Such genetic maps are now a starting point for sequencing whole genomes.
D. Morgan coined the term linkage to describe the generation of non-parental gene combinations. — False. Morgan used linkage for the physical association of genes on a chromosome. The generation of non-parental combinations he called recombination.
NCERT Extract:
Linkage and Recombination – “… For example 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).”
Linkage and Recombination – “… Morgan and his group knew that the genes were located on the X chromosome (Section 4.4) 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 (Figure 4.11).”
Linkage and Recombination – “… His student Alfred Sturtevant 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.”