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Introduction to Morgan's Experiments

Three Mark Qu...

Question

Three Mark Questions :
During his studies on genes in Drosophila that were sex linked, T. H. Morgan found $F_{2}$ population phenotypic ratios deviated from expected 9 : 3 :3 :1. Explain the conclusion he arrived at.

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Solution

Morgan’s experiment:- Dihybrid cross was carried out on fruit flies. Yellow bodied, white-eyed females were crossed with brown bodied, red-eyed males. F progeny was obtained, which were inter-crossed. F progeny was obtained and the F ratio was observed. F ratio was observed to be significantly different from 9:3:3:1 as observed in Mendelian dihybrid cross. Explanation of deviation from Mendelian ratio: Genes involved are located on X chromosome. When two genes are located on the same chromosome, the proportions of parental gene combinations were much higher than those of non-parental. Linkage − Physical association of genes on a chromosome. Recombination − Non-parental gene combination.

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Q. Morgan carried out several dihybrid crosses in Drosophila and found

F_{2}

-ratios deviated very significantly from the expected Mendelian ratio. Explain his findings with the help of an example.

Q. Which of the following statements is incorrect with respect to studies on linkage?

I. Morgan was the first to report linkage based on his studies on Drosophila.
II. Morgan observed that the two genes segregate independently of each other and the

F_{2}

phenotypic ratio is 9:3:3:1.
III. Morgan coined the term ‘recombination’ to describe the physical association of genes on a chromosome.
IV. Bateson and Punnett were the first to report linkage based on their studies on Lathyrus odoratus.

Q. The phenotype ratio 9:7 obtained in F2 generation instead of 9:3: :3:1 is due to which type of genes? Explain.

Q. Assertion : When yellow bodied, white eyed Drosophila females were hybridised with brown-bodied, red eyed males; and F

_{1}

progeny were intercrossed, F

_{2}

ratio deviated from 9:3:3:1.
Reason : When two genes in a dihybrid are on the same chromosome, the proportion of parental gene combinations in the offsprings are much higher than the non-parental type.

Q. Given below are some statements related to the dihybrid cross performed by Morgan (considering the body colour and eye colour as the two characters for study). Which of the following would you choose to find out the location of the gene for body colour?

I. Deviation of

F_{2}

progeny from the usual 9:3:3:1 ratio
II. Appearance of more parental combinations in

F_{2}

generation
III. Appearance of more non-parental combinations in

F_{2}

generation
IV. Linked inheritance of body colour and eye colour in Drosophila

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