4. In Thomas Hunt Morgan’s experiment with fruit flies, he utilized a white-eyed

Question

4. In Thomas Hunt Morgan’s experiment with fruit flies, he utilized a white-eyed (w), miniature-winged (m) female crossed with a wild-type male.

A. Construct a Punnett square that gives the results of the F1 offspring. Provide both the phenotypes and genotypes.

B. Construct a Punnett square that shows the results of the mating between a wild-type female and a white-eyed, miniature-winged male from the F1 offspring above. Provide both the phenotypes and genotypes.

C. Using the knowledge you have about Mendelian genetics, what would you expect the F2 ratio to be?

D. However, Morgan observed that only 37 percent of the offspring were recombinants. Which of the above offspring (from part B) would be classified as recombinants?

Explanation / Answer

A. When a white eyed miniature winged female crossed with wild type male, F1 females were white eyed miniature winged.

B. In Complete linkage, the genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. Complete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic organisms.

If a red eyed normal winged (wild type) female Drosophila is crossed to white eyed and miniature winged male. The progeny or F1 generation individuals are heterozygous red eyed and normal winged. When F1 males are test crossed to homozygous recessive female (white eyed and miniature winged), only two types of individuals are produced— red eyed normal winged and white eye miniature winged in the ratio of 1 : 1 (parental phenotypes only). Similarly during inbreeding of F1individuals, recombinant types are absent. In practice, this 1: 1 test ratio is never achieved because total linkage is rare.

While in Incomplete linkage, the genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assortment. In independent assortment all the four types (two parental types and two recombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.

If a red eyed normal winged or wild type female Drosophila is crossed to white eyed and miniature winged male. The progeny or F1 individuals are heterozygous red eyed and normal winged. F1 female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1: 1: 1: 1. Instead the ratio comes out to be 9: 1: 1: 8. This shows that the two genes did not segregate independently of each other. Only 9.3% recombinant types were observed which is quite different from 50% recom­binants in case of independent assortment. This shows that in the oocytes of the F1, genera­tion only some of the chromatids undergo cross-over while the majority is preserved intact. This produces 90.7% parental types in the progeny.

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