A kidney bean shaped eye is produced by a recessive allele [k] on the third chro

Question

A kidney bean shaped eye is produced by a recessive allele [k] on the third chromosome of Drosophila. Orange eye color, called "cardinal" is produced by the recessive allele [cd] on the same chromosome. On this same chromosome is a third locus with a recessive allele [e] that produces ebony body color. Homozygous kidney, cardinal females are mated to homozygous ebony males. The trihybrid F1 females are then test crossed to produce F2. The following progeny were observed:

1761 kidney cardinal
128 kidney, ebony
1761 ebony
97 kidney
138 cardinal
89 ebony, cardinal
6 kidney, ebony, cardinal
8 triple wild type

a. Determine the genotypes of the P and the F1.
b. Construct an accurate genetic map of chromosome 3 in Drosophila.

Explanation / Answer

a. Determination of the parental genotypes : The genotypes found most frequently are the parental genotypes. From the table it is clear that the k cd + & + + e genotypes were the parental genotypes.

b. F1 phenotypes : + + + / k cd e. & k cd e / k cd e.

c. GENE MAP :

Gene order : k e cd.

The linkage distance is calculated by dividing the total number of recombinant gametes into the total number of gametes.

So, the distance between k & e = [(128+132+9+4)/3942]*100 = 6.92 cM.

The distance between e & cd = [(92+85+9+4)/3942]*100 = 4.81 cM.

GENE MAP :-

k

F1 gametes observed number Type of gamete Kidney, cardinal - k cd + 1761 parental Kidney, ebony - k + e 128 single crossover between k & e. ebony - + + e 1731 parental Kidney - k + + 92 single crossover between e & cd. Cardinal - + cd + 132 single crossover between k & e. Cardinal, ebony - + cd e 85 single crossover between e & cd. Kidney, cardinal, ebony - k cd e 9 Double crossover. Wild type. - + + + 4 Double crossover.

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