The recessive alleles k (kidney-shaped eyes), c (cardinal-colored eyes), and e (

Question

The recessive alleles k (kidney-shaped eyes), c (cardinal-colored eyes), and e (ebony body color) identify three genes on the same chromosome in Drosophila. Females with kidney-shaped, cardinal-colored eyes were mated with ebony males. The F1 were all wild-type. When F1 females were testcrossed with ebony males with kidney-shaped, cardinal-colored eyes, the following progeny phenotypes were obtained:

+ c e 149

k + + 144

k c + 784

+ + e 790

+ + + 3

k c e 6

+ c + 68

k + e 70

a) Determine the order of genes and the map distances between them.

b) How much interference in double-recombination is there?

Explanation / Answer

(a)

Based on the given data, the cross is as follows:

From the above cross observed genotypes and number of progenies are as follows:

So, the parental genotypes are as follows:

The genotypes of double cross over as follows:

From these sums it is clear that the “+/e” locus lies between the “+/k” locus and the “+/c” locus.

There is 3 + 149 + 144 + 6 = 302 progeny show recombination between genes +/e and +/k.

Thus, the gene distance between +/e and +/k is 14.9 map units (m.u).

There is 3 + 68 + 70 + 6 = 147 progeny show recombination between genes +/e and +/c.

Thus, the gene distance between +/e and +/c is 7.2 (m.u).

The genetic map of the three genes is as follows:

Thus, the gene distance between +/k and +/c is (14.9+7.2 = 22.1m.u).

Here, the total recombination frequency of all these genes is less than 50%, hence the they are Linked.

(b)

There are 302 progeny show recombination between genes +/e and +/k and 147 progeny show recombination between genes +/e and +/c. Therefore the expected (e) rate of double recombination (D.C.O) is:

But, there are actually only 6+3 = 9 are observed (o) double recombinants. Therefore the coefficient of coincidence (c.o.c.) is:

The Interference = 1- (c.o.c.)

Thus, the interference in double-recombination is 0.1 or 10%.

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