You are tracking three loci in the fruit fly, locis A, B, and C, and want to kno

Question

You are tracking three loci in the fruit fly, locis A, B, and C, and want to know whether the loci are linked and if so what their physical distances are from one another as well as the physical order they are in. To find out this information you perform a three-point testcross with a fly that is heterozygous for all three alleles. The progeny of this cross are:

Genotype                Number of Individuals

a/a . B/b . C/c           389

A/a .b/b . c/c           410

a/a . b/c . C/c           39

A/a . B/b . c/c           44

a/a . b/b . c/c            77

A/a . B/b . C/c       83

a/a . B/b . c/c         6

A/a . b/b . C/c        5

Total 1053           

(a) What is the recombination frequency between loci A and B?

(b) What is the recombination frequency between loci B and C?

(c) What is the recombination frequency between loci A and C?

(d) Are these loci linked?

(e) Demonstrate the physical arrangement of the genes on the chromosome, i.e. the order of the genes on a chromsosome as well as their map distances.

(f) Write out the genotypes of the parents for this cross, indicating which alleles are linked with one another for each parent?

________________________________ X ____________________________________

Explanation / Answer

Based on the given data,

So, the parental genotypes are as follows:

a/a. B/b. C/c

A/a .b/b. c/c

The genotypes of double cross over as follows:

a/a. B/b. c/c

A/a. b/b. C/c

From these sums it is clear that the “C/c” locus lies between the “A/a” locus and the “B/b” locus.

There is 5 + 77 + 83 + 6 = 171 progeny show recombination between genes A and C.

Thus, the gene distance between A and C is 16.2 map units (m.u).

There is 39 + 44 + 5 + 6 = 94 progeny show recombination between genes B and C.

Thus, the gene distance between B and C is 8.9 (m.u).

The genetic map of the three genes is as follows:

Thus, the gene distance between A and B is (16.2+8.9= 25.1 m.u).

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

The genotypes of parents are:

a/a. B/b. C/c

A/a .b/b. c/c

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