You are tracking three loci in the fruit fly, locis A, B, and C, and want to kno
ID: 49923 • Letter: Y
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
389
410
39
44
77
83
6
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 ____________________________________
Genotype
Number of individuals
a/a B/b C/c
389
A/a b/b c/c
410
a/a b/b 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
Explanation / Answer
* The recombination frequency between A and B is number of recombinants for these two loci divided by total number of progeny, that is 810/1053 = 0.77 or 77%.
* The recombination frequency between B and C is 94/1053 = 0.09 or 9%.
* The recombination frequency between A and C is 882/1053 = 0.84 or 84%.
A recomibnatin frequency above 50 percent indicates the genes are unlinked. Hence, it can be concluded that Genes A and B, A and C are unlinked, they are sufficiently distnt from each other. On the other hand, genes B and C are linked.
* The physical arrangement of these genes can be represented as shown: A___________B_C (the distance between A and C is more than the distance between A and B).
* The genotypes of the parents are as follows: A/a * B/b * C/c and a/a * b/b * c/c
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