Question

Purple eyes (pr), black body (b) and vestigial wings (vg) are recessive alleles in Drosophila melanogaster. Apr pr b' bvg' vg trihybrid is crossed to a fly homozygous for the recessive allele at each locus. The following numbers of offspring are observed. Phenotype Number of progeny Unordered genotypes Ordered genotypes Wild type 188 1241 vg pr, b pr, vg 1243 b, vg 190 88 pr, b, vg Total: 2990 1. Write the full, unordered genotypes of each category (omit the genotype of the tester, it is the same for everything), to the right of the number of progeny column. 2. Which are double crossovers? Which are non-recombinants? 3. The non-recombinants represent the two chromosomes present in the trihybrid. Using linkage notation, write three possible orderings of the alleles on the trihybrid parent; each ordering must have a different locus in the middle. 4. Which ordering produces double crossover progeny consistent with the actually observed double crossover classes? 5. Rewrite all genotypes in the correct order, to the right of the unordered genotypes. Compare each ordered genotype to the non-recombinant genotypes to deduce the location of crossover, and indicate those crossovers with a"" on the ordered genotypes. 6. Calculate recombination frequency, and draw a map. 7. Calculate the coefficient of coincidence and the interference.

Answer 1

1. The unordered genotypes are pr,b; pr,vg; b,vg

2. The double crossovers are the least occuring thus vg and pr,b.

The recombinants are not the parentals thus pr:b:pr,vg: b,vg.

3.The possible orderings are pr,vg,b/ b,pr,vg/ pr,b,vg.

4.The exact orderings according to the observed double crossovers are pr,vg,b.

5. Unordered geneotypes are pr ,b, vg

6.Recombination frequency (single cross over between pr and vg)= (190+188+15)/2990 ×100=13.1%

Single crossover betwwen vg and b = (1241+1243+15)/2990 ×100= 83.57%

Linkage map

13.1mu. 83.57 mu

Pr_______________vg_________________________b

7.coefficient of coincidence = % of observed double crossover / % of expected double crossover = 0.501/10.87

=0.0460

Interference = 1- coefficient of coincidence

= 0.954