B) He decides to map one of these mutants- white 1 (wl-). He finds that the muta

Question

B) He decides to map one of these mutants- white 1 (wl-). He finds that the mutation maps to chromosome2. To map this mutation precisely he uses two RFLP loci located on chromosome2 and performs a three point cross. RFLP locusA gives either a 2kb (A2) or a 3kb band (A3) on a Southern blot with probeA. RFLP Locus B gives either a 5kb (B5) or a 7kb (B7) band on a Southern with probeB. A heterozygous slug for all three loci (w1-/W1+, A2/A3, B5/B7) is test crossed to a w1/w1, A2/A2, B5/B5 homozygous recessive slug. The following progeny are obtained: (The RFLP marker is only scored for band size in the heterozygous slug) What is the correct order of these three loci? Also what are the map distances between the W1 gene and the two RFLPs

Explanation / Answer

Parental genotypes and number of offspring :

w1- A3 B7 : 410

W1+ A2 B5 : 390

Double recombinant genotypes and number of offspring :

w1- A2 B5 : 9

W1+ A3 B7 : 11

When parental genotypes are compared with double recombinant genotypes, it can be inferred that recombination happened between A and B. So, W should be in the middle for double recombination to happen.

Correct order of three loci : A W B

Map distance between W and A :

Double recombinants + Single recombinants between W and A /Total offspring

9 + 11 + 50 + 50 = 120 / 1000 = 0.12 x 100 = 12 m.u

Map distance between W and B :

9 + 11 + 39 + 41 = 100 / 1000 = 0.10 x 100 = 10 m.u

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