Three corn seed traits are C for red, c for white ,S for plump, s for shrunken, W for normal, w for waxy. Use the data from the testcross to map the distances and order of these three loci. Show calculations and illustrate the gene map.
Parents: CCssWW(CsW/CsW) × ccSSww(cSw/cSw)
Trihybrid: CcSsWw(CsW/cSw) × ccssww(csw/csw)
To map the distances and order of the three loci, we will perform a test cross between the trihybrid (CcSsWw) and a homozygous recessive strain (ccssww). We will analyze the progeny to determine the recombination frequencies for each pair of loci.
Trihybrid: CcSsWw (CsW/cSw) × ccssww (csw/csw)
Gametes produced by the trihybrid:
1. CsW
2. cSw
Gametes produced by the homozygous recessive strain:
3. csw
Progeny genotypes and phenotypes:
1. CcSsWw (CsW/csw - red, plump, normal)
2. ccSsWw (cSw/csw - white, plump, normal)
Recombination frequencies:
C and S loci:
- Parental combinations: CsW, csw
- Recombinant combinations: cSw
- Recombination frequency = (Number of recombinants) / (Total number of progeny) = 1/2 = 0.5 or 50%
C and W loci:
- Parental combinations: CsW, csw
- Recombinant combinations: cSw
- Recombination frequency = (Number of recombinants) / (Total number of progeny) = 1/2 = 0.5 or 50%
S and W loci:
- Parental combinations: CsW, cSw
- Recombinant combinations: csw
- Recombination frequency = (Number of recombinants) / (Total number of progeny) = 1/2 = 0.5 or 50%
Now that we have the recombination frequencies, we can convert them to map distances. Map distances are generally calculated as recombination frequency x 100 (centimorgans, cM).
C-S: 50% * 100 = 50 cM
C-W: 50% * 100 = 50 cM
S-W: 50% * 100 = 50 cM
Gene map (each '-' indicates 50 cM):
C-W-S
Thus, the map distances and order of the three loci are C-W-S, with 50 cM between each pair of loci.