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)
Test cross offspring
Seed trait. Gamete from. Number
Trihybrid
Red, shrunken, CsW. 2777
normal
White, plump,. cSw. 2708
Waxy
Red, plump,. CSw 116
Waxy
White, shrunken,. csW. 123
Normal
Red, shrunken,. Csw. 643
Waxy
White, plump,. cSW. 626
Normal
Red, plump,. CSW. 4
Normal
White, shrunken,. csw. 3
Waxy
Total number of progeny. 7000
To determine the distances and order of the three loci, we first need to find the recombinant frequencies for each pair of genes. Recombination frequencies are determined by the number of recombinant offspring divided by the total offspring. They help to determine how far apart genes are on a chromosome.
We can first evaluate pairs of genes:
1. C and S loci:
Non-recombinant offspring: 2777 (CsW) + 2708 (cSw) = 5485
Recombinant offspring: 116 (CSw) + 123 (csW) = 239
Recombination frequency for C-S: 239/7000 = 0.0341 (3.41%)
2. C and W loci:
Non-recombinant offspring: 2777 (CsW) + 643 (Csw) = 3420
Recombinant offspring: 116 (CSw) + 4 (CSW) = 120
Recombination frequency for C-W: 120/7000 = 0.0171 (1.71%)
3. S and W loci:
Non-recombinant offspring: 2708 (cSw) + 123 (csW) = 2831
Recombinant offspring: 116 (CSw) + 3 (csw) = 119
Recombination frequency for S-W: 119/7000 = 0.0170 (1.70%)
Since the C and W loci have the lowest recombination frequency (1.71%), they are the closest together on the chromosome. The S locus has a higher recombination frequency with both C (3.41%) and W (1.70%), indicating that it is further apart from these two loci.
To determine the order of loci, first place C and W as the adjacent genes on the gene map, and calculate the sum of the distances of S to C and to W (3.41% + 1.70% = 5.11%). If the results are equal to the distance between C and S, then the order would be C-W-S. If not, the order would be C-S-W.
Taking the recombination frequency of C-W (1.71%), we observe that this is less than the combined distance of S to C and to W (5.11%). Therefore, the correct order of the three genes on the chromosome is C-W-S.
The gene map would therefore look like this:
C---1.71%---W---3.40%---S (or in map units: C---1.71 m.u.---W---3.40 m.u.---S)