Can you please help me with this problem:
a). If the three genes are assorted independently, the trihybrid testcross progeny would be equal proportions. As the numbers are not equal, hence, the three genes are linked.
b).
Hint: Always non-recombinant genotypes are large numbered than the recombinant genotypes.
Hence, the parental (non-recombinant) triple heterozygous genotypes is b+ wx+ cn / b wx cn
c).
Wild type = b+ wx+ cn+ = 5
Black, wavy, cinnabar = b wx cn = 6
waxy, cinnabar = b+ wx cn = 69
Black = b wx+ cn+ = 67
Cinnabar = b+ wx+ cn = 382
Black, waxy = b wx cn+ = 379
Waxy= b+ wx cn+ = 48
Black, cinnabar = b wx+ cn = 44
Total 1000
1).
If single crossover occurs between b & wx..
Normal combination: b+ wx+ / b wx
After crossover: b+ wx / b wx+
b+ wx progeny= 69+48 = 117
b wx+ progeny = 67+44 = 111
Total this progeny = 228
Total progeny = 1000
The recombination frequency between b & wx = (number of recombinants/Total progeny) 100
RF = (228/1000)100 = 22.8%
2).
If single crossover occurs between wx & cn..
Normal combination: wx + cn / wx cn+
After crossover: wx+ cn+ / wx cn
wx+ cn+ progeny= 5+67 = 72
wx cn progeny = 6+69 = 75
Total this progeny = 147
The recombination frequency between wx & cn = (number of recombinants/Total progeny) 100
RF = (147/10000)100 = 14.7%
3).
If single crossover occurs between b & cn..
Normal combination: b+ cn / b cn+
After crossover: b+ cn+ / b cn
b+ cn+ progeny= 5+48 = 53
b cn progeny = 6+44 = 50
Total this progeny = 103
Total progeny = 1000
The recombination frequency between b & cn = (number of recombinants/Total progeny) 100
RF = (103/1000)100 = 10.3%
Recombination frequency (%) = Distance between the genes (mu)
Order of genes = b---cn---wx
Gene map = b----------10.3mu--------cn---------14.7mu--------------wx
d).
Expected double crossover frequency = (RF between b & cn) * (RF between cn & wx)
= 10.3% * 14.7% = 0.015
The observed double crossover frequency = 6+5 / 1000 = 0.011
Coefficient of Coincidence (COC) = Observed double crossover frequency / Expected double crossover frequency
= 0.011 / 0.015
= 0.73
Interference = 1-COC
= 1- 0.73 = 0.27
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