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Can you please help me with this problem:

3. The allele b gives Drosophila flies a black body, and b+ gives brown, the wild type phenotype. The allele wx of a separate

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Answer #1

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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