2. In Drosophila, brittle (b), ebony body (e) and gaucho (g) are three recessive
ID: 196177 • Letter: 2
Question
2. In Drosophila, brittle (b), ebony body (e) and gaucho (g) are three recessive genes. If brittle, ebony females are crossed with homozygous gaucho body males, the resulting F1 progeny are all wild-type. If heterozygous F1 females are mated with brittle, ebony body, gaucho males, the following 1,000 progeny appear: mahea Ban brittle, ebony 390 ucho 420 britle Gaucho ss ony brittle 22 ebony, gaucho 28 ild-type 8 rittle, gaucho, ebony 12 a) Draw the chromosomal composition of the F1 females. b) Derive a map for the three (3) genes. Ann E. Reynolds Lee M. SilverExplanation / Answer
Answer:
a). F1 genotype: bGe / BgE
b). b-------------14cM-----------g---7cM----e
Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.
Brittle, ebony / gaucho = bGe / BgE
Hence, the parental (non-recombinant) genotypes is bGe / BgE
Brittle, ebony—390---bGe
Gaucho—420---BgE
Brittle, gaucho—65---bgE
Ebony-55--BGe
Brittle—22--bGE
Ebony, gaucho—28--Bge
Wild-type—8--BGE
Brittle, gaucho, ebony—12--bge
Total = 1000
1).
If single crossover occurs between b & G..
Normal combination: bG / Bg
After crossover: bg/BG
bg progeny= 65+12=77
BG progeny = 55+8=63
Total this progeny = 140
The recombination frequency between b&G = (number of recombinants/Total progeny) 100
RF = (140/1000)100 = 14%
2).
If single crossover occurs between G & e..
Normal combination: Ge / gE
After crossover: GE / ge
GE progeny= 22+8=30
ge progeny = 28+12=40
Total this progeny = 70
The recombination frequency between G&e = (number of recombinants/Total progeny) 100
RF = (70/1000)100 = 7%
3).
If single crossover occurs between b & e..
Normal combination: be / BE
After crossover: bE/bE
bE progeny= 65+22=77
Be progeny = 55+28=83
Total this progeny = 160
The recombination frequency between b&e = (number of recombinants/Total progeny) 100
RF = (160/1000)100 = 16%
Recombination frequency (%) = Distance between the genes (cM)
b-------------14cM-----------g---7cM----e
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