7. Several mutant genes have been mapped to the third chromosome of Drosophila m
ID: 195868 • Letter: 7
Question
7. Several mutant genes have been mapped to the third chromosome of Drosophila melanogaster. Three well characterized examples are Dichaete (D), a dominant mutation which affects wing shape (i.e. dd gives normal wings), and two recessive mutations: ebony (ee), which affects body color, and pink (pp) which affects eye color. Heterozygous F1 flies of unknown parentage were test-crossed to pink ebony homozygotes, with these results:
Genotypes Number
ddEePp 824
Ddeepp 878
ddeePp 214
DdEepp 187
ddeepp 28
DdEePp 31
ddEepp 4
DdeePp 7
Give the phenotypes for the above genotypes. Determine the order and distance between these genes on chromosome three, and the arrangement of alleles in both the parental and the F1 generations.
Explanation / Answer
Answer:
The phenotypes of given genotypes:
ddEePp 824--Wild
Ddeepp 878---Dichaete, ebony, pink
ddeePp 214---ebony
DdEepp 187--Dichaete, pink
ddeepp 28----ebony, pink
DdEePp 31---Dichaete
ddEepp 4--Pink
DdeePp 7---Dichaete, ebony
The test cross parent phenotype is pink and ebony and its genotype type would be ddeepp (as it is homozygous for all three traits).
Parental and F1 progeny genotypes:
The triple heterozygous genotype would be DdEePp. The test cross parent (ddeepp) produces only one type of gametes, dep. We deduce gamete from each genotype, we will the the gamete geneotypes. The large number geneotypes are the non-recombinant (parental) gametes. Finally the configuration of triple heterozygote is dEP/Dep and its parental genotypes are ddEEPP & DDeepp.
ddEEPP x DDeepp -----P1
dEP/Dep ----------F1
Order of genes:
Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.
Hence, the parental (non-recombinant) genotypes is DdEePp (f1) x ddeepp (pink, ebony)
We need to remove the “dep”alleles from the each given genotypes.
dEP---824 824
Dep--878 878
deP--214 214
DEp--187 187
dep--28 28
DEP--31 31
dEp--4 4
DeP--7
1). dEP / Dep (F1)
If single crossover occurs between d & E..
Normal combination: dE / De
After crossover: de/DE
de progeny= 214+28=242
DE progeny = 187+31=218
Total this progeny = 460
Total progeny= 2173
The recombination frequency between d&E = (number of recombinants/Total progeny) 100
RF = (460/2173)100 = 21.17%
2).
If single crossover occurs between E & P..
Normal combination: EP / ep
After crossover: Ep/eP
Ep progeny= 187+4=191
eP progeny = 214+7=221
Total this progeny =412
The recombination frequency between E&P = (number of recombinants/Total progeny) 100
RF = (412/2173)100 = 18.96%
3).
If single crossover occurs between d & P..
Normal combination: dP / Dp
After crossover: dp/DP
dp progeny= 4+28=32
DP progeny = 7+31=38
Total this progeny = 70
The recombination frequency between d&P = (number of recombinants/Total progeny) 100
RF = (70/2173)100 = 3.22%
Recombination frequency (%) = Distance between the genes (cM)
d----------3.22cM----p-----------18.96cM--------------e
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