You want to determine the linear order and genetic distance between three genes
ID: 161768 • Letter: Y
Question
You want to determine the linear order and genetic distance between three genes on chromosome II in Drosophila. The recessive mutant alleles are g, m, and o. You cross heterozygous females to homozygous recessive males, score the phenotypes of progeny and obtain the following:
540 g m +
560 + + o
162 + m +
170 g + o
245 g + +
240 + m o
46 g m o
37 + + +
a) Determine the orientation of mutant vs. wild type alleles on the female parent’s chromosomes. (1pt)
b) Determine gene order (remember, what matters is which gene is in the middle). (1pt)
c) Calculate the distance between the three genes and draw the genetic map. (1pt)
d) Calculate interference and provide a brief verbal interpretation of your result. (1pt)
Explanation / Answer
a). Female heterozygous genotype= g + m/+ o +
Genes g, + & m are on one chromosome and + o + genes are on the homologous chromosome.b).
b). Gene order= g--o--m
Middle gene is "o"
c). g ---------20.55 m.u.---------- o -----------28.4 m.u.----------m
d).
Interference= 1-coc= 1-0.72=0.28
Interference is positive
There was slight difference between observed double crossover progeny (83) and expected double cross over progeny (116)
Explanation
genotypes
Number of progeny
g m +
540
+ + o
560
+ m +
162
g + o
170
g + +
245
+ m o
240
g m o
40
+ + +
37
Total
2000
Parental combinations are more so, the parental combinations are g m + & + + o
1. If single cross over (SCO) occurs between g & m
Normal order= g ---------m & + ----- +
After cross over= g ----- + & +------m
g ----- + recombinants are 170+245= 415
+ ------m recombinants are 162+240=402
Total recombinants = 817
RF = (817/2000.)*100 =40.85%
2. If single cross over (SCO) occurs between m & +
Normal order= m --------- + & + --------- o
After cross over= m --------- o & + ------ +
m --------- o recombinants are 240+46= 286
+ ------ + recombinants are 245+37= 282
Total recombinants = 568
RF = (568/2000)*100 = 28.4%
3. If single cross over (SCO) occurs between g & +
Normal order= g --------- + & +------o
After cross over= g --------- o & + --------- +
g --------- o recombinants are 170+46=216
+ --------- + recombinants are 162+37=199
Total recombinants = 411
RF = (411/2000)*100 = 20.55%
% RF = Map unit distance
The order of gene is -----
g ---------20.55 m.u.---------- o -----------28.4 m.u.----------m
Expected double cross over frequency (EDCO) = RF b/e g & o * RF b/e o & m
= 0.205 * 0.284= 0.058
Expected double crossover progeny= 0.058*2000= 116
Observed double cross over frequency (ODCO)= (46+37)/2000 =83/2000= 0.042
Coefficient of Coincidence (coc)= EDCO/ODCO = 0.042/0.058=0.72
Interference= 1-coc= 1-0.72=0.28
Interference is positive
genotypes
Number of progeny
g m +
540
+ + o
560
+ m +
162
g + o
170
g + +
245
+ m o
240
g m o
40
+ + +
37
Total
2000
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