Below are data from a three-point linkage experiment in Drosophila melanogaster
ID: 163055 • Letter: B
Question
Below are data from a three-point linkage experiment in Drosophila melanogaster. These data represent the offspring from a testcross between flies expressing the recessive mutant phenotypes for black body, vestigial wings and brown eyes and flies that express normal wild type phenotypes but are heterozygous for all three traits. All the heterozygous wild type parental flies in this cross start out with mutant alleles in the cis configuration. Answer the following questions based on these data and show your work for question 1 (a-d).
Body
Wing
Eye
Phenotype
Count
b+
vg+
bw+
Gray body, normal wings, red eyes
103
b
vg
bw
Black body, vestigial wings, brown eyes
95
b+
vg
bw
Gray body, vestigial wings, brown eyes
32
b
vg+
bw+
Black body, normal wings, red eyes
29
b+
vg+
bw
Gray body, normal wings, brown eyes
71
b
vg
bw+
Black body, vestigial wings, red eyes
68
b+
vg
bw+
Gray body, vestigial wings, red eyes
10
b
vg+
bw
Black body, normal wings, brown eyes
13
1a)Which combinations of phenotypes are only possible in those offspring derived from gametes resulting from double crossover events?
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1b)What is the distance between the body color (gray vs. black) and wing loci (normal vs. vestigial)?
1c)What is the distance between the body color (gray vs. black) and eye color loci (red vs. brown eyes)?
1d)What is the coefficient of coincidence for double crossovers?
Body
Wing
Eye
Phenotype
Count
b+
vg+
bw+
Gray body, normal wings, red eyes
103
b
vg
bw
Black body, vestigial wings, brown eyes
95
b+
vg
bw
Gray body, vestigial wings, brown eyes
32
b
vg+
bw+
Black body, normal wings, red eyes
29
b+
vg+
bw
Gray body, normal wings, brown eyes
71
b
vg
bw+
Black body, vestigial wings, red eyes
68
b+
vg
bw+
Gray body, vestigial wings, red eyes
10
b
vg+
bw
Black body, normal wings, brown eyes
13
Explanation / Answer
1.
a. Double crossover events give rise to phenotypes with lower numbers and from the table, we could infer that Gray body, vestigial wings, red eyes (10) and Black body, normal wings, brown eyes (13) are the products of double crossover events.
b. The gene order of these alleles on the chromosome is b+-----vg+-----bw+
The distance between body colour and wing loci can be calculated using
No of single recombinants + number of double crossovers / Total progeny x 100
32 + 29 + 10 + 13 / 421 x 100 = 19.9 cM
c. The distance between wing loci and eye colour can be calculated using
No of single recombinants + number of double crossovers / Total progeny x 100
71 + 68 + 10 + 13 / 421 x 100 = 38.4 cM
d. Coefficient of incidence is actual double recombinant frequency / expected double recombinant frequency
Actual double recombination frequency is
0.19 x 0.38 = 0.0722 or 72.2
c.o.c = 23/72.2 = 0.32
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