A geneticist discovers a new mutation in D. melanogaster that causes the flies t
ID: 189489 • Letter: A
Question
A geneticist discovers a new mutation in D. melanogaster that causes the flies to shake and quiver. She calls this mutation spastic (sps) and determines that spastic is due to an autosomal recessive. She wants to determine it the spastic locus is linked to the vestigial wings locus (recessive allele is vg). She crosses a fly homozygous for spastic and vestigial traits with a fly homozygous for the wild-type traits and then uses the resulting F1 females in a testcross. She obtains the following flies from this testcross. WT vg sps 230 224 97 sps Are the vestigial wings and spastic loci linked? First, use the chi-square test to ask if the loci assorted independently. Chi d.f Ac or Rej? If Rej, what are the parental classes and the recombinant vg What is the genotype for vg and sps for each phenotype class: WT What is the map distance between the two loci in cM?Explanation / Answer
Null hypothesis: The observed values are not deviating from the dihybrid test cross ratio 1:1:1:1.
Test static: Chisquare test:
Category
WT
vg. Sps
vg
sps
Total
Observed values
230
224
97
99
650
Exptected Ratio
1
1
1
1
4
Exprected Values
162.5
162.5
162.5
162.5
Deviation
67.5
61.5
-65.5
-63.5
D^2
4556.25
3782.25
4290.25
4032.25
D^2/E
28.03846
23.27538
26.40154
24.81385
102.5292
X^2
102.5292
Degrees of freedom
4
-
1
3
Inference: The calculated chisquare value i.e. 102.52 is greater than the table value i.e. 7.82 at 3 DF and 0.05 probability, hence the null hypothesis is rejected. So, the genes are not segregated independly. Which means the genes vg and sps is linked.
In general, the parental combinations are high in frequency and recombinant are low in frequency. So, the categories with 230 and 224 that is WT and vg.sps are belong to parental combinations and vg and sps categores belong to recombinant categories.
The Distance between the genes = Frequency of recombinants/Total number of progeny X 100
= 196/650*100 = 30.15
Category
WT
vg. Sps
vg
sps
Total
Observed values
230
224
97
99
650
Exptected Ratio
1
1
1
1
4
Exprected Values
162.5
162.5
162.5
162.5
Deviation
67.5
61.5
-65.5
-63.5
D^2
4556.25
3782.25
4290.25
4032.25
D^2/E
28.03846
23.27538
26.40154
24.81385
102.5292
X^2
102.5292
Degrees of freedom
4
-
1
3
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