1. In Drosophlla, two recessive alleles for the sepla locus cause flles to have
ID: 203515 • Letter: 1
Question
1. In Drosophlla, two recessive alleles for the sepla locus cause flles to have very dark colored eyes. Two recessive alleles at the halry locus cause the fly to have very dense bristles on Its legs (halry legs). A male from a true breeding llne of sepla-eyed, hairy-legged flles is crossed to a female from a true breeding line of red-eyed, normal legged fles ("wild-type") to create F1 progeny. Next, an F1 female is crossed to a male from the sepla-eyed, halry-legged line. The phenotypes of 1000 progeny from this test cross were: Eyes Red Sepia Red Sepia Legs normal normal hairy hairy Number observed 315 210 187 288 a) You are expecting the sepia and hairy alleles to be unlinked? State your null hypothesis (5 pts). b) Calculate the Chi-squared value, and degrees of freedom. (10 pts) c) Can you reject your null hypothesis, and Why? What does this mean? (use the attached table) (10pts)Explanation / Answer
a. Null hypothesis : The observed values are not deviating from the mendilian dihybrid test cross ratio i.e. 1:1:1:1.
b. Test static:
Category
Red Normal
Sepia Normal
Red hairy
Sepia Hairy
Total
Observed values (O)
315
210
187
288
1000
Exptected Ratio (ER)
1
1
1
1
4
Exprected Values (E)
250
250
250
250
Deviation (O-E)
65
-40
-63
38
D^2
4225
1600
3969
1444
D^2/E
16.9
6.4
15.876
5.776
44.952
X^2
44.952
Degrees of freedom
4
-
1
3
c. Inference: As the calculated chisquare value i.e. 44.95 is greater than the table value i.e. 7.82 at 3 DF and 0.05 probability, hence the null hypothesis is rejected. Which means the genes are linked.
Category
Red Normal
Sepia Normal
Red hairy
Sepia Hairy
Total
Observed values (O)
315
210
187
288
1000
Exptected Ratio (ER)
1
1
1
1
4
Exprected Values (E)
250
250
250
250
Deviation (O-E)
65
-40
-63
38
D^2
4225
1600
3969
1444
D^2/E
16.9
6.4
15.876
5.776
44.952
X^2
44.952
Degrees of freedom
4
-
1
3
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