Homework Sourse3 In Drosophila singed bristles sn and carnation eyes car ar
3) In Drosophila, singed bristles (sn) and carnation eyes (car) are both caused by recessive, X-linked alleles. The wild-type alleles (sn+ and car+) are responsible for straight bristles and red eyes, respectively. A sn car female is mated to a sn+ car+ male and the F1 progeny are testcrossed. The F2 are distributed as follows
sn car
55
sn car+
45
sn+ car
45
sn+ car+
55
200
What is the value of X2 for a test of the hypothesis that the sn and car genes are unlinked?
| 3) In Drosophila, singed bristles (sn) and carnation eyes (car) are both caused by recessive, X-linked alleles. The wild-type alleles (sn+ and car+) are responsible for straight bristles and red eyes, respectively. A sn car female is mated to a sn+ car+ male and the F1 progeny are testcrossed. The F2 are distributed as follows
What is the value of X2 for a test of the hypothesis that the sn and car genes are unlinked? |
Solution
Bristles (sn) and carnation eyes (car) are both caused by recessive, X-linked alleles
The wild-type alleles (sn+ and car+) are responsible for straight bristles and red eyes
A sn car female is mated to a sn+ car+male
A sn car (female) X sn+ car+ (male
F1 progeny will be sn/sn+ car/car+
Now you did test cross.
F1 progeny sn/sn+ car/car+ X sn/sn car/car (homozygous recessive- Tester)
For test cross, always you need to get a ratio of 1:1:1:1 if the genes are unlinked and assorting independently.
.sn+car+
.sn+car
.sncar+
.sncar
.sncar
sn+/sn car+/car
sn+/sn car/car
sn/sn car+/car
sn/sn car/car
Progeny
So, out of 200 progeny, you got sn car=55, sn car+=45, sn+ car=45, sn+ car+=55
Observed (O)
Expected (E)
(O-E)
(O-E) 2/E
sn car
55
50
5
0.5
sn car+
45
50
-5
0.5
sn+ c
45
50
-5
0.5
sn+ car+
55
50
5
0.5
Total
200
2=2
Degrees of freedom=n-1=4-1=3
The obtained chi-square value is 0.90>2<0.10. This clearly shows that the genes are not linked.
| .sn+car+ | .sn+car | .sncar+ | .sncar | ||
| .sncar | sn+/sn car+/car | sn+/sn car/car | sn/sn car+/car | sn/sn car/car | Progeny |
