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 interbred.The F₂ males are distributed as follows.What is the value of ?² for a test of the hypothesis that the sn and car genes are unlinked? $$\begin{array} { | l | l | } \hline s n \text { car } ^ { + } & 55 \ \hline s n \text { car } ^ { + } & 45 \ \hline s n ^ { + } \mathrm { car } ^ { + } & 45 \ \hline \mathrm { sn } ^ { + } \mathrm { car } ^ { + } & 55 \ \hline \end{array}$$ A) 0.5 B) 1.0 C) 2.0 D) 0.4 E) 20

Question

Quizplus · Accepted Answer

The hypothesis of unlinked genes suggests that the offspring should exhibit a 1:1:1:1 ratio for the four phenotypes. Given that there are 200 total offspring (55+45+45+55), we expect 50 of each phenotype under the hypothesis. The chi-square test value ($\chi^2$) is calculated as $\sum \frac{(O-E)^2}{E}$, where O is the observed frequency and E is the expected frequency. For each phenotype, the calculation is $\frac{(55-50)^2}{50} + \frac{(45-50)^2}{50} + \frac{(45-50)^2}{50} + \frac{(55-50)^2}{50} = \frac{25}{50} + \frac{25}{50} + \frac{25}{50} + \frac{25}{50} = 2.0$.

In Drosophila,singed Bristles (Sn)and Carnation Eyes (Car)are Both Caused by Recessive,X-Linked