Question

In corn, a dihybrid for the recessive genes a and b is test-crossed. The distribution of the phenotypes is as follows:

A B	122
A b	118
a B	81
a b	79

a) What appears to be the distance between genes a and b?

b) Test your hypothesis with a chi-squared test. Do the genes appear to be sorting independently?

Answer 1

Answer:

a).

Expected testcross progeny would be 1:1:1:1 ratio according to Mendel experiments. But the given numbers of the progeny are not similar to 1:1:1:1 ratio. So, the genes are not in assorted independently and they are linked. Each expected progeny = 1/4

Recombination frequency (RF) = (no. of recombinant progeny / Total progeny)100

RF = (81+79/400)100 = 40%

RF(%) = Distance between the genes (m.u.)

The distance between the two genes = 40 m.u.

b).

Phenotype	Observed(O)	Expected (E)	O-E	(O-E)2	(O-E)2/E
AB	122	100.00	22.000	484.000	4.840
Ab	118	100.00	18.000	324.000	3.240
aB	81	100.00	-19.000	361.000	3.610
ab	79	100.00	-21.000	441.000	4.410
Total	400	400			16.100

Chi-square value = 16.10

Degrees of freedom = Number of phenotypes – 1

Df = 4-1=3

Critical value = 7.81

The chi-square value of 16.10 is greater than the critical value of 7.81. We can reject null hypothesis and the two genes are not assorted independently.