Question

Dominant alleles at two different loci affect the tail of mice. These genes are linked and both are lethal in the embryo when homozygous. One of the tail genes is called brachyury (T), and these mice have short tails. A third gene, histocompatibility-2 (H-2), is linked to the tail genes and is concerned with tissue transplantation. Mice that are H-2/+ will accept tissue grafts, unlike +/+. A cross was carried out between male T H-2/ + + and female + +/+ + mice, and produced the following baby mice.

Brachy tail, accepts graft	105
Normal tail, rejects graft	115
Brachy tail, rejects graft	14
Normal tail, accepts graft	16
Total	250

Determine if the linkage between H-2 and T is significant using the chi-square test.

Answer 1

Null hypothesis - the two genes are not linked and show independent assortment

Expected ratio = 1 : 1 : 1 : 1

Total # progenies = 250

Expected # of progenies = 250/2 = 62.5

	#observed	#expected	(O-E)^2/E
Brachy, accepts	105	62.5	28.9
Normal, rejects	115	62.5	44.1
Brachy, rejects	14	62.5	37.6
Normal, accepts	16	62.5	34.6

Chi square value = 145.2

Degrees of freedom = 4 - 1 = 3

P value < 0.05

Null hypothesis rejected. Hence, the genes are linked.

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