Question

You cross two individual striped mice multiple times. The resulting F1 generation offspring all have striped coats. You cross F1 mice multiple times and observe the following phenotypes among the F2 offspring:

145 striped mice,

37 spotted mice,

13 mice with no patterns.

From these observations you suspect that coat patterning is under the control of two bi-allelic genes experiencing dominant epistasis.

Answer the questions below (Q 1-3) and use a Chi-square test to support or reject your hypothesis, then answer the remaining question (Q4).

Q1 What is the expected ratio of offspring phenotypes under dominant epistasis?

Q2 Assuming that striped mice, spotted mice, and mice with no patterns represent the exact order of the phenotypes in the ratio you selected above, what is your calculated chi-square value?

Q3 Do you reject or support your initial hypothesis?

Q4 What other type of epistasis between two loci can lead to the expression of three distinct phenotypes?

Answer 1

1).

The phenotypic ratio = 12:3:1

2).

Phenotype	Observed(O)	Expected (E)	O-E	(O-E)2	(O-E)2/E
Striped mice	145	146.25	-1.25	1.56	0.0107
spotted mice	37	36.5625	0.4375	0.19	0.0052
mice with no pattern	13	12.1875	0.8125	0.66	0.0542
Total	195	195			0.0701

Chi-square value = 0.07

Degrees of freedom = number of genotype – 1

Df = 3-1=2

p-value = 5.99

The X^2 value of 0.07 is less than the critical value of 5.99. So we can accept or support the hypothesis.

4). Recessive epistasis can also lead the production of three distinct phenotypes with 9:3:4 ratio.