Pink eye is a recessive trait found in deer mice. In a cross between two individuals, both of whom are heterozygous for the pink eye mutation, the following offspring are obtained:

Normal 18

Pink eye 2

Use the chi-square test to determine if the observed numbers of offspring fit the expected numbers of offspring from this cross. The value of is: ____________

If pink eye is recessive, it means that the normal eye is the dominant trait. According to Mendelian law, the ratio of normal to pink eye in a cross involving two heterozygous mice should be 3:1.

Chi-square = [tex]\frac{(O - E)^2}{E}[/tex] where O = observed frequency and E = expected frequency.

Phenotype Observed Expected Chi-square

Normal 18 3/4 x 20 = 15 [tex]\frac{(18 - 15)^2}{15}[/tex] = 0.6

Pink 2 1/4 x 20 = 5 [tex]\frac{(2 - 5)^2}{5}[/tex] = 1.8

Total Chi-square = 0.6 + 1.8 = 2.4

Degree of freedom = 2 - 1 = 1

Critical Chi-square value at 95% = 3.841

The critical Chi-square value is more than the calculated value. Hence, the observed number of offspring fits the expected number of offspring.