Pink-eye and albino are two recessive traits found in the deer mouse Peromyscus
ID: 54587 • Letter: P
Question
Pink-eye and albino are two recessive traits found in the deer mouse Peromyscus maniculatus. F.H. Clark crossed pink-eyed mice with albino mice; the resulting F1 had normal coloration in their fur and eyes. He then crossed these F1 mice with mice that were pink-eyed and albino and obtained the following mice. It is very hard to distinguish between mice that are albino and mice that are both pink-eye and albino, so he combined these two phenotypes together.
Phenotypes - Number of Progeny
Wild type fur; wild type eye - 12
Wild type fur; pink-eye - 62
Albino + Albino; pink-eye - 78
1. Give the expected numbers of progeny with each phenotype if the genes for pink-eye and albino assort independently.
2. Use a Chi-square test to determine if the observed numbers of progeny fit the number expected with independent asortment. Do you accept or reject your hypothesis? Please explain.
Explanation / Answer
1. For fur- Let's use A for and a for albino fur.
For eye color- Let's take P for wild-type eye color and p for red eye color.
In the first cross,
Pink-eyed-mice were crossed with albino mice. It is very likely that both the mice were homozygous wild-type for the allele. The cross will be:
Pink- eye (AA pp) X Albino (aa PP)
F1- Normal eyes and fur(Aa Pp)
These F1 mice were then crossed with pink-eyed and albino (aa pp)
The F2 resulted in:
wild-type, wild-type fur with pink eyes, both albino fur with wild-type eyes and albino fur with pink eyes
These were difficult to distinguish between. If the two pairs of alleles are assorting independently, we would expect that each phenotypic category will occur 1/4 of the time.
Thus, the result for the cross Wild-type(Aa Pp) X Albino, pink-eye(aa pp) will be:
1/4 wild-type
1/4 wild-type fur and pink eyes
1/4 albino fur and wild-type eyes
1/4 albino fur and pink eye
Since we cannot distinguish between albinos with wild-type eyes and albinos with pink eyes, we will combine the expected phenotypic categories. This will result in a combined ratio of albinos to be 1/2.
2. Chi square test will be:
Wild-type
Since there are 3 phenotypic classes, so the degrees of freedom = 3 – 1= 2.
The chi-square value is greater than 10.597 for a probability value less than.005 or 0.5%. The observed number of progeny do not fit with the expected numbers due to independent assortment. Thus, the hypothesis is rejected and the progeny do not fit the number expected with independent assortment.
Phenotype Observed(O) Expected(E) (O-E)2/EWild-type
12 38 17.7 Wild-type pink eyes 62 38 15.2 Albino wild-type, albino pink eyes 78 76 .05 Total 152 19.5 32.95