Question

A geneticist discovers a new mutation in Drosophila melanogaster that causes the flies to shake and quiver. She calls this mutation quiver (qu) and determines that it is due to an autosomal recessive gene. She wants to determine if the gene encoding quiver is linked to the recessive gene for vestigial wings (vg). She crosses a fly homozygous for quiver and vestigial traits with a fly homozygous for the wild-type traits and then uses the resulting F1 females in a testcross. She obtains the following flies from this testcross.

vg+ sps+ 230
vg sps 224
vg sps+ 97
vg+ sps 99
total 650
1. Are the genes that cause vestigial wings and the spastic mutation linked?
2. Do a chi-square test of independence to determine whether the genes have assorted independently.
3. List the chi-square value, degree of freedom, P value, and whether you accept or reject the hypothesis of independent assortment.

Answer 1

1. First calculate the distance of the map for trace (eg) and spastic mutation (sps); Recombinant frequency:

vg + sps + vg sps + = 99 + 97 = 196

we are going to divide 196 by 650 (total descendants) = 0.3.

in the next step we are going to calculate the recombinant frequency 0.3 * 100 = 30 mu or cM or 30%

this shows that there is an association between vestige (eg) and spastic mutation (sps) since the recombinant frequency is less than 50%.

The squareness test is not valid because the combining frequency is far from 50%.

2. Results:

1. Genes are linked.

3. The test rejected the independent assortment law because it is not true that there is a 1: 1: 1: 1 relationship between descendants