Question

Three Drosophila genes on the X chromosome occupy three different loci.

The mutant versions of these genes are

y for yellow body

ct for cut wing

ec for echinus (spiky eyes)

When a cross is between female X, and male Y flies, the P and F1 have the following genotype

+ + +/y ct ec

When the F1 is selfed to create the F2, most wild type genes remain linked on the same chromosome, as do the mutant genes on their chromosome. A few recombinants occur, for example

y + +/+ ct ec

where y is unexpectedly found on the same chromosome as the wild-type genes + + + due to crossing over

The percentage of these cross-overs were y/ct 24.9%, y/ec 5.4%, ct /ec 20.3 %. That is the number of times, as a percentage when the two mutant alleles in a pair ended up on different chromatids, rather than on the same chromatid.

What is the order of the loci on the X chromosome? Explain why. [3]

Answer 1

If two genes are distantly located, crossing over percentage between those two genes increases. So, increased crossing over percentage means two genes are distantly located. Here y and ct have highest crossing over frequency (24.9%). so y and ct are two terminal genes among these three genes. so, middle gene is ec. The crossing over percentage between y and ec is lowest so these genes are closely located than the others.

So the order of the gene will be = y------ec----------------------------------ct