Question

In fruit flies, a grey body is dominant to a black body, and normal wings are dominant to vestigial wings. Flies are heterozygous for both grey bodies and normal wings were crossed with flies that had black bodies and vestigial wings. The following results were obtained:

Phenotype	Number of flies
Grey body/Normal wings	482
Black body/Vestigial wings	472
Grey body/Vestigial wings	103
Black body/Normal wings	92

The results indicate that the genes for wings and body color are on the same chromosome. The recombinant offspring are a result of crossing over.

How many map units (expressed as a percentage to the nearest whole number) apart are the two genes?

Answer 1

According to the question,

Grey body color (g⁺) is dominant over recessive black body (g) and normal wings (vg⁺) is dominant over vestigial wings (vg).

Flies that are heterozygous for both alleles (g⁺g vg⁺vg) were crossed with black body and vestigial wings (gg vgvg). The cross can be represented as follows:

                             g⁺g vg⁺vg X gg vgvg

Gametes	g+ vg+	g+ vg	g vg+	g vg
g vg	g+g vg+vg Grey body/ normal wings	g+g vgvg Grey body/ vestigial wings	gg vg+vg Black body/ normal wings	gg vgvg Black body/ vestigial wings
No: of offsprings	482	103	92	472

Thus, four classes of offsprings were produced. Of this, grey body/ normal wings and black body/ vestigial wings are parental genotypes and grey body/ vestigial wings and black body/ normal wings are recombinant genotypes.

This shows that the genes for body color and wings are present on the same chromosome and are linked. In this case, parental configurations are more and recombinant configurations are less.

In order to measure linkage, we calculate the recombination frequency (RF) between the genes.

Recombination frequency is a measure of genetic linkage and is used in the creation of genetic linkage map.

RF can be calculated as follows:

RF   = (Number of recombinants / Total number of offsprings) X 100

Hence, for the above cross,

RF = [(103 + 92) / (482 + 103 + 92 + 472)] X 100

       = 16.97%

RF is not a direct measure of the distance between two genes on a chromosome, but is an estimation of the physical distance.

High RF implies the genes are far apart and low RF indicates that the genes are closer together.

Distance between two genes are expressed in centimorgans (cM) or map units.

1% RF = 1 cM or 1 map unit.

Therefore, 16.97% (rounded to 17%) = 17 cM or map units.

Thus, in this case, the genes of the wings and body color are 17 map units apart from each other.