Question

Suppose that we cross two flies: One is a female that is b+b vg+vg; the other is a male that is homozygous recessive. The female has grey (wild-type) body and normal (wild-type) wing length. The male is black-bodied with vestigial wings. The female's parents (I'm going to refer to them as the grandparents) were a true-breeding grey, normal fly and a true-breeding black, vestigial fly.

A) What is the frequency of phenotypes you would expect to see in their offspring? This can be written as A/N grey, normal : B/N grey, vestigial : C/N black, normal : D/N black, vestigial. (A/N, B/N, C/N, D/N are just the fraction of offspring with that phenotype).

B) A researcher named Thomas Hunt Morgan did these crosses. He found the following results in the offspring (I'm going to refer to these offspring as the grandkids since we crossed their parents and know something about the grandparents).

# offspring	phenotype	genotype (inferred)
965	b+b vg+ vg	grey, normal
206	b+ b vg vg	grey, vestigial
185	b b vg+ vg	black, normal
944	b b vg vg	black, vestigial

B) This result does NOT agree with the prediction (if they agree with the frequencies you predicted in part A, something is wrong in part A). What would best explain this? Why is it that the phenotypes of the mom's parents (the grandparents - grey, normal and black, vestigial) are more common in the offspring (grandkids) than the other phenotypes (grey, vestigial and black, normal)?

Answer 1

Answer:

The female's parents were a true-breeding grey, normal fly (b+ vg+ / b+ vg+) and a true-breeding black, vestigial fly (b vg / b vg)

Female genotype = b+ vg+ / b vg

Male genotype = b vg / b vg

b+ b vg+vg x b b vgvg ---Parents

Expected progeny frequencies:

	b vg
b+ vg+	b+b vg+vg –1/4 (grey, normal)
b+ vg	b+b vgvg—1/4 (grey, vestigial)
b vg+	bb vg+vg—1/4 (black, normal)
b vg	bb vgvg—1/4 (black, vestigial)

B).

Based on the progeny numbers, it is clear that the two genes are present on the same chromosome and are linked. When genes are linked, recombination frequency would be less than expected when genes are unlinked (as mentioned above). The linkage phenomenon reduces the chance of occurring recombinations or crossing over between genes. Hence, the progeny numbers are not same and recombinant phenotypes are less common than the non-recombinant phenotypes.

Linkage is indirectly proportional to recombination frequency (recombinants)

The phenotypes of the mom's parents (the grandparents - grey, normal and black, vestigial) are more common in the offspring (grandkids) because they are non-recombinants. The other phenotypes (grey, vestigial and black, normal) are recombinant so that they are less numbered.