In the fruit fly, drosophila melanogaster, a spineless (no wing bristles) female fly is mated to a male that is claret (dark eyes) and hairless (no thoracic bristles). all of the resulting flies are phenotypically wild-type (assume all three mutant genes are recessive). the wild-type f1 female progeny were mated to fully homozygous (mutant) males, and the following progeny (1000 total) were observed: phenotypes number observed spineless 321 wild 38 claret, spineless 130 claret 18 claret, hairless 309 hairless, claret, spineless 32 hairless 140 hairless, spineless 12 what is the correct gene map for these genes?

Explanation:

We have three genes in the same chromosome and are trying to determine their order and relative distance to each other.

The genes and possible alleles are:

All mutations are recessive: two copies of the mutant allele are needed for the fly to show that trait.

Parental cross

Each parent can produce 1 type of gamete only, so the F1 will be homogeneous:

F1

During meiosis, the F1 females can produce 8 types of gametes: 2 parentals and 6 recombinants (two of them, the result of a double crossing over).

If they are test crossed to homozygous recessive males (which can only produce a gamete), the following phenotypes are obtained (I just write the alleles they inherited from the female fly, as the ones that came from the male are the same for all of them):

The phenotypes observed in the highest frequency are always the parentals, and the ones in the lowest frequency are always the double recombinants.

To determine the order of the genes:

In this problem, only if the middle gene is h+/h the double crossing over gives us the observed double recombinant gametes, therefore hairless is the middle gene.

Double recombinants:

To determine the distance between the genes:

Genetic distance (m.u.) = Recombination Frequency x 100

spineless -----------------hairless ---------------------------claret

                   10 m.u.                            30 m.u.

explanation:

the fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus eohippus.

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