In Drosophila, sepia eyes (se), curled wings (cu) and ebony body (e) are found, in this order, on chromosome 3. Sepia is 22 map units from curled, which is 12 map units from ebony. A true-breeding female with sepia eyes and ebony body is crossed to a true- breeding male who has curled wings. The resulting F1 progeny from this cross are all wild type. An F1 female is test crossed to a male expressing all three traits. Assuming a Coefficient of coincidence of 1:

a. What proportion of the F2 progeny will express only the sepia trait?
b. If there are 1000 total flies, what number will express curved only?

a. 6%

b. 316.8 or roughly 317

Explanation:

Distance between two genes in map units = recombination frequency (RF) between them. So,

RF between se and cu gene = 22%

RF between cu and e gene = 12%

Double crossover frequency (All three genes involved) = 0.22 * 0.12 = 0.0264 or 2.64%

Since coefficient of coincidence is 1, observed double crossover frequency is same as expected double crossover frequency

Female = true breeding sepia and ebony = seseCUCUee

Male = true breeding curled wings = SESEcucuEE

seseCUCUee X SESEcucuEE = SEseCUcuEe (F1)

Next is a test cross:

SEseCUcuEe X sesecucuee = F2 generation

a) Given, the genes are in the order se-cu-e on the chromosome. According to this gene order, a single crossover between genes cu and e will give recombinant gametes seCUE and SEcue. Out of them, seCUE will result into F2 progeny with only sepia eyes ( Other two traits will be wild type). Since the recombination frequency between cu and e genes is 12%, 6% will be seCUE or the one with only sepia eyes.

b) Total RF = Single RF between se and cu + Single RF between cu and e + Double crossover frequency

= 22 + 12 + 2.64 = 36.64%

Parental combinations = 100 - 36.64 = 63.36%

The two parental combinations are seseCUCUee and SESEcucuEE. Out of them, seseCUCUee will produce only curled wings ( Other two traits will be the mutant type). Since total parental combinations are 63.36%, curled wings will be = 63.36/2 = 31.68%

Total number of flies in F2 = 1000

Curled only flies = 1000*0.3168 = 316.8 or 317 flies.

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