2) 2 points. Drosophila melanogaster females heterozygous for three linked autos

Question

2) 2 points. Drosophila melanogaster females heterozygous for three linked autosomal genes (a, b, and c) are testcrossed with triple homozygous recessive males. The map distances are: a-b 10 map units, b-c 20 map units, a-c 30 map units, and the genes are in coupling, that is: all the recessive genes are on the same chromosome. Cross: AaBbCc X aabbcc Write down the possible gametes that will be produced by the heterozygous females. Predict the frequencies of all the phenotypes expected in the offspring. Hint: determine the gene order, then figure out the double-crossover frequencies. Show that your figures give the correct recombination frequencies. I.e. sum the recombination frequencies of each pair to achieve the RFs given above.

Explanation / Answer

a---10cM--b-------20cM----C

Gametes

1. A B C parent
   a b c parent
   A B c Single cross over B and C
   a b C Single cross over B and C
   a B C Single cross over A and B
   A b c Single cross over A and B
   a B c Double cross over
   A b C Double cross over

2. Consider Total 100 progeny

   DCO = 0.1 X 0.2 = 0.02
Progeny in DCO = 0.02 X 100 = 2

a B c - 1
A b C - 1

RF between A and B = 2 X SCO between A and B + DCO / 100
       0.1 = 2 X x / 100
      100 X 0.1 = 2x
       x=5

a B C = 5
A b c = 5

So for RF between B and C

A B c = 10
a b C = 10

Remaining are parents = 100-32 = 68

A B C = 34
a b c = 34

D. Coefficient of coincidence = Observed DCO / Expected DCO
   Observed DCO = 2/100 = 0.02
   Expected DCO = 0.1 X 0.2 = 0.02
   COC = 1
   Interference = 1-COC
                = 1-1 = 0

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