5. In Drosophila melanogaster, black body (b) is recessive to gray body (b+), pu

Question

5. In Drosophila melanogaster, black body (b) is recessive to gray body (b+), purple eyes (pr) are recessive to red eyes (pr+) and vestigial wings (vg) are recessive to normal wings (vg+). The loci encoding these traits are linked, with the following map distances:

b--- pr --- vg

(6) (13)

The interference among the genes is 0.5. A fly with a black body, purple eyes, and vestigial wings is crossed with a fly homozygous for a gray body, red eyes, and normal wings. The female progeny are then crossed with males that have a black body, purple eyes and vestigial wings. If 1000 progeny are produced from this testcross, what will be the phenotypes and proportions of the progeny ?

Explanation / Answer

Answer

Black body (b) is recessive to gray body (b+),

Purple eyes (pr) are recessive to red eyes (pr+)

Vestigial wings (vg) are recessive to normal wings (vg+)

b-----pr-------vg

   6          13

Given interference is 0.5

Coefficient of coincidence = 1- interference

Coefficient of coincidence (C.O.C) = 0.5

As, the recombinant frequency comes from both single and double crossover progenies first calculate the number of double crossover progeny:

Recombinant frequency b/w b and pr is 6 or 0.06%

Recombinant frequency b/w pr and vg is 13 or 0.13%

We know that coefficient of coincidence is 0.5

C.O.C = observed double crossovers/expected double cross overs (1000)

0.5 = observed double crossovers/ (0.06) (0.13) (1000)

0.5 = observed double crossovers/7.8

Observed double crossovers = (0.5) (7.8)

Observed double crossovers = 3.9 round it to 4

As parents are either homozygous recessive or homozygous dominant for all the three alleles, the F 1 heterozygote fly has chromosomes with b pr vg and b+ pr+ vg+ and thus, these are nonrecombinant progeny.

The expected double crossover progeny should be b+ pr vg+ and b pr+ vg. As there are 4 double crossover progeny, there should be 2 progeny flies of each double crossover.

Recombination frequency b/w b and pr is 6% or 0.06, this recombination frequency arises from the sum of single crossover (SCO) b/w b and pr, the double crossover progeny is:

SCO (b and pr) + DCOs = 0.06 (1000)

SCO (b and pr) + 4 = 60

SCO (b and pr) = 60 – 4

SCO (b and pr) = 56

So, the SCO progeny b/w b and pr are b pr+ vg+ and b+ pr vg. As, the total progeny are 56, or there would be 28 each.

Similarly, SCos b/w pr and vg is 13% or 0.13:

SCO (pr and vg) + DCOs = 0.13 (1000)

SCO (pr and vg) + 4 = 130

SCO (pr and vg) = 130 - 4

SCO (pr and vg) = 126

Progeny b/w pr and vg are b pr vg+ and b+ pr+ vg. These are 126 in total so, or there would be 63 each.

The number of parental progeny can be calculated as following:

= 1000 - (126+ 56+ 4)

= 1000 - 186

= 814

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