Question

Fruit flies with one allele for curly wings (Cy) and one allele for normal wings (Cy ) have curly wings. When two curly-winged flies were crossed, 203 curly-winged and 98 normal-winged flies were obtained. In fact, all crosses between curly-winged flies produce nearly the same curly : normal ratio among the progeny. If a curly-winged fly was mated to a normal-winged fly, how many curly-winged flies would you expect among 270 total offspring

Answer 1

If there are 270 total offspring, we would expect

• 135 curly-winged flies

As flies with one curly allele (Cy) & one normal allele (Cy+) have curly wing, curly wing is dominant to normal wing.

Two curly-winged flies produce 203 curly-winged & 98 normal-winged flies. This is almost in 2:1 ratio for curly wings: normal wings. But expected ratio in offspring when two heterozygous flies are crossed is 3:1.

Only possibility is that homo-zygous for curly gene is lethal, i.e., CyCy genotype is lethal. So, if a curly-winged fly is survived its genotype will be Cy Cy+. Genotype of normal-winged fly will be Cy+ Cy+

The cross between one curly-winged & one normal-winged fly is shown in below Punnett square.

Gametes                     Cy+                              Cy+

Cy               Cy Cy+ (Curly wing)               Cy Cy+ (Curly wing)

Cy+             Cy+ Cy+ (Normal wing)      Cy+ Cy+ (Normal wing)

We find that expected ratio for curly & normal-winged fly is 1:1. Thus, among 270 offspring we will expect $\frac{270}{2}$ = 135 offspring of curly-winged.

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