7. in fruit flies, the gene for wing shape has an unusual allele called ‘curly’ (designated ‘cy’). the normal (wild type) allele is designated ‘cy.’ a fly homozygous for cy (cy cy) has normal, straight wings. the heterozygote (cy cy) has wings which curl up on the ends (and, incidentally, can’t really fly). the homozygote for the cy allele (cy cy) never hatches out of the egg. in other words, this allele is lethal in the homozygous condition. if two curly winged flies are mated, and the female lays 100 eggs, predict the following, showing
appropriate work:
a. how many eggs will produce living offspring?

b. how many straight winged flies do you expect among the living offspring?

c. what percentage of the living offspring do you expect to be curly winged like the parents?

A) The answer is 75
b) The answer is 25.
c) The answer is 66%

The genotype of a curly winged fly is Cycy. So the cross between two curly winged flies would look like:
Parents:          Cycy         x         Cycy
Offspring:    CyCy      Cycy   Cycy       cycy

So, 1 of 4 (25%) will be dominant homozygotes (CyCy) which is lethal, 2 of 4 (50%) will be with curly wings (Cycy), and 1 of 4 (25%) will be recessive homozygotes (cycy) with normal wings. 

a) How many eggs will produce living offspring?
Of total 4 combinations, only one of them is lethal (CyCy), thus, 3 of 4 combinations (2 Cycy and 1 cycy) will hatch out. Therefore, 75% of the eggs will survive. If there are 100 eggs, 75% is 75 eggs:
100 eggs : 100% = x : 75%
x = 100 eggs * 75% : 100%
x = 75 eggs

b) How many straight winged flies do you expect among the living offspring?
Of the total 100 eggs, 75 eggs will survive. According to the crossing square, 3 combinations will survive - 2 Cycy and 1 cycy. Among those living offspring 1 is with straight wings. So, it is 1 of 3 or ≈ 33%. Since we have 75 eggs as 100% of living offspring, 33% is 25:
75 eggs : 100% = x : 33%
x = 75 eggs * 33% : 100%
x ≈ 25 eggs

c) What percentage of the living offspring do you expect to be curly winged like the parents?
We have already calculated that 75 eggs will survive. Among the living offspring (3 combinations), 2 of them are with curly wings. So, the percentage is 2 of 3 or 2/3 or 66%