Question

Fruit flies with one allele for curly wings (CM 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. a. Whet is the approximate phenotypic ratio in these offspring? b. Suggest an explanation for these date. c. If a curly-winged fly was mated to e normal- winged fly. how many flies of each type would you expect among 180 total offspring?

Answer 1

a) 2:1 (Curly:normal); b) The mutation is lethal in homozygosis; c) 90 will have curly wings, 90 will have normal wings.

Step-by-step explanation:

The mutation for curly wings Cy is dominant over the recessive wild-type allele Cy+.

In order to see normal-winged flies in the progeny, the curly-winged parentals must have been heterozygous.

The cross was:

CyCy+ x CyCy+

The expected offspring (obtained from a Punnett Square of the cross) is:

• 1/4 CyCy curly winged
• 2/4 CyCy+ curly winged
• 1/4 Cy+Cy+ normal winged

The expected phenotypic ratios are 3:1.

However, those were not the observed proportions.

a) Out of a total of 301 progeny, 203 were curly-winged and 98 were normal-winged. 203/301 is approximately 2/3, and 98/301 is approximately 1/3, so the observed phenotypic ratio is 2:1.

b) The observed phenotypic ratio in all crosses suggests that the genotype CyCy is lethal and never appears in adult flies.

c) From a Punnett Square we can determine that a cross between the flies CyCy+ (curly winged) and Cy+Cy+ (normal winged) would give the offpsring:

• 1/2 CyCy+ (Curly winged)
• 1/2 Cy+Cy+ (Normal winged)

Among 180 total offspring, half of them (90) will have curly wings, and half (90) will have normal wings.