Fruit flies with one allele for Curly wings (Cy) and one allele for normal wings

Question

Fruit flies with one allele for Curly wings (Cy) and one allele for normal wings (Cy+) have normal curly wings. When two curly-winged flies are crossed, 203 curly-winged and 98 normal winged flies are obtained. In fact, all crosses between curly-winged flied produce the same curly : normal ratio among the progeny. (a) What is the approximate phenotypic ratio in these offspring? (b) Suggest an explanation for these data. (c) If a curly-winged fly was mated to a normal winged fly, how many flies of each type
would you expect among 180 total offspring?

Explanation / Answer

For the purposes of this answer... C=curly and c=normal... because it's easier. Since having a heterogeneous genotype (Cc) produces the curly phenotype we know that the curly allele is dominant. 203 ~= 200 and 98 ~= 100 a) So the ratio is 2:1 b) "2 : 1 Phenotypic Ratio The second major modification of the classical monohybrid ratio (e.g., 3: 1) is produced by alleles whose effect is sufficiently drastic to kill the bearer of certain genotypes in the prenatal or postnatal period prior to sexual maturity. Such alleles are called lethal alleles." http://www.microbiologyprocedure.com/genetics/phenotypic-ratio/2-1-phenotypic-ratio.htm It seems as though being homogeneous (CC) with the curly allele is lethal, because we can see that a cross of a heterogeneous (Cc) pair (curly) would produce two heterogeneous (Cc) (curly), one homogeneous (cc) (normal), and one homogeneous (CC) (dead) in a punnet square. c) 90 normal (cc) and 90 curly (Cc)

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