Question

Cinnabar eyes (cn) and reduced bristles (rd) are autosomal recessive characters in Drosophila. A homozygous wild-type female was crossed to a reduced, cinnabar male, and the F1 males were then crossed to the F1 females to obtain the F2. Of the 400 F2 offspring obtained, 292 were wild type, 9 were cinnabar, 7 were reduced, and 92 were reduced and cinnabar.

Answer 1

Remainder of question

If these genes assorted independently, what phenotypic ratio would you expect for f2?

Answer:

For Mendelian inheritance of 2 traits where alleles are independently assorted, we would expect a ratio of 9:3:3:1.

Explanation:

Lets denote the dominant allele as cn+ and the recessive as cn (and rd+ and rn)

The F1 generation would all be cn+cn / rd+rd

There are 4 possible gamete variations for each parent: cn+rd+, cnrd, cn+rd or cnrd+

Independent assortment of these gametes produces 16 possible diploid variations. 9 of these will have the wild type (WT) phenotype, 3 will be WT eyes but reduced bristles, 3 will be WT bristles but cinnabar eyes, and 1 will have reduced bristles and cinnabar eyes.

This can be demonstrated using a punnet square for a dihybrid cross.

For 400 offspring, that means there will be:

WT: 400/ 16 * 9 = 225

WT eyes but reduced bristles: 400/16 * 3 = 75

WT bristles but cinnabar eyes: 400/16 * 3 = 75

reduced bristles and cinnabar eyes: 400/16 * 1 = 25