Question

Suppose that a dominant allele (P) codes for a polka-dot tail and a recessive allele (p) codes for a solid colored tail. In addition, suppose that a dominant allele (L) codes for long eyelashes and a recessive allele (l) codes for short eyelashes. If two individuals heterozygous for both traits (tail color and eyelash length) mate, what's the probability of the phenotypic combinations of the offspring? A. 1:1 ratio (half are polka-dot tails and short eyelashes; half are solid tails and long eyelashes) B. 9:3:3:1 ratio (9 polka-dot tails and short eyelashes, 3 polka-dot tails and long eyelashes, 3 solid tails and short eyelashes, 1 solid tail and long eyelash) C. 1:1 ratio (half are polka-dot tails and long eyelashes; half are solid tails and short eyelashes) D. 9:3:3:1 ratio (9 polka-dot tails and long eyelashes, 3 polka-dot tails and short eyelashes, 3 solid tails and long eyelashes, 1 solid tail and short eyelash)

Answer 1

This is a di-hybrid cross example. You can use a large punnet square to work out all the possible phenotype combinations in the offspring, however the shortcut is the 9:3:3:1 ratio. The phenotype outcomes of this ratio can be inferred directly with dominant alleles filling the 9 ratio. The 3 ratios are one dominant and one recessive for each of the two genes, and finaly the 1 ratio is recessive allele for both genes.

Like so:
9 Polka dot tail and long eye lashes
3 Polka dot tail and short eyelashes
3 Solid tail and long eye lashes
1 Solid tail and short eye lashes

The answer is D.