Discussion
In humans pointed eyebrows are dominant over smooth eyebrows and downward pointed frontal hairline is dominant over continuous hairline. What phenotypic ratio would you expect in the offspring from a cross between an individual heterozygous for both genes and an individual homozygous recessive for both genes ?
Answer: C
The expected ratio is 1 : 1 : 1 : 1.
This is a dihybrid test cross. The double heterozygote (PpHh) makes four kinds of gamete in equal numbers, PH, Ph, pH and ph, and the double recessive (pphh) makes only ph. Each offspring phenotype therefore reflects one gamete of the heterozygote:
\[ \text{PpHh} \times \text{pphh} \to 1 : 1 : 1 : 1 \]
9 : 3 : 3 : 1 is the ratio from crossing two double heterozygotes. 9 : 3 : 4 (recessive epistasis) and 9 : 7 (complementary genes) are modified dihybrid ratios that need gene interaction, which the question does not describe.
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