Final answer:
OPTION B.
The expected phenotypic ratio of offspring from parents both heterozygous for brown hair and detached earlobes is 9:3:3:1, as derived from a dihybrid cross that applies the product rule.
Step-by-step explanation:
In humans, brown hair is dominant over blonde hair, and detached earlobes are dominant over attached earlobes. If a male that is heterozygous for both traits has children with a female that is also heterozygous for both traits, the expected phenotypic ratio of their offspring is 9:3:3:1. This ratio comes from a dihybrid cross where each parent contributes two heterozygous alleles (Bb for hair color and Ff for earlobe attachment).
Using a Punnett square, we can predict the genotypic outcomes and thus the phenotypes of the offspring. When we separate the traits and look at them individually, each follows a 3:1 dominant to recessive phenotypic ratio characteristic of monohybrid crosses. However, when combined in a dihybrid cross, the product rule applies and the phenotypic proportions arise from the product of the individual trait proportions: (3/4 dominant hair color × 3/4 dominant earlobe shape) yields 9/16, representing both dominant traits; (3/4 dominant hair color × 1/4 recessive earlobe shape) and (1/4 recessive hair color × 3/4 dominant earlobe shape) each yield 3/16, representing one dominant and one recessive trait; and (1/4 recessive hair color × 1/4 recessive earlobe shape) yields 1/16, representing both recessive traits.
The correct answer to the given question is therefore option B, which is 9:3:3:1.