Assume that long earlobes in humans are an autosomal dominant trait that exhibits 30% penetrance. A person who is heterozygous for long earlobes mates with a person who is homozygous for normal earlobes. What is the probability that their first child will have long earlobes

15%.

Explanation:

The autosomal dominant is trait that has ability to express itself even in the heterozygous condition as well. Earlobe is the autosomal dominant trait with the penetrance of 30%. Penetrance is the ability to express it phenotype in the population.

The heterozygous individual with long earlobes (Ee) is mated with person that are homozygous for normal lobes (ee). The cross between them results in the 50% progeny with the long earlobes (Ee) and 50 % progeny with normal earlobes (ee). The probability of first child of ear lobe is 50 % or 0.50 with the penetrance of 30% or 0.30. The probability = 0.50 * 0.30 = 0.15 or 15%.

Thus, the answer is 15%.