Which is a heterozygous genotype?

Which is a heterozygous genotype?

• A. AA.
• B. aa.
• C. BB.
• D. Aa.

Answer: (D)

Heterozygosity means having two different alleles for a gene. For a gene with alleles A and a, the only combination that has two different alleles is Aa, meaning one comes from each parent. The other options pair identical alleles (AA, aa) or two of the same allele (BB), so they are homozygous. If A is dominant, the Aa genotype would show the A-trait in phenotype despite having one of each allele.

Heterozygosity means having two different alleles for a gene. For a gene with alleles A and a, the only combination that has two different alleles is Aa, meaning one comes from each parent. The other options pair identical alleles (AA, aa) or two of the same allele (BB), so they are homozygous. If A is dominant, the Aa genotype would show the A-trait in phenotype despite having one of each allele.