Which of the following represents a homozygous genotype?

• A. Aa
• B. AA
• C. AB
• D. XY

Answer: (B)

A homozygous genotype consists of two identical alleles for a specific gene. This means that both alleles on the homologous chromosomes are the same, whether they are both dominant or both recessive. In this case, the choice indicating "AA" is a clear example of a homozygous genotype because both alleles are the same dominant allele.

The other options demonstrate heterozygosity or different characteristics. For instance, "Aa" shows one dominant and one recessive allele, indicating a heterozygous genotype. The choice "AB" represents two different alleles which also signifies heterozygosity. Lastly, "XY" refers to sex chromosomes rather than a genotype for a specific trait, as it denotes male biological characteristics rather than genetic makeup of a specific gene. Thus, "AA" stands out as the only option that clearly meets the criteria for a homozygous genotype.