What is the difference between homozygous and heterozygous alleles?

• A. Homozygous alleles are identical (AA or aa), while heterozygous alleles are different (Aa).
• B. Homozygous means two different alleles.
• C. Heterozygous means identical alleles.
• D. Homozygous refers to the expression of multiple genes.

Answer: (A)

When talking about a gene, you have two copies, one from each parent. If those two copies are the same allele, the genotype is homozygous for that gene (examples are AA or aa). If the two copies are different, the genotype is heterozygous (example Aa). This distinction is about the genotype, not the visible trait by itself, though dominance can affect what phenotype appears in a heterozygote. The other statements mix up the terms: having two different alleles is not homozygous, having two identical alleles is not heterozygous, and homozygous does not refer to the expression of multiple genes.