What does heterozygous mean?

• A. A mix of several genes
• B. Two different alleles
• C. One allele present
• D. Two identical alleles

Answer: (B)

Heterozygous means having two different alleles at the same gene locus. In a diploid organism, you inherit one allele from each parent for every gene. If those two alleles are different, you’re heterozygous for that gene (for example, Aa). This contrasts with homozygous, where the two alleles are the same (AA or aa). The idea is about the pair of alleles at a single gene, not about multiple genes or a single copy. So having two different alleles directly describes heterozygosity.