What does homozygous mean?

• A. Two different alleles
• B. Two identical alleles
• C. No alleles
• D. One allele present

Answer: (B)

Homozygous means having two identical alleles for a gene. In a diploid organism, you inherit one allele from each parent, so you have two copies of every gene. If both copies are the same version, you’re homozygous (for example, both alleles are the dominant version or both are recessive). If the two copies are different, you’re heterozygous. So the idea here is two identical alleles. The other descriptions describe different situations: two different alleles is heterozygous, having no alleles isn’t a normal state for a gene in a typical organism, and having only one allele present isn’t how genes are usually organized in a diploid individual.