What describes a homozygous individual for a gene?

• A. Two different alleles.
• B. One allele present.
• C. Half the gene is expressed.
• D. Two identical alleles.

Answer: (D)

Two identical alleles for a gene define a homozygous individual. In diploid organisms, each gene has two copies, one on each chromosome. If those two copies are the same—such as AA or aa—the genotype is homozygous. If they are different, the genotype is heterozygous (Aa). This is why “two identical alleles” correctly describes homozygosity. Descriptions like two different alleles fit the heterozygous case, saying only one allele is present isn’t accurate for a typical gene in a diploid organism, and expressing half the gene isn’t how allele identity or gene copies determine genotype.