An organism possessing two identical alleles for a given gene is described as:

• A. Heterozygous
• B. Hemizygous
• C. Monohybrid
• D. Homozygous

Answer: (D)

Having two identical alleles for a gene means the organism’s genotype is homozygous. In diploid organisms you inherit one allele from each parent, and when both copies are the same—whether the trait is expressed dominantly or recessively—the genotype is called homozygous (for example, YY or yy). If the two copies were different, it would be heterozygous (such as Yy), which often results in the dominant trait being shown in the phenotype. Hemizygous describes having only one allele for a gene in a diploid context (as with many X-linked genes in males), not two identical copies. Monohybrid refers to a type of genetic cross examining one gene, not the arrangement of alleles within an individual. So two identical alleles for a gene describe a homozygous genotype.