Which term refers to having only one allele of a gene instead of two?

The term that describes having only one allele of a gene instead of two is hemizygous. In a hemizygous condition, an organism has only one copy of a gene instead of the typical two copies that are present in diploid organisms. This situation often occurs in males for genes located on the X chromosome, since males have one X and one Y chromosome, leading to a hemizygous state for X-linked genes.

In contrast, homozygous refers to having two identical alleles of a gene, while heterozygous denotes having two different alleles for a gene. The term genotype, on the other hand, encompasses the genetic constitution of an individual with respect to a specific trait but does not specifically indicate the number of alleles present for a gene. Understanding these distinctions is crucial for studying genetic inheritance patterns and their implications in various biological contexts.