What term describes an animal carrying a transgene on one chromosome but not on the homologous chromosome?

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

Answer: (C)

When a gene or transgene is present on only one chromosome of a pair, there is just a single copy of that locus in the genome. That situation is described as hemizygous. It captures the idea of having one allele for the gene across the two homologous chromosomes, rather than a pair of identical or different alleles.

Homozygous would mean both chromosomes carry the same allele (both with the transgene or both without). Heterozygous would mean the two chromosomes carry two different alleles at that locus (one transgene allele and one wild-type allele). The term haptozygous isn’t used in standard genetics.