Which description describes an animal that carries the transgene in only one copy of a chromosome pair (and lacks it on the homolog)?

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

Answer: (C)

Having a transgene on only one chromosome of a pair, with no copy on the homolog, means you possess a single allele for that gene in the diploid genome. This condition is called hemizygous. In a diploid organism, homozygous would require two identical copies on both homologs, while heterozygous would involve two different alleles on the paired chromosomes, so there are still two copies present. Haplotypic refers to a set of alleles inherited together on one chromosome, not the number of copies. A practical example is a male animal for X-linked genes: it has only one X chromosome, so any gene on X is present in a single copy (hemizygous).