If a transgene is integrated in one of the chromosomes of a pair, the animal is said to be:

• A. heterozygous
• B. hemizygous
• C. homozygous
• D. haptozygous

Answer: (B)

Having the transgene on only one chromosome means there is a single copy of that gene in the genome. In a diploid animal, most genes have two copies—one on each homolog. When the transgene sits on just one chromosome and not on the other, the animal is hemizygous for that transgene. This term emphasizes that there is only one copy per genome, not two identical copies as in the homozygous state. Breeding can segregate this so offspring may or may not inherit the transgene; to make a line homozygous for the transgene, you'd need both chromosomes to carry it.