The mice resulting from the mating of a wild type with a chimera having a transgene in its germ cells will be __ for this mutation.

• A. null
• B. homozygous
• C. heterozygous
• D. dominant

Answer: (C)

When one parent carries the transgene in its germ cells and the other is wild-type, each offspring inherits one copy of the transgene from the chimera and one normal allele from the wild-type parent. That makes the offspring heterozygous for the transgene. They can’t be null because they acquire the transgene allele from the chimera, and they can’t be homozygous because the wild-type parent does not contribute a second transgene allele. Dominant describes how the trait is expressed, not the zygosity, so the genotype here is heterozygous.