If two hemizygous transgenic mice are crossed, what fraction of offspring carry the transgene?

• A. All offspring
• B. Half of the offspring
• C. One quarter
• D. None

Answer: (B)

When two organisms each carry a single copy of a transgene (hemizygous), each parent can pass on either the transgene-bearing chromosome or the normal chromosome to its offspring, with equal probability. Consequently, the mating can produce four equally likely combinations: two copies of the transgene (one from each parent), one copy (transgene from only one parent), or no copies (from neither parent). Exactly half of the offspring will inherit the transgene from one parent only, giving them a single copy, while the other half will lack the transgene or have two copies. Since the question uses “carry” to mean having a single-copy transgene (hemizygous), half of the offspring carry it.