Question

Achondroplasia is the most common form of dwarfism in miniature horses. Achondroplasia dwarf horses have normal sized bodies and heads, but noticeably smaller upper legs and ears. This form of dwarfism is an autosomal dominant trait. The pedigree shows the offspring of one horse without dwarfism, Individual 1, and one with dwarfism, Individual 2. The horses with achondroplasia are shorter.
Based on the pedigree, which of the following is an accurate claim about the genotype of Individual 2?

Answer 1

Individual 2 must be heterozygous (Aa).

Reason: achondroplasia is autosomal dominant, so the unaffected Individual 1 is aa. Because the pedigree shows some normal (non-dwarf) offspring, Individual 2 cannot be AA (which would make all offspring affected when crossed with aa). Therefore Individual 2 is Aa.