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.
Which of the following is the correct pairing of phenotype and genotype of Individual 5 in the pedigree?

Answer 1

Individual 5 is normal (not dwarf) and genotype dd (homozygous recessive).

Reason: The unaffected parent must be dd. Because the mating produced both affected and unaffected offspring, the affected parent must be heterozygous Dd. A normal (unaffected) offspring therefore has genotype dd.