Achondroplasia, a form of dwarfism is inherited as an autosomal dominant. Two dwarfs both of whom had one parent of normal height, marry and plan to have a child. What is the probability that their first child will be a dwarf?
Both parents would be heterozygous for Achondroplasia. That means they each have one gene for Achondroplasia and one gene for normal height. Let A stand for Achondroplasia and lower case a stand for normal height gene. Following Mendelian genetics principles, Aa x Aa, the genotypes of the offspring are as follows: 25% AA, 50% Aa and 25% aa. This will produce 75% chance of their offspring having Achondroplasia and 25% chance of them having normal height. It is a 3:1 ratio. Remember that since Achondroplasia is a dominant gene, a person only needs to inherit one copy of that gene to be expressed in the phenotype.