Achondroplasia is an autosomal dominant form of dwarfism. people with achondroplasia are heterozygotes because carrying two copies of the dominant, mutant allele is fatal during embryogenesis. two people with achondroplasia decide to start a family.

Two people with achondroplasia will have 50% of chance to have a child with achondroplasia. According to Mendelian inheritance, we can indicate dominant allele by A and recessive allele by a, so dominant homozygotes are AA, recessive homozygotes aa and heterozygotes Aa.

Two heterozygotes Aa will have 50% of chances to have a child with achondroplasia (AA), 25% of chances to have a recessive homozygote child (aa) and 25% of chance to concieve a child with two dominant mutant allele (AA):
Parental generation: Aa x Aa
F1 generation: AA, Aa, Aa, aa
Ratio:                1  :     2    :  1

Cotton was the very first