Homozygosity

Homozygosity for dominant genes is uncommon, occurring
only when two people with the same disorder have children
together. This may happen preferentially with certain
conditions, such as achondroplasia. Homozygous
achondroplasia is a lethal condition and the risks to the
offspring of two affected parents are 25% for being an affected
homozygote (lethal), 50% for being an affected heterozygote,
and 25% for being an unaffected homozygote. Thus two out of
three surviving children will be affected.