Final answer:
If two parents with achondroplasia (both heterozygous Aa) have a child, there is a 50% chance the child will have achondroplasia, a 25% chance the child will have average height (aa), and a 25% chance of a nonviable fetus with two dominant alleles (AA).
Step-by-step explanation:
If two people with achondroplasia have a baby, the likelihood of their child also having achondroplasia depends on the genotypes of the parents. Achondroplasia is an autosomal dominant condition. This means that a person with one copy of the mutated gene will express the trait. Generally, individuals with achondroplasia are heterozygous, meaning they have one mutated allele and one normal allele (Aa).
When two heterozygous individuals conceive, there is a 75% chance the child will inherit at least one dominant allele and therefore have achondroplasia and a 25% chance the child will inherit two recessive alleles, resulting in an average height phenotype. However, it is important to note that if a child inherits two dominant alleles (AA), they typically do not survive, which is known as a lethal genotype.
The adjusted calculations for two heterozygous people with achondroplasia (each with an Aa genotype) would have a 50% chance of having a child with achondroplasia (Aa), a 25% chance of having a child with an average height (aa), and a 25% chance that the pregnancy will result in a nonviable fetus with two dominant alleles (AA).