Question

Achondroplasia is a common form of dwarfism caused by an autosomal dominant mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. Two copies of the mutant gene are invariably fatal before or shortly after birth. If a person with achondroplasia marries a person of normal height, what is the probability that both their first child and second child will have achondroplasia?

Answer 1

Answer:

1/4

Explanation:

Achondroplasia is caused by an autosomal dominant mutation. Two copies of the mutant allele are fatal.

The possible genotypes and phenotypes for this gene are:

• AA: lethal
• Aa: achondroplasia
• aa: normal height

If a person with achondroplasia marries a person of normal height: the corss is Aa x aa.

Their children can have the following genotypes:

• 1/2 Aa (achondroplasia)
• 1/2 aa (normal)

Each time the couple has children, the gametes that combine do so randomly and independently. According to the rule of multiplication, the probability of occurrence of two independent events can be calculated as the multiplication of the probability of each one.

Therefore, the probability of having the first two children with achondroplasia will be 1/2 × 1/2 = 1/4.