Question

achondroplasia is caused by an autosomal dominant mutation in the fibroblast growth factor receptor 3 (fgfr3) gene. a person who carries this mutation has shorter limbs than average. two copies of the mutant gene are invariably fatal before or shortly after birth. if a person with achondroplasia has children a person of average height, what is the probability that both their first child and second child will have achondroplasia?
multiple choice
1/8
1
1/4
1/2

Explanation:

Step1: Determine genotypes of parents

Person with achondroplasia: heterozygous (let’s say Aa, since AA is fatal). Average height person: homozygous recessive (aa).

Step2: Calculate probability per child

Gametes from Aa: A or a (50% each). Gametes from aa: only a. Offspring genotypes: Aa (achondroplasia) or aa (average height). Probability per child having achondroplasia = $\frac{1}{2}$.

Step3: Calculate joint probability

Probability both first and second child have achondroplasia = $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.

Answer:

1/4