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?
A)1/8
B)1/4
C)1/2
D)1

Question

Quizplus · Accepted Answer

Since achondroplasia is caused by an autosomal dominant mutation, the affected person must have at least one copy of the mutant gene. Therefore, the affected person must be heterozygous with one mutant allele and one normal allele. 
When the affected person's normal height spouse is added to the equation, we can create a Punnett square to determine the probability of their offspring inheriting achondroplasia:

|   | A | a | <- Parent 1 (A = mutant allele, a = normal allele)
|---|---|---|
| a | Aa| aa|
| a | Aa| aa|

There is a 50% chance that the affected person will pass on the mutant allele (A) to each of their offspring, since they are heterozygous. There is also a 50% chance that the unaffected spouse will pass on a normal allele (a) to each of their offspring. Therefore, the probability that both children will have achondroplasia is:
0.5 (chance of inheriting mutant allele from affected parent) x 0.5 (chance of inheriting normal allele from unaffected parent) x 0.5 (chance of inheriting mutant allele from affected parent again) x 0.5 (chance of inheriting normal allele from unaffected parent again) = 0.125 or 1/4. So the answer is B) 1/4.

Achondroplasia Is a Common Form of Dwarfism Caused by an Autosomal