Achondroplasia is a form of dwarfism in humans. It is caused by a mutant allele of the fibroblast growth factor receptor gene (FGFR) that produces an overactive protein. Having one copy of the mutant allele results in dwarfism. Two copies of the mutant allele results in death before birth. If two people with achondroplasia have a child together, what is the probability that their child will also have achondroplasia?

According to the question, achondroplasia, a form of dwarfism, is caused by a mutant allele of the fibroblast growth factor receptor (FGFR) encoding gene.
This achondroplasia trait is dominant in nature. This is because presence of a single copy of the mutant allele results in the development of a dwarf phenotype.
Let the dominant mutant allele for the trait be represented by: A.
Let the recessive normal allele for the trait be represented by: a.
The individual with aa genotype is normal.
The individual with Aa genotype shows dwarfism.
The individual with AA genotype dies before birth.
As per the question, two people having achondroplasia have the genotype Aa.
Gametes produced by each of the individuals are, A and a.
Mating these individuals, we get the following possible offspring,

A a

A AA Aa

Phenotype Dies Dwarf

a Aa aa

Phenotype Dwarf Normal

The genotypes and the phenotypes of the possible offspring are represent in the Punnett square above.
The probability of the child to have achondroplasia is: [tex]= \frac{2}{3}[/tex].
Since the child with AA genotype dies before birth so three (genotypes Aa and aa) out of four possible children survives.