What is the probable explanation for achondroplasia in their child?
Achondroplasia results from a point mutation in the gene coding for fibroblast growth factor receptor 3 on the short arm of chromosome 4. The result is abnormal chondroid production resulting in decreased linear bone growth, among other functions; hence, the decreased stature in those affected. In over 80% of cases, the condition occurs due to sporadic, or de novo, mutation. Thus, a child with achondroplasia can be born to healthy parents with no family history of the disorder, as in our case. A risk factor for producing offspring with the de novo mutation is advanced paternal age, due to mutation occuring during spermatogenesis.
What is the risk of recurrence in other children of this couple?
1/443, or 0.02% (Source: Mettler G, Fraser FC. Recurrence risk for sibs of children with "sporadic" achondroplasia. Am J Med Genet. 2000 Jan 31;90(3):250-1. PMID: 10678665)
If the parents are of average stature, the risk to sibs of having achondroplasia is very low but appears to exceed that in the general, comparable population because of the possibility of parental germline mosaicism
If the husband has another child by a different mother, what would the risk of achondroplasia be in that child?
The risk to half-sibs of having achondroplasia is very low and appears to equal that in the general, comparable population, since this is a de novo mutation
What is the risk that any offspring of the affected child will also have achondroplasia?
The risk to offspring of an individual with achondroplasia of inheriting the FGFR3 pathogenic variant is 50%. An individual with achondroplasia who has a partner of average stature is at 50% risk of having a child with achondroplasia. When both parents have achondroplasia, their offspring have a 25% chance of having average stature, a 50% chance of having achondroplasia, and a 25% chance of having homozygous achondroplasia (a lethal condition).