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How is achondroplasia inherited?
Achondroplasia is inherited as an autosomal dominant trait whereby only a single copy of the abnormal gene
(mutation) is required to cause achondroplasia. The gene for achondroplasia is fully penetrant, meaning that everyone who possesses it has achondroplasia. No one with the gene escapes achondroplasia. However, there is some variation in expression of the gene, meaning that children with achondroplasia are not carbon copies of each other, although they may look alike to the untutored eye.
In only about an eighth of cases is the gene inherited from a parent who has achondroplasia. Rather, about seven-eighths of cases are due to a new mutation (a new change in the gene). This means that most cases of achondroplasia occur sporadically (out of the blue) and are the result of a new mutation in a sperm or ovum of one of the normal- appearing parents. The chance of a new mutation rises with the age of the father. As early as 1912 it was noted that sporadic (new) cases were more often last-born than first-born children. This fits with the fact that the chance of an achondroplastic birth has been shown to increase with paternal age (age of the father).