Pycnodysostosis (cont.)

Medical Reviewer:

How is pycnodysostosis inherited?

Pycnodysostosis is an autosomal recessive condition. It is autosomal in that the gene for pycnodysostosis is situated on one of the non- sex chromosomes (autosomes). Pycnodysostosis is recessive in that a single edition of the pycnodysostosis gene is not enough to cause the disease. Two copies of the abnormal gene (one from each parent) must be present for the disease to develop. The presence of a normal gene from one or both parents prevents the disease.

What are the risks of pycnodysostosis?

Pycnodysostosis is a rare disease so that, if there is no family history of the disease, the risk for a child to have pycnodysostosis is very low.

However, with a inbred (consanguineous) union, the risk of pycnodysostosis rises a little. This means that if one parent carries a rare gene like that for pycnodysostosis, the chance that a mate of theirs who is actually related also carries the same rare gene is increased compared to an unrelated mate.

In a family with a child with pycnodysostosis, the risks are much higher. The parents of a child with pycnodysostosis have no signs of the disease themselves but they each carry a single edition of the pycnodysostosis gene and, with a given pregnancy, each has a half chance of transmitting the pycnodysostosis gene (versus the normal paired gene). It is like tossing a coin. The chance for the child to have pycnodysostosis is one-half (from one parent) times one-half (from the other parent). The overall risk of pycnodysostosis in the offspring of parents carrying the gene is, therefore, one-quarter (25%). Just as coins have no memory of a prior toss, the 25% odds of having a pycnodysostosis child apply to every pregnancy conceived together by these parents, irrespective of the status of any of their other children.

Medically Reviewed by a Doctor on 12/20/2013

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