Inheritance: autosomal dominant with high (100%) penetrance
Frequency: 1 in 15,000 to 1 in 40,000 live births
Parental status patterns:
(1) neither parent has clinical achondroplasia
(2) one parent normal, one parent with achondroplasia
(3) both parents with achondroplasia
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Parental Status
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Recurrence Risk
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neither parent with achondroplasia (both normal)
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99.98% normal
0.02% with achondroplasia
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one parent with achondroplasia, second normal
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50% normal
50% heterozygous for achondroplasia,
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both parents with achondroplasia
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25% normal
50% heterozygous for achondroplasia
25% homozygous
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where:
• The risk of recurrence in "sporadic" cases with phenotypically normal parents depends on the mutation rate and the occurrence of gonadal mosaicism (Mettler and Fraser). Because of gonadal mosaicism, the recurrence rate should be higher than twice that of the mutation rate (a rate of 1 in 20,000 would be 0.005%; 2 times this 0.01%).
• The risk of recurrence of "sporadic" cases appears related to a paternal age > 35 years of age.
• Homozygous achondroplasia is lethal as a neonate.
