Usually a heterozygote. Achondroplasia - Affected mother with affected son. Achondroplasia - Unaffected deceased parents. This is an example of a new mutation. Often the case in severe autosomal dominant disease 6.
Achondroplasia is fully penetrant and shows no variation of expression.
This is not the only explanation for such a pedigree pattern in autosomal dominant disease. Concepts of penetrance, age de pendent penetrance and variability of expression. Concept of reproductive fitness important in understanding gene frequencies for autosomal dominant diseases.