achondroplasia
Long bones in children grow at special zones near their ends, where cartilage is steadily turned into bone like an assembly line lengthening the limb. In achondroplasia that assembly line runs too slowly because a growth-control switch is stuck in the on position, so the long bones of the arms and legs stay short. It is the most common form of disproportionate short stature, with a characteristic body shape and average-sized trunk.
Achondroplasia is autosomal dominant: a single altered copy of the FGFR3 gene is enough. FGFR3 encodes a receptor that normally restrains bone growth; the causative mutation is a gain-of-function change that makes the receptor overactive. Remarkably, the very same single base change (a specific substitution) accounts for the great majority of cases worldwide, making it one of the most uniform of all genetic disorders.
Most cases arise from a new mutation in a child born to average-height parents, and the chance of a new mutation rises with the father's age. People with achondroplasia have normal intelligence and lifespan; this entry uses the encyclopedic medical term and is educational reference, not a value judgment or medical advice.
A baby with shortened limbs and average trunk length is born to two average-height parents; genetic testing finds the typical FGFR3 substitution, present in neither parent, confirming a new mutation.
A new dominant mutation can appear in a child whose parents do not carry it.
Achondroplasia is a striking example of mutational uniformity: one particular substitution at one position in FGFR3 causes nearly all cases, partly because that site is unusually prone to that change.