X ray showing the skull and spine of a person with achondroplasia Achondroplasia Definition Achondroplasia, or short-limb dwarfism, is the most common form of abnormally short stature in adults. It is caused by a mutation in a single gene on chromosome 4 that regulates the conversion of cartilage to bone. This gene is the only gene that is known to be associated with achondroplasia. Description Achondroplasia is basically a disorder of bone development. The skeleton of a human fetus is composed primarily of cartilage, a dense and somewhat elastic form of connective tissue that gradually turns to bone during normal development. In a person with achondroplasia, a gene that is involved in the process of bone formation produces too much of a protein that limits bone growth. As a result, the person with achondroplasia has unusually short bones in the arms and legs and other skeletal abnormalities. They also usually have difficulties with posture, joint disorders, and breathing problems
