Who does achondroplasia affect?

It's a genetic condition that in general, will occur in half of the children of those who have achondroplasia.

What is the cause of achondroplasia?
Achondroplasia is caused by a mutation in a gene (called fibroblast growth factor receptor 3) that is located on chromosome 4 (6, 7). In a minority of cases, a child inherits achondroplasia from a parent who also has the condition. If one parent has the condition and the other does not, there is a 50 percent chance that their child will be affected. If both parents have achondroplasia, there is a 50 percent chance that the child will inherit the condition, a 25 percent chance that the child will not have it, and a 25 percent chance that the child will inherit one abnormal gene from each parent and have severe skeletal abnormalities that lead to early death. (When both expectant parents have achondroplasia, they are usually offered prenatal tests to diagnose or rule out this fatal form of the disease.) A child who does not inherit the condition cannot pass it on to his or her own children.

In more than 80 percent of cases, however, achondroplasia is not inherited but results from a new mutation that occurs in the egg or sperm cell that forms the embryo(4). The parents of children with achondroplasia resulting from a new mutation are usually normal sized. Typically, these parents have no other children with achondroplasia, and the chances of their having a second affected child are extremely small.

Geneticists have observed that older-than-average fathers (age 40 and older) are more likely to have children with achondroplasia and certain other autosomal dominant conditions (disorders that occur when one gene in a gene pair is abnormal) caused by new mutations. Individuals with achondroplasia resulting from new mutations transmit the disorder to their children as described above.

Can achondroplasia be prevented?
There is no way to prevent the majority of cases of achondroplasia because they result from totally unexpected gene mutations in unaffected parents. Genetic counseling can help adults with achondroplasia and unaffected individuals who have had an affected child make informed decisions about family planning.

its a defect in the formation of cartillage.