What is Achondroplasia Genetics?

Achondroplasia is a genetic disorder which causes disproportionate dwarfism. Literally translated, the word means “without cartilage formation.” However, this is a bit misleading. People with this disorder have normal cartilage in most of their body. The exception to this is the cartilage which grows on the ends of the long bones of the arms and legs. The bodies of people with achondroplasia are not able to convert this cartilage into bone. This makes them disproportionately short-statured.

This condition is caused by a random genetic mutation. Most babies born with achondroplasia are from average height, non-achondroplastic parents. The genetic mutation is an autosomal dominant trait, meaning that if someone has the genetic mutation they will always be found to have the disorder and are able to pass it to their children.

People affected by achondroplasia generally have average intelligence as well as an average lifespan. All have larger than average heads, big foreheads, under development of the mid-face, and a low nasal bridge with narrow nasal passages. The average height ranges from 4 feet, 1 inch (about 1.25 meters) to 4 feet, 10 inches (about 1.47 meters) and is largely determined by the natural height of the parents.

There are a few physical issues which can be seen in most people affected with achondroplasia. One of the most common issues, especially with achondroplastic children, is middle ear infections. Early detection and treatment of middle ear infections can prevent permanent hearing loss.

Another common issue among people affected with the condition is bowing of the legs caused by the way the long leg bones form. Doctors often recommend surgical straightening of bowed legs to prevent it from potentially damaging hip and knee joints.

Dental issues are also common in people with achondroplasia. These are caused by the way the bones in the mouth form. Most people with this disorder need orthodontic correction.

Because the organs of an achondroplastic person are of average size, weight issues are common later in life. This must be controlled because of the added strain extra weight can add to the hips and other joints.

Two potentially fatal physical issues of people affected by achondroplasia are foramen magnum compression and hydrocephalus. The foramen magnum is a hole at the base of the skull where the spinal cord attaches to the brain. In many people with this disorder, this hole is smaller than average and can compress the spinal cord. Also, because of the smaller foramen magnum it is possible for spinal fluid to back up into the skull which causes hydrocephalus, a condition more commonly known as “water on the brain.” Death can occur if the spine is compressed or the spinal fluid cannot flow freely.

Those with the genetic mutation have a 50% chance of having a child of their own with the disorder. If two achondroplastic dwarfs have a child together they have a 50% chance of having a dwarf baby, a 25% chance of having a baby of average height, and a 25% chance that the baby will have what is commonly referred to as “double dominance” achondroplasia. This last form is always fatal in infancy. Babies born with double dominance suffer from very small rib cages, as well as serious brain abnormalities.

Genetic testing is now readily available for this condition, since the discovery of the genetic mutation in 1994, and is always recommended when both parents have it. Testing can be conducted through either amniocentesis or chronic villas sampling.

Most adults with this disorder live and work just as those without the dwarfing condition do.