First of all, achondroplasia is due to a single base change and is passed down as an autosomal dominant trait. Therefore, a person needs to only inherit one allele for the condition to occur. Often this occurs as a new mutation that occurs spontaneously. The region in the genome where this gene is located has a high rate of mutation. Therefore, two parents with no history of this disease can produce an offspring with achondroplasia and in other cases, it can be genetically transmitted from one or both parents who have the condition.
People with this condition has long trunks and short arms and legs which are disproportionate. They are small in stature.
Many of the infants born with this are stillborn or die very early on. Individuals with achondroplasia have high mortality and low fertility because many individuals will not replace themselves with offspring when they become adults.
As for the disease phenotype, much of the early cartilage in the infant doesn't harden to form bone due to the mutation they inherited in the FGFR3 gene. Genes direct the production of proteins in the body. A necessary protein needed to produce bone is overactive in a person with this mutation. This in turn interferes with the development of bone and the proper maintenance of the skeleton.
Other conditions associated with this are-- water on the brain, stenosis of spinal cord which can be very painful, difficulty in breathing, delays in motor skills and a large head in relation to the body size. There can be curvature to the spine in some individuals.
For people with this disorder, as conditions related to their spine arise, they can be tended to however, there is really no treatment for this disorder.