Achondroplasia (left) versus normal growth in the six-year-old child
Achondroplasia can be evident at birth, but it is not always obvious to the layperson. As the child grows, however, the distinct shortness and mild hydrocephaly is clearly visible, especially when next to their full-sized peers.
The genetic anomaly that causes this disorder prevents the fibroblasts that create cartilage from forming correctly, and without the continuous growth and ossification of cartilage at the ends of the long bones, the limbs grow very little. The fact that the mutation only causes anomalous cartilage is also why the torsos of people with achondroplasia are typically normal-length - the spinal column doesn’t lengthen by way of distal extension, and since other ossification factors are normal in the body, it isn’t affected.
While most cases of achondroplasia are sporadic mutations (around 75%), it can also be inherited as an autosomal dominant disorder from an achondroplasic parent. With an achondroplasic parent and a parent of normal height (or who doesn’t have a mutated FGFR3 gene), the chances of a child being born with the condition are 1 in 4. With two achondroplasic parents, or an achondroplasic and a hypochondroplasic parent (caused by different mutations, but on the same gene), the chances of a child being born with some form of dwarfism rises to 3 in 4 - though one of those three will inherit two copies of the disordered gene, and with no functional allele, will inevitably be stillborn or die early in life due to almost completely absent skeletal integrity.
Diseases of Children. Herman B. Sheffield, 1921.