Hypochondrogenesis is an autosomal dominant disease, usually occurring sporadically (in rarer cases familial). Usually, causative mutations are detected in the COL2A1 gene which lead to a deficiency of type II collagen. Newborns with hypochondrogenesis are usually characterized by a short upper body and short extremities. The head is often over-proportionally large and has an oval shape; the face is flat with widely-spaced eyes. Distinguishing hypochondrogenesis from achondrogenesis is difficult, since it is most likely the same syndrome with a different degree of severity. Along with achondrogenesis, spondyloepiphyseal dysplasia, Kniest syndrome and Stickler syndrome, these various types of connective tissue disorders are also termed type II collagen disorders. Collagen type II is an important component of bones, cartilage and connective tissue; disorders in the synthesis or in the integration of collagen II usually result in skeletal dysplasia.
The molecular cause of all the above-mentioned diseases is mutations in the COL2A1 gene. Although it is currently difficult to establish a clear genotype-phenotype correlation, hypochondrogenesis is mainly caused by glycine substitutions in the triple-helix domain.