Affected infants have short arms and legs, a small chest with short ribs, and underdeveloped lungs. The spinal bones (vertebrae) in the neck and part of the pelvis (the sacrum) do not harden, or ossify, properly. The face appears flat and oval-shaped, with widely spaced eyes, a small chin, and, in some cases, an opening in the roof of the mouth called a cleft palate. The abdomen is enlarged, and excess fluid may build up in the body before birth (a condition called hydrops fetalis).

As a result of these serious health problems, infants are usually premature and stillborn or die shortly after birth from respiratory failure. Some infants have lived for a time, however, with intensive medical support. Babies who live past the newborn period are usually reclassified as having spondyloepiphyseal dysplasia congenita, a related disorder on the spectrum of abnormal bone growth.

Hypochondrogenesis is one of the most severe conditions in a spectrum of disorders caused by mutations in the "COL2A1" gene. The protein made by this gene forms type II collagen, a molecule found mostly in cartilage and in the clear gel that fills the eyeball (the vitreous). Type II collagen is essential for the normal development of bones and other connective tissues (tissues that form the body's supportive framework). Mutations in the "COL2A1" gene interfere with the assembly of type II collagen molecules, which prevents bones from developing properly.

This condition is caused by new mutations in the COL2A1 gene. Hypochondrogenesis is considered an autosomal dominant disorder because the affected gene is located on an autosome, and only one copy of the altered gene is necessary to cause the condition. The disorder is not passed on to the next generation, however, because affected individuals do not live long enough to have children.

A recent article in 2015 reported a persistent notochord in a fetus at 23 weeks of gestation. The fetus had an abnormal spine, shortened long bones and a left clubfoot. After running postmortem tests and ultrasound, the researchers believed that the fetus suffered from hypochondrogenesis. Hypochondrogenesis is caused when type II collagen is abnormally formed due to a mutation in the COL2A1 gene. Normally, the cartilaginous notochord develops into the bony vertebrae in a human body. The COL2A1 gene results in malformed type II collagen, which is essential in the transition from collagen to bone. This is the first time that researchers found a persistent notochord in a human body due to a COL2A1 mutation.

Spondyloepiphyseal dysplasia congenita is one of a spectrum of skeletal disorders caused by mutations in the "COL2A1" gene. The protein made by this gene forms type II collagen, a molecule found mostly in cartilage and in the clear gel that fills the eyeball (the vitreous). Type II collagen is essential for the normal development of bones and other connective tissues. Mutations in the "COL2A1" gene interfere with the assembly of type II collagen molecules, which prevents bones from developing properly and causes the signs and symptoms of this condition.

Spondyloepiphyseal dysplasia congenita is inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to cause the disorder.

Spondyloepiphyseal dysplasia congenita (abbreviated to SED more often than SDC) is a rare disorder of bone growth that results in dwarfism, characteristic skeletal abnormalities, and occasionally problems with vision and hearing. The name of the condition indicates that it affects the bones of the spine (spondylo-) and the ends of bones (epiphyses), and that it is present from birth (congenital). The signs and symptoms of spondyloepiphyseal dysplasia congenita are similar to, but milder than, the related skeletal disorders achondrogenesis type 2 and hypochondrogenesis. Spondyloepiphyseal dysplasia congenita is a subtype of collagenopathy, types II and XI.

Kniest Dysplasia is an autosomal dominant condition. This means that the person only needs to have one copy of the mutated gene in order to have the condition. People with a family history are at a higher risk of having the disease than people with no family history. A random mutation in the gene can cause a person with no family history to also have the condition.

Infants with achondrogenesis, type 2 have short arms and legs, a small chest with short ribs, and underdeveloped lungs. Achondrogenesis, type 2 is a subtype of collagenopathy, types II and XI. This condition is also associated with a lack of bone formation (ossification) in the spine and pelvis. Typical facial features include a prominent forehead, a small chin, and, in some cases, an opening in the roof of the mouth (a cleft palate). The abdomen is enlarged, and affected infants often have a condition called hydrops fetalis in which excess fluid builds up in the body before birth. The skull bones may be soft, but they often appear normal on X-ray images. In contrast, bones in the spine (vertebrae) and pelvis do not harden.

Achondrogenesis, type 2 and hypochondrogenesis (a similar skeletal disorder) together affect 1 in 40,000 to 60,000 births. Achondrogenesis, type 2 is one of several skeletal disorders caused by mutations in the "COL2A1" gene. This gene provides instructions for making a protein that forms type II collagen. This type of collagen is found mostly in cartilage and in the clear gel that fills the eyeball (the vitreous). It is essential for the normal development of bones and other tissues that form the body's supportive framework (connective tissues). Mutations in the "COL2A1" gene interfere with the assembly of type II collagen molecules, which prevents bones and other connective tissues from developing properly.

Achondrogenesis, type 2 is considered an autosomal dominant disorder because one copy of the altered gene in each cell is sufficient to cause the condition. The disorder is not passed on to the next generation, however, because affected individuals hardly survive past puberty.