Mermaid Syndrome Genetic
Mermaid syndrome develops early in foetal development when blood is diverted from cartilage, bones, muscles, nerves and blood vessels (the caudal structures) to the placenta. It's this lack of blood flow that causes the limbs to fuse, as the lower extremities are starved of oxygen and nutrients. Why this happens is still a mystery.
At the time of writing (November 2009) it is not known what causes Mermaid syndrome. Several ideas have been put forward and these include poor health of an expectant mother, exposure to large amounts of radiation that creates genetic mutations, and/or a major illness suffered by the expectant mother.
There may well be a series of genetic mutations, but if there are, they have not been identified. However, it has been established that sirenomelia can have a genetic basis in mice. Sirens have been observed when some strains have been crossed and as a consequence of mutations that boost the levels of retinoic acid. Researchers have also reported that sirenomelia has been caused by a loss of bone morphogenic protein 7 (Bmp 7) in combination with a complete loss or half dose of another Bmp protein - called twisted gastrulation (Tsg). Bone morphogenic proteins are multi-functional growth factors that play crucial roles in bone and cartilage development.
All incidences of Mermaid syndrome have occurred as isolated cases in families. It could be that each case represents a new spontaneous mutation.
There is also a possibility that sirenomelia does not have a genetic component; some doctors postulate that an environmental trigger could cause the malfunctions with the blood circulatory system that result in organs not developing correctly.
Another idea is that it could be caused by direct damage to the caudal region of the embryo by excessive distention of the neural tube.