Opitz Trigonocephaly Syndrome is an autosomal recessive inherited syndrome, meaning that it happens on the non-sex chromosomes, and two recessive genes must be present. It is highly uncommon, though the exact numbers are not known (apparently the number is something to the line of .0000167 percent of the population, or around 17 out of every million births). Most children born with this disease do not live past their first year.
There are a dozens of defects associated with it, though the most predominant is trigonocelphaly, or a triangular-shaped head. Children with this disease also tend to have short necks, loose skin, congenital heart defects, a deeply furrowed palate, and deformations of the elbows, wrists, and fingers. Many exhibit polydactyly, or multiple fingers (well, we do too), but may have six fingers and toes on each limb, as well as angulated ears.
The strange thing about this disease is that the "aliens" in the pictures of the Roswell autopsy actually may have been human children with this syndrome. They had the trigonocephaly and most of the other symptoms (see grays, the "aliens" that beared resemblance), although looking at the pictures on the Opitz Trigonocephaly Syndrome Family Network website (http://www.geocities.com/HotSprings/Villa/1407/), the patients actually look almost completely normal as compared to normal humans.
Opitz Trigonocephaly Syndrome is also called C-Syndrome or Opitz-C, C being the first initial of the first patient diagnosed with this disease.
Trigonocephaly (triangulation of head)
Forehead narrow and pointed
Polysyndactyly (six fingers and toes)
Flexion deformity of elbows, wrists, and fingers (the limbs tend to have problems flexing)
Hypoplastic nasal root
Congenital heart defect
Prominent clitoris and labia majora
Epicanthus (fold below eye, like in Asian and some other races)
Palpebral fissures upslanted
Palate deeply furrowed
Resources: Why People Believe Weird Things by Michael Schermer (book); and the Opitz Trigonocephaly Syndrome Family Network (website)