Missing upper beak: a new lethal mutation in domestic fowl.

A new condition affecting the facial structure of domestic fowl is described. It is controlled by an autosomal recessive gene having complete penetrance. The mutation has been named missing upper beak and the gene symbol mub is proposed. The condition is an obligate lethal. Mortality occurs on the twelfth day of incubation or during the hatching process. One chick hatched unaided and five were assisted from the shell; none of these survived beyond 11 days of age. They had partial vision and hearing, they could vocalize and had normal balance, but they were unable to eat. Major phenotypic effects of the mutation are absence of the upper beak and absence of eyelids. All or part of the upper eyelid is always missing; the lower eyelid and nictitating membrane may be present or absent. Premaxilla and nasal bones are greatly reduced causing the upper beak to be rudimentary. Other membranous bones of the head are reduced causing the face to be shortened. Cartilaginous bones appear not to be affected. It is postulated that the condition results from a defect in membrane formation during embryonic development.