The Chicken Health Handbook by Gail Damerow
"Dark Cornish carry a similar short-leg gene that causes death at the time of hatch. Signs of "Cornish lethal" include short beaks and wings, and bulging eyes. New Hampshires carry a lethal that causes death in the twentieth and twenty-first day of hatch. Signs are crooked necks, short upper beaks, and shriveled leg muscles. The Silver Gray Dorking has a lethal that causes death in the ninth day of incubation. Embryos have short necks and beaks. A Barnvelder lethal causes "Donald Duck syndrome", in which the upper beak curls upward, the lower beak curls downward, and death occurs in the last days of incubation. Congenital tremor is a lethal gene found in a number of breeds including Ancona, Plymouth Rock, Rhode Island Red, white Leghorn, and white Wyandotte. Chicks hatch but can't control their neck muscles. When a chick tries to stand, its head falls over and the bird falls down. Unable to eat or drink, it dies soon after hatching. This is by no means a complete list of all the possible lethal genes. Among other lethals are those found in the black Minorca (short legs with extra toes), Rhode Island Red (short legs, wings, and beaks), white Leghorn (short legs and parrot-like beaks), and white Wyandotte (early embryonic death). Two common genetic factors that don't qualify as lethal genes, but that do reduce hatchability, are frizzledness and rumplessness."
International Registry of Poultry Genetic Stocks - University of Connecticut - College of Agriculture and Natural Resources
http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1028&context=saes
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It doesn't specify which lethal gene but it does say that it is an Embryonic Lethal Mutation
