some genes in poultry are what is called lethal genes; If an animal has 2 copies of the lethal gene they die. Some examples are the (Cr)crested gene in mallard derivitive ducks, these die in the shell, ducks with a sinle copy may or may not have a crest; the (Cr) creeper gene is lethal in 1 doses and in 1 produces the short legs as in the Japanese bantams; another is the (Et) ear tufts as in the Araucana fowl.

Some other genes can be deleterious, or can have some bad side effects/. Examples could be the (F)frizzle gene. Some birds are bred for this type of feather in their fowl intentiuoally. not only does this gene cause a abnormally twisted feather which is thought to be attractive to some, it can cause internal abnormalities also such as enlarged heart, gizzards and alimentary canals. (Rp) and (rp^2) are the rumpless genes which cause no or reduced tail vertabrae and can reduce hatchability and roachback. The (Lav)lavender or self blue gene is associated with poor feather quality. There are others but just thought I would touch on a few for fun.