Faded shaker, a lethal pigment and neurological mutation in the chicken.

Faded shaker (fs) is a lethal condition in chickens resulting in a congenital tremor and a dilution of down and feather melanin. It is inherited as an autosomal recessive gene with the homozygote showing incomplete penetrance, the apparent result of a single dominant gene masking the effects of the fs/fs genotype. Mortality of the mutants occurred between 18 days of incubation and three months of age; most died before one week of age with half of the mutants dying before hatching. No specific cause of death was observed. The tremor had a frequency of 6 to 10 vibrations per second, was more pronounced in the posterior end of the bird and was constant over time within an individual. Weight of the faded shaker cerebellum was reduced by 7.6%, presumably due to an observed deficiency of myelin. The down and feather color of faded shakers ranged from almost normal to near white, but was constant over several feather generations within an individual. Faded shaker melanosomes were both deficient in number and incompletely melanized. Melanocyte dendrition and melanosome distribution were both normal. Skin transplants showed that the dilution was a result of a defect located in the fs/fs skin. Pigmentation of the retinal pigment epithelium also was abnormal.