Graduation date: 2007Hatchability of the fertilized avian egg is impacted by factors which can be categorized as being either genetic or environmental in nature. An elevated incubation temperature, > 100 F reduces development time (time, days to hatch), but increases embryonic mortality and lowers the numbers of fertile eggs that will hatch. Embryonic failures, which affect hatchability are expressed at varying times in development. One such embryonic period is associated with down formation. In the mid-shaft portion of the developing feather plumule a condition referred to as clubbed down has been described. This condition was recognized in 1937, but the precise origins of this disorder remain unclear. Genetic studies conducted in this thesis have revealed that clubbed down in Coturnix quail is the expression of an autosomal recessive gene in the homozygous state whose expression is modulated by incubation at a temperature of 102 F. Results of selected matings of carriers for clubbed down suggest that the condition is expressed in embryos that are homozygous for the recessive gene. The responsible gene appears to be temperature sensitive in its expression.