"The ability of a gene to express itself uniformly in all the individual with same genotype" is defined as

"The ability of a gene to express itself uniformly in all the individual with same genotype" is defined as Expressivity.
Expressivity is the degree to which trait expression differs among individuals.
As with reduced penetrance, variable expressivity is probably caused by a combination of genetic, environmental, and lifestyle factors, most of which have not been identified.