<unk>  refers to variations of a phenotype in genetics.  The term is used to qualitatively characterize the variance or extent of the phenotype.  For example a quantitative trait - like body height - might have large variance and therefore can make prediction of the phenotype difficult.  Like penetrance in most instances<unk>  is related to an allele or a mutation or more generally a genotype.  In these instances<unk>  denotes how well the phenotype can be predicted, given the genotype.
Expressivity
Hysteresivity
Non-mendelian inheritance
Homology 
Heredity
Sexual dimorphism
Pronunciation
Dominance relationship