Locus Heterogeneity
One disease phenotype caused by mutations in distinctly different genes [makes it difficult to determine the causative gene] [Ex: there are different causes for elevated phenylalanine]
Allelic Heterogeneity
Many loci contain multiple mutant alleles in a population - mutations in a gene can be ordered along a continuum of severity [Ex: different alleles of the PAH gene caused by different mutations in the PAH gene result in varying severity of PKU disease]
Heterogenic (or clinical) Heritability
Different mutations in the same gene causing completely different diseases
Penetrance
probability that a mutant gene will have any phenotypic expression [for our cases assume 100% penetrance]
Expressivity
Severity of expression of the phenotype among individuals with the same disease causing genotype (variable expressivity - the severity of the disease differs in people who have the same genotype)
