Penetrance

Penetrance is the conditional probablity that P(xg) of being affected with disease x given a specific genotype g.

Penetrance is different than "Variable Expressivity" which means variations in the the degrees of manifestation of a disease.

Anticipation is a special case of variable expressivity which is that the severity of the expression becomes stronger and manifests earlier in successive generations

Age-dependent Penetrance is a phenomenon in which the penetrance depneds on the age of the individual, that is, not all individuals carrying the disease get affected but the older they are the, the higher the probability that they will be affected. Age dependent penetrance is important in genetic counselling in which not only it is important to know the probability of affecting a disease but also the probability of when will someone be affected by the disease. Age-dependent penetrance is also important in differentiating the etiologies of a disease (at least potentially).

Genetic Marker

A Genetic Marker is a special DNA locus with at least one base being different between at least two individuals. In principle, only the features of being detectable and having a known location in the human genome are required for a locus to serve as a marker.