Penetrance

Penetrance is the proportion of individuals with a particular genotype who exhibit its associated phenotype. It may be complete or incomplete and can vary with age and environment.

Genotype–phenotype relationship: The association between an organism’s genetic makeup and its observable traits. Penetrance measures how often a particular genotype produces its associated phenotype.
Genotype: The genetic constitution of an organism, often described at one or more loci. Penetrance is calculated for individuals sharing a specified genotype.
Heritability: The proportion of variation in a trait within a population attributable to genetic differences under specified conditions. Heritability concerns population-level variation, whereas penetrance concerns outcomes for a genotype.
Expressivity: The degree or pattern in which a genotype-associated phenotype is manifested. Expressivity describes how strongly a phenotype appears; penetrance asks whether it appears.
Incomplete penetrance: A condition in which some individuals with a disease-associated genotype do not show the associated phenotype. It is the defining case in which penetrance is below 100 percent.
Phenotype: The observable characteristics of an organism, shaped by genes and environment. A phenotype is the outcome counted when estimating penetrance.
Genetic counseling: The communication of genetic information to help people understand inherited risks and choices. Counselors use penetrance to explain why carrying a variant does not guarantee disease.
Risk ratio: The ratio of the probability of an outcome in one group to that in another. A risk ratio compares groups, while penetrance is a proportion within a genotype-defined group.
Age-dependent penetrance: A pattern in which the probability of showing a genotype-associated phenotype changes with age. A genotype may appear nonpenetrant early in life but manifest later.
Allele: One of several alternative versions of a genetic sequence at a locus. A penetrance estimate usually concerns a genotype defined by particular alleles.

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