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Penetrance measures the proportion of individuals in a population who carry a specific gene and express the related trait. Therefore, penetrance is a measurement of the relationship between a genotype and phenotype: in other words, the relationship between the specific genes in an organism and the observable physical properties of the organism.

In cases of complete penetrance, all of the individuals in a population who carry a specific genotype express the corresponding phenotype. However, some genes do not show complete penetrance. In cases of incomplete penetrance, less than 100% of the individuals who bear a particular genotype express the corresponding phenotype. For accurate measurements of penetrance, it is necessary to record genotypes and phenotypes in a large population. It is important to note that penetrance only describes whether or not individuals express a trait or not. It is not used to describe individual variations in the degree of expression of a particular gene, which is known as expressivity.

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