Expressivity is a term used in genetics to refer to variations in a phenotype among individuals carrying a particular genotype. It is determined by the proportion of individuals with a given genotype who also possess the associated phenotype. This differs from penetrance, which refers to the likelihood of the gene generating its associated phenotype at all. In contrast, expressivity refers to the influence of an expressed gene at the level of particular individuals. Expressivity can therefore be used to characterize qualitatively or quantitatively the extent of phenotypic variation within a particular genotype. The term is analogous to the severity of a condition in clinical medicine.
For example, the volume of blood ejected from the pumping heart with each contraction, relative to the total amount of blood contained in the heart's chamber can be quantified by echocardiography and is called the ejection fraction. If a specific genotype is associated with the development of congestive heart failure, the expressivity would be represented by the range of ejection fractions seen in patients that have that genotype. As a more qualitative example, in the hypothetical example of a "blue" gene, the gene might have an expressivity of 25% for individuals that express the "blue" gene and appear light blue, while for individuals who express the "blue" gene and appear dark blue, the expressivity might be 75%. Expressivity is measured only when there is 100% penetrance.
Variable expressivity occurs when a phenotype is expressed to a different degree among individuals with the same genotype. For example, individuals with the same allele for a gene involved in a quantitative trait like body height might have large variance (some are taller than others), making prediction of the phenotype from a particular genotype alone difficult. The expression of a phenotype may be modified by the effects of aging, other genetic loci, or environmental factors. Another example is neurofibromatosis, where patients with the same genetic mutation show different signs and symptoms of the disease.
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