What is "expressivity"

n. 1 (context uncountable English) The quality of being expressive. 2 (context genetics English) The degree to which a gene affects or can affect an organism.

In genetics, expressivity quantifies variation in a non-binary phenotype across individuals carrying a particular genotype. It is equal to the proportion of individuals carriers of a genotype for a trait who show the trait to a specifiable extent (as in the "shades of blue" example below). This differs from penetrance, the term that measures how often a gene generates its associated phenotype to any extent that makes an individual carrier different from the wild type. Expressivity therefore characterizes non-binary qualitatively or quantitatively the extent of phenotypic variation within a particular genotype. With binary phenotypes (e.g., albino vs. wild type) expressivity and penetrance are the same. 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.

Expressivity, expressiveness, expressive power, etc., may refer to:

• A degree of an expression or a quality of being expressive
• Expressivity (genetics), variations in a phenotype among individuals carrying a particular genotype
• Expressive power (computer science) of a programming language
• Expressive potential (electronic music), a control in a musical device for tuning musical expression