Difference Between Penetrance and Expressivity

The key difference between penetrance and expressivity is that the penetrance refers to the proportion of genotypes that actually show expected phenotypes in a population while the expressivity is the degree to which trait expression differs among individuals.

Penetrance and expressivity are two terms very commonly used in genetic analysis. Though alleles are alternative forms of a gene, they may show different expression rates since there are many factors affecting the gene expression. Though there are Mendelian principles to explain the inheritance of traits in offspring, there are some factors that cause expression of traits to deviate from the patterns as described by Mendel’s laws of inheritance. Hence, penetrance and expressivity are similar in nature yet very confusing for students. They explain why the specific genetic code of a trait or illness is not expressed. The purpose of this article is to explain the difference between penetrance and expressivity by highlighting their features.

CONTENTS

1. Overview and Key Difference
2. What is Penetrance
3. What is Expressivity
4. Similarities Between Penetrance and Expressivity
5. Side by Side Comparison – Penetrance vs Expressivity in Tabular Form
6. Summary

What is Penetrance?

Penetrance is how often a gene finds expression in a population. It expresses as a percentage of the population has the gene that develops corresponding phenotype. If there is a gene with low penetrance, it may not be expressed even when the trait is dominant. Moreover, it may not be expressed when the trait is recessive, and the gene responsible for the trait is present on both the chromosomes. Though many persons may be carrying the gene, penetrance may vary from person to person and its expression may also dependent upon a person’s age. So, if an abnormal allele is not visible in a person but he is a carrier, he can pass the allele to children who may have an abnormality.

Penetrance will be 100% if all the individuals in the population show the expected phenotype. When it is below than 100%, we call it incomplete penetrance. Incomplete penetrance is very common. Though everyone in a population carries the same allele, not all individuals are capable of showing the expected phenotype. It may be due to several reasons such as modifiers, epistatic genes, or suppressors in the rest of the genome or because of a modifying effect of the environment etc.

What is Expressivity?

Expressivity is the intensity of the expression of a gene in an individual. In simple words, expressivity refers to the extent to which a gene expresses in a single individual. It is also a percentage measurement. For example, if a gene has 75% expressivity means, the individual only shows a ¾ of the features of that trait. On the other hand, an individual showing 100% expressivity means, the correct phenotype with all the features is present in an individual.

Hence, expressivity determines how much the trait affects or the extent to which the features of a trait are apparent in an individual. Therefore, expressivity can range from zero to 100% and is dependent upon many factors such as genetic makeup, environment (exposure or intake of harmful substances), and even the age of the person.

Furthermore, down syndrome is a disease condition that describes the principle of expressivity. It’s a genetic disorder arising due to trisomy 21. Some individuals with this genotype show the disease while some others are affected only to a limited extent.

What are the Similarities Between Penetrance and Expressivity?

• Penetrance and expressivity are two terms used in genetics.
• These concepts quantify the modification of gene expression by varying environment and genetic background.
• Also, both are related to gene expressions, and they describe the range of phenotypic expression.
• Besides, both are measured in percentage values.

What is the Difference Between Penetrance and Expressivity?

Penetrance and expressivity are two measures that determine the variability of genotype and phenotype relationship. Although the genotype is present, the phenotype may not appear, or it may appear only in some individuals in a population. Also, the intensity may vary. Penetrance measures the proportion of genotypes that actually show expected phenotypes. On the other hand, expressivity measures the intensity of the phenotype expression in a single individual. Hence, this is the key difference between penetrance and expressivity.

Furthermore, another difference between penetrance and expressivity is that the penetrance describes statistical variability among a population of genotypes while the expressivity describes the individual variability. Following is a tabulation of the difference between penetrance and expressivity.

Summary – Penetrance vs Expressivity

Penetrance and expressivity are concepts that explain showing up of abnormal traits defying Mendelian principles. Penetrance is the percentage between phenotypes and genotypes of a particular trait in a population. Hence, it measures how often a particular phenotype expressed in a population despite having the genotype. On the other hand, expressivity is the percentage of the intensity of a phenotype in an individual. Penetrance is a measure of a population while the expressivity is a measure of an individual. Thus, this summarizes the difference between penetrance and expressivity.

Reference:

1. Griffiths, Anthony JF. “Penetrance and Expressivity.” Current Neurology and Neuroscience Reports., U.S. National Library of Medicine, 1 Jan. 1970. Available here  
2. Nature News, Nature Publishing Group. Available here  

Image Courtesy:

1.”13081113344″ by Genomics Education Programme (CC BY 2.0) via Flickr
2.”Micrograph (expressivity)”By Thày Bùi Phúc Trạch (CC BY-SA 4.0) via Commons Wikimedia