Difference Between Penetrance and Expressivity

Main Difference – Penetrance vs Expressivity               

Although some types of inheritance in genetics are simple as autosomal dominant conditions, there can also be complicated types of inheritance. When dominant alleles are inherited, they are expected to express a particular trait or set of traits which constitute a particular condition or a syndrome. However, in reality, dominant allele affect different people in different ways. On that account, a specific genotype may exhibit phenotypic variability or a range of different phenotypes. Penetrance and expressivity are two measurements that describe the range of phenotypic expression of a particular genotype in individuals. The main difference between penetrance and expressivity is that penetrance is a quantitative measurement, describing the levels of expression of a particular phenotype, which corresponds to a dominant genotype whereas expressivity is the extent of a given genotype expressed at the phenotypic level.  

Key Areas Covered

1. What is Penetrance
      – Definition, Significance, Examples
2. What is Expressivity
      – Definition, Significance, Examples
3. What is the difference between Penetrance and Expressivity
      – Comparison of Key Differences

Key Terms: Penetrance, Expressivity, Genotype, Phenotype, Gene Expression, Dominant Allele, Phenotypic Variability, Genetic syndromes

What is Penetrance

Penetrance is the percentage of individuals with a given phenotype, who exhibit the phenotype associated with that particular genotype. In other words, it explains the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype. It is a quantitative measurement, describing the expression variations on the level of phenotype.

In some conditions, the dominant allele exhibit the dominant phenotype, following the Mendelian genetics. In this case, the percentage of penetrance is 100%. In addition, many dominant genotypes exhibit incomplete penetrance. In incomplete penetrance, the percentage of penetrance is less than 100%. The percentage of the penetrance is estimated from family history data and by DNA sequencing. The genotypes and the corresponding phenotypes should be recorded in a large population in order to obtain an accurate measurement. Individuals with zero expression of a particular gene exhibit reduced penetrance. The reduced penetrance usually occurs in familial cancer syndromes. Mutations in the BRCA1 and BRCA2 genes develop cancers during the lifetime of some people, but not in the others. The reduced penetrance may depend on a combination of genetic, environmental, and lifestyle factors.

The penetrance of a particular gene may vary from person to person and with person’s age. The factors affecting penetrance are shown in figure 1.

Figure 1: Factors Affecting Penetrance

What is Expressivity

Expressivity is the extent of a given genotype expressed at the phenotypic level. The expressivity of gene can also be expressed as a percentage. When the expressivity of a particular gene is 50%, only the half of the characters of that gene are present in that particular individual. The extent of the expression of a particular gene depends on environmental factors. Therefore, the same gene may have variable expressions among individuals. Variable expressivity shows how mild or severe the phenotype is.

Many genetic conditions are identified as a set of characteristics, occurring together. A group of recognizable characteristics, which occur due to a common cause is called a syndrome. Genetic syndromes are quite variable in their expressivity. Though the mutation of Marfan syndrome occurs in FBN1, the characteristics of the syndrome widely vary among people. Another example for expressivity is polydactyly (the occurrence of extra toes) in cats. The relationship between penetrance and expressivity is shown in figure 2.

Figure 2: Relationship between Penetrance and Expressivity

Difference Between Penetrance and Expressivity

Definition

Penetrance: Penetrance is the percentage of individuals of a given genotype, exhibiting the phenotype associated with that particular genotype.

Expressivity: Expressivity is the extent of a given genotype, expressing at the phenotypic level.

Significance

Penetrance: Penetrance explains how often the gene is expressed.

Expressivity: Expressivity explains the extent of gene expression.

Qualitative/Quantitative

Penetrance: Penetrance is a quantitative measurement.

Expressivity: Expressivity is a qualitative measurement.

In Diseases

Penetrance: Penetrance explains whether the disease shows up.

Expressivity: Expressivity explains how a disease shows up.

Type of Measurement

Penetrance: Penetrance describes the expression of a single gene.

Expressivity: Expressivity describes the expression of a group of characters or a syndrome.

Associated Phenomenon Involved in Genetic Analysis

Penetrance: Incomplete penetrance is a phenomenon that makes genetic analysis more difficult.

Expressivity: Variable expressivity is a phenomenon that makes genetic analysis more difficult.

Examples

Penetrance: The expression of BRCA1 and BRCA2 genes develop cancers in some individuals but not in others as a result of penetrance.

Expressivity: The characteristics of Marfan syndrome widely vary among individuals as a result of expressivity.

Conclusion

Penetrance and expressivity describe the relationship between a dominance genotype and its associated phenotype. Penetrance is the proportion of a particular genotype, expressing its associated phenotype. It is a quantitative measurement of the amount of the expression of a particular gene. Expressivity describes the variations in gene expression of a particular genotype. It is a qualitative measurement, which correlates with the extent of gene expression. Therefore, the main difference between penetrance and expressivity is in their parameters.

Reference:

1. Griffiths, Anthony JF. “Penetrance and expressivity.” An Introduction to Genetic Analysis. 7th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. Available here. 04 June 2017. 
2.”Phenotype Variability: Penetrance and Expressivity.” Nature News. Nature Publishing Group, n.d. Web. Available here.  04 June 2017. 
3.”What are reduced penetrance and variable expressivity? – Genetics Home Reference.” U.S. National Library of Medicine. National Institutes of Health, n.d. Web. Available here. 05 June 2017. 
4.” Genetic analysis of a single gene.” Open Genetics – CuboCube. N.p., n.d. Web. Available here. 05 June 2017. 

Image Courtesy:

1. “Factors affecting penetrance” by Genomics Education Programme (CC BY 2.0) via Flickr