What is Penetrance?

Definition of Penetrance:

Penetrance can be defined as the proportion of a population of individuals that have a specific form of a gene, an allele that can be phenotypically expressed. Research on genetic penetrance is useful since it can provide some idea of the likelihood of someone developing a disease.

Causes of Penetrance:

There has been much scientific research done on possible factors that can cause penetrance to occur. There are now many scientists and researchers that now believe that epistatic genes, suppressor genes and modifier genes can all influence and cause penetrance to occur in the genome. A further possible cause of penetrance is thought to be various environmental factors.

Types of Penetrance:

The amount of penetrance of a specific allele can vary considerably. A gene can, in fact, have complete total penetrance in which case every person who has the gene expresses it phenotypically. Such alleles are dominant and are autosomal, which means they are found in the body cells (autosomes). Penetrance is given as a percentage so for instance, an allele with 50% penetrance means that it is expressed in half the people who carry it. There are quite a few alleles that do not have complete penetrance. Those genes with incomplete penetrance are not always expressed phenotypically. In terms of disease, this can mean it is difficult to exactly determine if a person will become ill. Rather a risk can be estimated based on the penetrance. A person can be carrying an allele for a type of disease but it means that they may not actually get the disease.

Examples of Penetrance:

A good example of complete penetrance is the gene that causes achondroplasia. Every person who carries the gene shows the condition. In other words, the person has a 100% chance of developing achondroplasia if they have this allele. Mutations in certain genes have been shown to be linked to an increased risk of breast cancer. In fact, the genes involved, BRCA1 and BRCA2 are considered high penetrance alleles, yet they are not examples of complete penetrance. This means that not everybody with these mutations will develop breast cancer. A further example of incomplete penetrance is evident in Huntington’s disease and in the genes COL1A1 and COL1A2 that can cause osteogenesis imperfecta.

What is Expressivity?

Definition of Expressivity:

Expressivity can be defined as the proportion of a population of individuals that do display or show a specific trait or traits to some extent. This is a non-quantitative measure of gene expression and shows variability of that expression. 

Causes:

Genetic expressivity is very common and is thought to be caused to some extent by modifier genes. Scientists also think that environmental factors may also play a role in influencing the extent to which a particular genotype is expressed.

Types of Expressivity:

Expressivity can vary a great deal and the amount of expressivity may depend on environmental factors acting in combination with genetics and lifestyle. Expressivity can be seen to vary among individuals having a specific disease. There is often great variation in the types of symptoms that different people will show. The occurrence of expressivity in human illnesses that are caused by genetic mutations is actually very high. 

Examples of Expressivity:

Marfan syndrome is an example of variable expressivity. Some people with Marfan syndrome have very severe symptoms including heart problems that require surgical intervention, while other people have very mild symptoms such as simply having long fingers and being tall. People with Marfan syndrome all have a mutation in the FBN1 gene. Neurofibromatosis is another example of expressivity in that individuals who have the genetic mutation of the neurofibromin gene can show a range of severity in symptoms. Some people will have worse symptoms than others.

Difference between Penetrance and Expressivity?

1. Definition

Penetrance is defined as the proportion of individuals in a population that have a specific allele or gene that can be expressed. Expressivity is defined as the proportion of individuals in a population that exhibit or show a trait or traits.

2. Quantitative Measure

Penetrance can be determined quantitatively as a percentage, while expressivity cannot be determined quantitatively.

3. Statistically Calculated

Penetrance is a measurement that can be statistically determined while expressivity cannot be statistically determined.

4. Causes

Penetrance can be caused by factors such as epistatic genes, modifier genes, suppressor genes, and possibly environmental factors. Expressivity is thought to be caused by the action of modifier genes and also possibly by environmental factors.

5. Types

There are two types of penetrance: complete and incomplete. Expressivity is variable and can occur to several different degrees.

6. Examples

Examples of penetrance include mutations of the breast cancer genes BRCA1 and BRCA2, achondroplasia, Huntington’s disease, and osteogenesis imperfecta. Examples of expressivity include Marfan syndrome and neurofibromatosis.

Table Comparing Penetrance and Expressivity

Summary of Penetrance Vs. Expressivity

• Penetrance is the proportion of individuals in a population that carry a specific allele that can potentially be expressed phenotypically.
• Expressivity is the proportion of individuals in a population that exhibit a trait or set of traits. 
• Penetrance can be complete as in the case of achondroplasia, but many times it is incomplete.
• Expressivity shows more of a gradation in expression when compared with penetrance.
• While penetrance is a quantitative measurement that can be used to calculate disease risk, expressivity is a qualitative measurement that is not used to assess disease risk.
• Many human genetic conditions and diseases have penetrance and expressivity, which really illustrates that gene expression is complex and involves many factors.
• Much of the cause of penetrance and expressivity is thought to be due to the influence of modifier genes and various environmental factors.

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