From the Genetics Home Reference "Help Me Understand Genetics" Handbook http://www.ghr.nlm.nih.gov/handbook/inheritance?show=all 

What are reduced penetrance and variable expressivity?

Reduced penetrance and variable expressivity are factors that influence the effects of particular genetic changes. These factors usually affect disorders that have an autosomal dominant pattern of inheritance, although they are occasionally seen in disorders with an autosomal recessive inheritance pattern.

Reduced penetrance

Penetrance refers to the proportion of people with a particular genetic change (such as a mutation in a specific gene) who exhibit signs and symptoms of a genetic disorder. If some people with the mutation do not develop features of the disorder, the condition is said to have reduced (or incomplete) penetrance. Reduced penetrance often occurs with familial cancer syndromes. For example, many people with a mutation in the BRCA1 or BRCA2 gene will develop cancer during their lifetime, but some people will not. Doctors cannot predict which people with these mutations will develop cancer or when the tumors will develop.

Reduced penetrance probably results from a combination of genetic, environmental, and lifestyle factors, many of which are unknown. This phenomenon can make it challenging for genetics professionals to interpret a person’s family medical history and predict the risk of passing a genetic condition to future generations.

Variable expressivity

Although some genetic disorders exhibit little variation, most have signs and symptoms that differ among affected individuals. Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic condition. For example, the features of Marfan syndrome vary widely— some people have only mild symptoms (such as being tall and thin with long, slender fingers), while others also experience life-threatening complications involving the heart and blood vessels. Although the features are highly variable, most people with this disorder have a mutation in the same gene ( FBN1).

As with reduced penetrance, variable expressivity is probably caused by a combination of genetic, environmental, and lifestyle factors, most of which have not been identified. If a genetic condition has highly variable signs and symptoms, it may be challenging to diagnose.

For more information about reduced penetrance and variable expressivity:

The PHG Foundation offers an interactive tutorial on penetrance that explains the differences between reduced penetrance and variable expressivity.

A more in-depth explanation of these concepts is available from the textbook Human Molecular Genetics 2 in chapter 3.2, Complications to the Basic Pedigree Patterns.

The Merck Manual Overview of Genetics: http://www.merckmanuals.com/professional/special_subjects/general_principles_of_medical_genetics/factors_affecting_gene_expression.html 

Penetrance and expressivity: Penetrance is how often a gene is expressed. It is defined as the percentage of people who have the gene and who develop the corresponding phenotype (see Fig. 6: General Principles of Medical Genetics: Penetrance and expressivity.). A gene with incomplete (low) penetrance may not be expressed even when the trait is dominant or when it is recessive and the gene responsible for that trait is present on both chromosomes. Penetrance of the same gene may vary from person to person and may depend on a person's age. Even when an abnormal allele is not expressed (nonpenetrance), the unaffected carrier of the abnormal allele can pass it to children, who may have the clinical abnormality. In such cases, the pedigree appears to skip a generation. However, some cases of apparent nonpenetrance are due to the examiner's unfamiliarity with or inability to recognize minor manifestations of the disorder. Patients with minimal expression are sometimes considered to have a forme fruste of the disorder.

Expressivity is the extent to which a gene is expressed in one person. It can be graded as a percentage; eg, when a gene has 50% expressivity, only half the features are present or the severity is only half of what can occur with full expression. Expressivity may be influenced by the environment and by other genes, so people with the same gene may vary in phenotype. Expressivity can vary even among members of the same family.

Fig. 6
Penetrance and expressivity.   How genotype is translated into phenotype depends on penetrance and expressivity.   Penetrance refers to whether the gene is expressed or not. That is, it refers to how many people with the gene have the trait associated with the gene. Penetrance may be complete (100%) or incomplete (eg, 50% when only half the people have the trait).   Expressivity determines how much the trait affects or how many features of the trait appear in the person. Expression, which can be stated as a percentage, ranges from complete to minimal, or it may not be present. Various factors, including genetic makeup, exposure to harmful substances, other environmental influences, and age, can affect expressivity.   Both penetrance and expressivity can vary: People with the gene may or may not have the trait, and in people with the trait, how the trait is expressed can vary.