Penetrance vs Expressivity
Penetrance and expressivity are terms that are used very commonly in genetic analysis. Not all alleles present in the genes are expressed at the same rate. There are many factors affecting gene expression. Though there are Mendelian principles to explain 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. Penetrance is how often a gene finds expression and is described as a percentage of the population having the gene that develops corresponding phenotype. Expressivity is the extent to which a gene is expressed in a single individual. It is also described in percentage for example when a gene has 75% expressivity. These two concepts namely Penetrance and expressivity are similar in nature and very confusing for students. This article will explain the differences between Penetrance and expressivity by highlighting their features.
These two concepts in genetic analysis explain why despite having genetic code for a specific trait or illness it is not expressed. As explained earlier, Penetrance is how often a gene is expressed in a population. If there is a gene with low Penetrance, it may not be expressed even when the trait is dominant. 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 depend upon person’s age. So, if an abnormal allele is not shown in a person but he is a carrier, he can pass the allele to children who may have abnormality.
Expressivity is the extent to which an abnormal trait is expressed in one person. It is gradable and hence expressed in terms of percentage. So only half of the features of a trait may be expressed in an individual or there may be an individual expressing 100% of the features of the gene. Expressivity determines how much the trait affects or the extent to which the features of a trait are apparent in an individual. 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 age of the person.
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Difference Between Penetrance and Expressivity • Penetrance and expressivity are concepts that explain showing up of abnormal traits defying Mendelian principles. • Penetrance is the frequency with which a dominant or recessive gene is expressed in a population • Expressivity is the extent to which a gene or trait is expressed in a single individual. • Down syndrome which is found in people with genotype (47, +21) describes the principle of expressivity. Some individuals with this genotype are greatly affected by the ailment while some others are affected only to a limited extent. • All beagles carry a white patch somewhere on their bodies. This can be described as 100% Penetrance.
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