Define the terms penetrance and expressivity and explain how they affect the expression of single gene traits

Question

What will be an ideal response?

Answer 1

The terms penetrance and expressivity define two different aspects of phenotypic variation. Penetrance is the probability that a disease phenotype will be present when the disease genotype is present. For example, if all individuals carrying the allele for a dominant disorder have the mutant phenotype, the gene has 100% penetrance. If only 25% of those who carry the mutant allele show the mutant phenotype, penetrance is 25%. If the phenotype of a trait is present in less than 100% of those with the related genotype, the trait is said to show incomplete penetrance. Expressivity refers to the range of phenotypes resulting from a given genotype. For example, a genetic disorder called camptodactyly causes an unmovable, bent little finger. All dominant allele homozygotes should have a bent little finger on both hands. However, some of those affected have only one bent finger or normal fingers on both hands, but must carry the mutant allele because it can be passed on to children, each of whom may have some level of expression.