what is incomplete penetrance

Incomplete penetrance is a phenomenon in genetics where some individuals carrying a particular variant of a gene do not express the associated trait, even though they carry the allele. In other words, the genotype does not always result in the expected phenotype. An allele is said to have complete penetrance if all individuals who have the disease-causing mutation have clinical symptoms of the disease. On the other hand, an allele is said to have incomplete or reduced penetrance if some individuals will not express the trait even though they carry the allele. Reduced penetrance often occurs with familial cancer syndromes. The penetrance of expression may also change in different age groups of a population. Incomplete penetrance is simply a qualitative description about a group of known genotypes.