- complete penetrance. The allele is said to have complete penetrance if all individuals who have the disease-causing mutation have clinical symptoms of the disease.
- highly penetrant. If an allele is highly penetrant, then the trait it produces will almost always be apparent in an individual carrying the allele.
- incomplete penetrance or reduced penetrance. Penetrance is said to be reduced or incomplete when some individuals fail to express the trait, even though they carry the allele.
- low penetrance. An allele with low penetrance will only sometimes produce the symptom or trait with which it has been associated at a detectable level. In cases of low penetrance, it is difficult to distinguish environmental from genetic factors.