When the underlying cause of a phenotype in a family member is different from the cause of the same phenotype in other family members.
Use in clinical context
Most phenotypes have several factors that contribute to them, including multiple genomic and environmental factors. This can lead to family members having the same disease but with different underlying causes. For example, a woman who develops breast cancer but does not have the BRCA1 variant found in other affected family members. In this case, the woman without the BRCA1 variant has a phenocopy of the familial breast cancer.