This is the term used to describe a condition which may be caused by mutations in more than one gene. Tuberous sclerosis again provides a good example of this, the identical disease is produced by mutations in either of two unrelated genes, TSC1 on chromosome 9 or TSC2 on chromosome 16. In such cases, presumably both genes act at different points in the same biochemical or regulatory pathway. Or perhaps one provides a ligand and one a receptor. The degree to which a disease may manifest itself can be very variable and, once again, tuberous sclerosis provides a good example.
Some individuals scarcely have any symptoms at all whereas others are severely affected.
Sometimes very mild symptoms may be overlooked and then a person may be wrongly classified as non-affected. Clearly this could have profound implications for genetic counselling. This is an extreme case of a low level of expressivity Some individuals who logically ought to show symptoms because of their genotype do not. In such cases even the most careful clinical examination has revealed no symptoms and a person may be misclassified until suddenly he or she transmits the gene to a child who is then affected. See lecture 5 .