Single gene defect causing multiple unrelated problems is known as:

Pleiotropism refers to the phenomenon in which a single mutant allele having widespread effects or expression in different tissues or organ systems. Eg is Marfans syndrome caused by a mutation in fibrillin gene causing multiple manifestations. Penetrance refers to the propoion of individuals with a paicular genotype that express the same phenotype. Penetrance is said to be complete when 100% of mutant individuals express the phenotype. Penetrance is incomplete when some mutant individuals do not express the phenotype. Ref: Tsaic A.C., Manchester D.K., Elias E.R. (2012). Chapter 37. Genetics & Dysmorphology. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.