Single gene defect causing multiple unrelated problems –
Pleiotropy occurs when a mutation in a single gene produces effects on more than one characteristic, that causes multiple mutant phenotypes.
In humans, this phenomenon is most obvious when mutations in single genes cause diseases with seemingly unrelated symptoms.

Some examples of this phenomenon are:

Transcription factor TB x 5 mutations that cause the cardiac and limb defects of Holt-Oram syndrome.
Glycosylation enzyme MP1 mutations that produce severe mental retardation and blood coagulation abnormalities of type lb congenital disorders of glycosylation.
DNA damage repairs protein NBSI mutations that lead to microcephaly, immunodeficiency and cancer predisposition in Nijmegen breakage syndrome.
Pleiotropism
Pseudodominance
Penetrance
Anticipation