From The School of Biomedical Sciences Wiki
A monogenic disease is caused by a defect or mutation in a single gene. They can be Dominant, Recessive or X-linked.
Dominant diseases (e.g. Huntington's) are monogenic disorders where there is damage to only one allele.
Recessive diseases such as Cystic Fibrosis, are monogenic disorders that occur due to damages in both alleles.
X-linked diseases (e.g. Duchenne Muscular Dystrophy) are caused by mutations on an X chromosome (N.B. women have XX, men have XY)
Although relatively rare, monogenic diseases affect millions of people world wide  as the condition can be passed on to subsequent generations.
- ↑ http://www.who.int/en/