A monogenic disease is caused by a defect or mutation in a single gene. They can be Dominant, Recessive or X-linked.
X-linked diseases (e.g. Duchenne Muscular Dystrophy) are caused by mutations on an X chromosome (NB 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.
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