We all carry genes that are potentially hazardous. Some are hidden in recessive form and we may never know that we carry them. Some will only exert their influence through interactions with environmental triggers. Others are manifest from or even before our birth.
Some conditions manifest themselves later in life.
The incidence of many Mendelian disorders varies from one ethnic group to another. The highest frequency of sickle cell anaemia is to be found in populations with a mid-African background, for example. Cystic fibrosis is most common amongst North Europeans and their descendants. The figures presented below represent UK populations in general. Furthermore, the list is restricted to conditions caused by single gene defects.