Incidence of genetic disorders

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.  A working party of the Royal College of Physicians has estimated that 2-3% of births result in babies with either congenital or genetically-determined abnormalities.

This means that approximately 13 000 births a year in the UK are so affected. Some conditions manifest themselves later in life. 5.5% of the population will have developed a genetic condition by age 25. Later in life, this figure rises to approximately 60% if we include conditions in which genetics plays some role.  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. Many comparatively common Mendelian forms of blindness, deafness and mental retardation are probably caused by a number of different genes. Late onset conditions are probably underestimated in the figures below as diagnosis at an early stage is often missed. 

Table showing approximate frequency of most common Mendelian disorders in the UK population

The genetics can become more complicated when the gene is either very rare (as hunting for genes often depends upon having a sufficiently large group of patients) or when a specific condition is only manifest when a number of genes interact with particular environmental triggers.

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