What is a complex disease?

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Many common diseases are influenced by a combination of multiple genes and environmental factors. These diseases are referred to as complex diseases.

  • Many of the most common diseases run in families and are caused by a combination of genetic, environmental and lifestyle factors.
  • These diseases are called complex diseases.
  • Although complex diseases tend to run in families, unlike single gene disorders, they do not have clear-cut patterns of inheritance so it is difficult to predict a person’s risk of inheriting or passing on these diseases.
  • Because they can be caused by both genetic and environmental factors, complex diseases can be difficult to treat.
  • Complex diseases include asthma, diabetes, epilepsy, hypertension, manic depression and schizophrenia.
  • Some developmental abnormalities are also included, such as cleft lip and congenital heart defects.
  • It is thought that the incidence of any complex disease is dependent on a balance of risks, too many negative genetic and environmental factors and the balance can be tipped towards disease.


Single nucleotide polymorphisms


  • Single nucleotide polymorphisms, or SNPs, are single letter changes in the DNA code.
  • SNPs are the most common type of genetic variation.
  • SNPs occur throughout the genome, on average there is an SNP every 300 bases.
  • Many genetic changes associated with complex diseases have been identified by looking to see if there are SNPs that occur more or less commonly in people with a disease, than people without the disease. This type of study is called a genome-wide association study or GWAS.
  • Identifying SNPs associated with particular diseases will enable scientists to predict an individual’s likelihood of developing a disease and how the disease runs in families.
  • These ‘risk SNPs’ can also help scientists to identify biological pathways underlying these diseases which can help with the development of treatments.

What about conditions that are caused by single genes? Find out more about single gene disorders