What is a complex disease?

• 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.