The role of genes in disease: beware of simplistic interpretations!

(1) Most diseases with a strong genetic component and a Mendelian pattern of inheritance are rare or very rare disorders; (2) The most common human diseases result from complex interactions between genetic and environmental factors. They are multifactorial; (3) The simple presence of genes that are associated with a disease is not generally sufficient to determine with certainty that the signs and symptoms that characterise the disease will develop in that individual; (4) The association of a disease with a gene does not mean that the gene causes the disease.