How do single-gene disorders differ from complex diseases involving many genes?

In human genetics, diseases are often categorized by the number of genes involved and the nature of their inheritance.

Single-Gene Disorders

These disorders result from defects in a single gene and typically produce discontinuous, distinct traits. They often follow Mendelian inheritance patterns, such as autosomal recessive or dominant inheritance, where the presence of a specific mutation directly dictates the phenotype. Some single-gene diseases exhibit non-Mendelian patterns, including those caused by triplet repeat expansions, mutations in mitochondrial DNA, or defects in imprinted genes. While environmental factors can influence the severity or manifestation of these conditions, the primary cause is a specific genetic alteration.123456

Complex Diseases

Complex diseases, often referred to as polygenic, are determined by the interaction of many genes, each contributing only a slight effect to the phenotype. Unlike single-gene disorders, these traits often vary continuously across a population—such as height, blood pressure, or intelligence. These conditions are significantly influenced by environmental factors, and the final result is a product of complex nature-nurture interactions. Many common disorders, including certain forms of cancer and obesity, show a familial tendency but lack a simple, formal pattern of inheritance.156