Its mark of success is a reproducing population whose individuals matter only in fulfilling the evolutionary imperative. Complex diseases generally are more frequent than single-gene disorders. Elevated frequency of single-gene disorders generally resulted from genetic drift or selective advantage. Both single-gene disorders and complex diseases are characterized by multiple genetic, developmental, and environmental factors. In single-gene disorders, however, one gene has a pronounced effect in producing the phenotype in question.
Complex diseases comprise the major contributors to morbidity and mortality in developed countries.
Although most people, including most health professionals, might not immediately recognize a genetic basis for infection, there is considerable evidence that genotype can affect susceptibility to infection by some pathogens. Given that infections are a consequence of a contest between two genotypes, with opportunities for variation in both, they resemble all other diseases in potential for susceptibility and resistance. Because of the salience of one gene, single-gene disorders may be expressed regardless of the environment, whether cellular or external. Others require specific stimuli, for example, phenylalanine in PKU, or many agents for the hemolytic anemia of G6PD deficiency. In complex diseases, in contrast, expression is influenced by the products of multiple genes interacting with environmental factors throughout development, maturation, and ageing.