Abstract and Keywords
Genetic influences on human disease can be understood in the context of essential and exacerbating causes. Alleles are essential causes when they are required for these diseases to occur. Severe diseases with essential genetic causes are rare unless they provide a compensating benefit in evolutionary fitness (accrued through increases in survival and/or reproduction) that offsets their fitness costs. Duchenne’s muscular dystrophy illustrates that upper limit of incidence for diseases that kill before reproduction and have essential genetic causes that provide no compensating benefit: about one death per 10,000 births. Sickle cell anaemia exceeds this limit in many populations because the causal allele confers a compensating benefit: protection against falciparum malaria. Alleles that contribute to common, severe diseases such as atherosclerosis and Alzheimer’s disease tend to be exacerbating causes, which act by increasing vulnerability to environmental causes. Inferring genetic causation from familial patterns is not straight-forward because genetic associations may be correlated with environmental exposures. Relatively high monozygotic twin cooncordances for schizophrenia, for example, are associated with in utero exposure to the parasite Toxoplasma gondii, which appears to cause a substantial portion of schizophrenia. Epigenetic contributions to disease involve relatively long-term but reversible modification of genetic influences that may occur in response to micro- or macroenvironmental exposures. Integration of these aspects of disease causation can be of practical use in the health sciences by clarifying interventions that can prevent, cure, or strongly ameliorate disease.
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