ABSTRACT
This chapter discusses the evolutionary genetics of human disease. The line between normal variation and disease can be blurred, and the precise boundary is often defined differently depending on the environment, society, and the ease of access to medical care. The chapter discusses simple human genetic diseases that are inherited in a Mendelian manner. For all of the simple genetic diseases discussed, other modifier genes, as well as the environment, affect their severity and nature. Simple genetic diseases are those where individuals carrying one disease allele (for dominant diseases) or two alleles (for recessive diseases) generally manifest the disease phenotype, despite variation due to stochasticity in the development of the phenotype, environmental effects, and the effects of other genes. Complex genetic diseases are those where a single disease allele (heterozygous or homozygous) does not cause the disease, and can be common in the population.
