when both alleles of a gene are similar (homozygous) a person has copies mutated or both normal. if both alleles have the mutation, it causes the full disease. in malaria prone areas, normal people die frequently of malaria often before they had children. those with both copies with sickle mutation die of sickle disease before they can reproduce. but the heterozygotes have a better survival (and have more children) than both homozygous groups. thus, the inheritance of the disease is an example of 'heterozygous advantage'.