"Balanced polymorphism is a situation in which two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone. The evolutionary process that maintains the two versions over time is called balancing selection."
"Most plants and animals have at least two copies of each chromosome, one inherited from each parent. The copies of the genes found on these homologous chromosomes may be identical or different; that is, the organism may carry two copies of one allele, or one each of two different alleles. In the first case, the organism is called homozygous for that gene, and, in the second, it is called heterozygous."
The significance of balanced polymorphism can be seen with sickle cell anemia. People who carry two copies of the sickle-cell allele suffer horribly and without medical intervention will die young. People who carry one copy of of the sickle-cell allele and one wild-type allele develop an immunity to malaria. The two allele balance each other off and create a benefit for the person.