In human genetics, having two different versions of the same gene at corresponding chromosome locations is called heterozygosity.
The two versions are called alleles, and the corresponding location on a chromosome is a locus. Because humans usually inherit one chromosome of each pair from each parent, they can carry two different alleles at a locus. A person with different alleles is heterozygous at that locus.
For example, a person may inherit one allele associated with a trait from one parent and a different allele from the other. The biological effect depends on the gene, the alleles involved, and their pattern of expression. Heterozygosity does not automatically mean that either allele is dominant.
The contrasting term is homozygosity, in which the two alleles at a locus are the same. Heterozygosity is also important in population genetics because genetic variation within a population can be measured partly by how often loci contain different alleles.