Alleles are different versions of the same gene or the same position in DNA. As we have two copies of each chromosome, we have two alleles of each gene, one inherited from our mother and the other from our father, except for the X and Y chromosomes in males and mitochondrial DNA.
If both alleles are identical, the person is homozygous for that gene; if they differ, the person is heterozygous. Some alleles function less effectively than the usual version or do not function at all, and may cause a health condition.