What are alleles?
Alleles are the different forms that the same gene can have. The human being has duplicate genetic material, we have received a copy from our father and another copy from our mother. Therefore, we also have duplicate genes, a maternal copy and a paternal copy. Each of these copies is an allele.
These two alleles, which are of the same gene, can be the same (same DNA sequence), and the individual is considered homozygous for that gene, or different (the DNA sequence is different between them, there are differences in their nucleotides), and the individual would be heterozygous for the gene. In all cases the alleles of the same gene carry information for the same protein or regulation, but the result may be different. Some alleles carry non-productive information, not being able to encode their protein or being more inefficient than the normal form, and causing disease.