What are alleles of a gene?
Diploid individuals have paired chromosomes. For instance in humans there are 23 pairs of chromosomes totalling 46 chromosomes. Each pair comprehends homologous chromosomes, one chromosome from the father and another from the mother; both of them have information related to the production of the similar proteins (with the exception of the sex chromosomes, which are partially heterologous). So in the diploid individual it is said that each gene has two alleles, one in each chromosome of the homologous pair.