What are alleles and what are their function?

Alleles are different forms of the same gene which are located on the same part of the chromosome. 

Genes are made up of information needed to produce different proteins, so alleles carry information to produce different versions of the same protein. 

We have 22 pairs of 'autosomal' or non-sex chromosomes and 1 pair of sex chromosomes (x and y). In each pair one copy comes from the mother and one from the father.

A particular allele will be in the equivalent place on each copy in the pair. So like the chromosome, we have one allele from the mother and one from the father for each gene.

The alleles may be the same (e.g. for the gene responsible for eye colour, both may be for brown eyes) - these alleles are called homozygous.

They may also be different (one allele e.g. from the mother may be for blue eyes and the other from the father for brown eyes) - these alleles are called heterozygous.