An allele is one of two or more versions of the same gene or gene locus. Two alleles for each gene, one from each parent, are passed on to offspring. Homozygous refers to two of the same alleles and heterozygous refers to two different alleles. Alleles can determine the observable phenotypic traits that are observable such as eye color.
Bailey, R. (n.d.) Allele- A genetics definition. Retrieved from http://biology.about.com/od/geneticsglossary/g/alleles.htm
Genetics Home reference (n.d.). Allele. Retrieved from http://ghr.nlm.nih.gov/glossary=allele