An allele is an alternative form of a gene that is located at a specific position on a specific chromosome. Alleles contain the DNA coding which determines the traits that can be passed on from parents to offspring Organisms have two alleles for each trait. When the alleles of a pair are heterozygous one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Therefore, heterozygous individuals have two different alleles for a single trait while homozygous individuals have two identical alleles for a single trait.