An allele is a DNA code that occupies a given position (locus) on a chromosome.

Allele
Allele, Scitable, By Nature Education, A good explanation on alleles plus links for further exploration.
http://www.nature.com/scitable/definition/allele-4...
How are different alleles created?
How are different alleles created?, Answered by Science Channel, It explains three ways different alleles are produced.
http://curiosity.discovery.com/question/how-are-di...
Properties of Alleles
Properties of Alleles, By Greg Chin, Education Portal Academy, Has a video and quiz.
http://education-portal.com/academy/lesson/dominan...