In genetics, an allele is one of any possible gene combinations that appear in the same location on a chromosome. As every individual has two sets of chromosomes, they can have identical alleles or different alleles of each gene. Their alleles may also be identical to the alleles of another individual (such as being shared with the parents, siblings or other close relatives) and may result in different characteristics such as skin color. However, having a different allele than another individual may result in no actual physical difference between the individuals.
Whether the two alleles are the same or different, only one will manifest itself in cellular development. This characteristic, known as "dominant-recessive", was first described and confirmed by Gregor Mendel in the 1860s. Recessive alleles will only manifest if both match, where dominant alleles will manifest whether there is a match or not.
Most genetic diseases can be tracked to a single allele, particularly on the X-chromosome in males. The X-Y combination is unique as there is no 1-1 correspondence between alleles between the two chromosomes, although there is some correpondence.