The definition of an allele is available which is a variant form of a given gene. However, I wonder what is the threshold to make a mutation/ a variant/ a polymorphism become an allele? I mean a mutation/ a variant/ a polymorphism can truly make some changes in the protein- the products of a given gene, thus there would be so many alleles for a given gene. However, people only note about 2 allele, the major and the minor. When I asked my teacher, he said because the frequency of those are the most, but I wonder what make the frequency of the major and the minor alleles are so different from the other allele? (Is it the natural selection?). Thank you