Alleles are variant forms of a particular gene. Each person carries two copies of each gene (one from their mother and one from their father). They may have two exact copies of a particular gene, or their two copies may vary from each other. The varied forms of a particular gene are called alleles. Sometimes alleles are called polymorphisms (many forms).

At the molecular level, alleles differ from each other in their DNA sequence. Thus, alleles may vary in the exact sequence of nucleotides, the length of the sequence of nucleotides, or the level of expression of the sequence of nucleotides. The interaction, or lack thereof, of the gene products of the different alleles will determine the phenotype of the individual with regard to that gene pair. Some alleles are dominant over other alleles, such that they determine the phenotype. An example of this is ABO blood types, whereby a person with one A allele and one O allele has type A blood. Recessive (or masked) alleles only show effects in the phenotype when no dominant allele is present. Thus, in blood types, the only way a person can have type O blood is to have two O alleles.  Some  alleles  can  interact  with  each  other to produce an intermediate or blended phenotype, or in some cases, both phenotypes are present. An example of this is AB type blood, which results when an individual carries both A and B alleles for this gene pair.

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Different alleles may have wide-ranging effects on the trait they influence. Some alleles have very small or mild effects, whereas others have large or even lethal effects on the individual. Some alleles are called mutant alleles because their expression (alone or with another mutant) results in a mutant phenotype (trait). For example, one form of dwarfism, achondroplasia, is the result of a single dominant mutant allele of the FGFR3 gene. On the other hand, cystic fibrosis results only when two recessive mutant alleles of the CFTR1 gene are present in an individual.

One particular type of allele that is receiving much attention is the single nucleotide polymorphism (SNP), in which there is a single nucleotide difference between the two alleles. Some of these changes can drastically alter the function of the gene, whereas some  have  no  effect.  There  is  a  concerted  effort by  geneticists  to  catalog  all  of  the  human  SNPs and figure out which of them have important phenotypic effects. Because many of these small differences affect how individuals respond to drugs, cataloging these particular differences may revolutionize the way that physicians treat patients. It is hoped that treatments can be tailored to the individual person with less risk for adverse side effects.


  1. Farlex, (n.d.). Allele. Retrieved from
  2. Pierce,  (2002).  Genetics:  A  conceptual  approach.  San Francisco: WH Freeman.