Thursday, March 4, 2021

What does ‘allele’ mean?


The word allele, in the study of inheritance, means a single copy of a gene. It is pronounced uh-LEEL.

Its usage implies that there are several different alleles for each gene. The term itself is irrelevant outside the field of inheritance.

For example, four different blood types exist for humans: A, B, AB, and O. These represent three different “versions” of the gene for blood type, each of which is defined as an allele: A, B, and O. Each of us has two alleles for blood type in our genome, because we have two copies of every one of our genes. The combination of these two alleles for blood type represents our genotype.

When an egg (or ovum) from the mother is fertilized by a sperm from the father, the resulting zygote gets one allele from the mother and one allele from the father. For every gene, we have two alleles, one from our mom and one from our dad.

  • If the two alleles we have are A and A, our blood type is A.
  • If the two alleles we have are A and B, our blood type is AB.
  • If the two alleles we have are A and O, our blood type is A.
  • If the two alleles we have are B and B, our blood type is B.
  • If the two alleles we have are B and O, our blood type is B.
  • If the two alleles we have are O and O, our blood type is O.

The genotype of an individual who has an A and a B allele would be written “AB” whereas the genotype for an individual who has two B alleles would be written “BB”.

The blood type is the physical trait that results directly from the genotype. The blood type is an example of a phenotype, which simply means the physical appearance or the expressed trait that can be observed. Just remember that Phenotype is Physical.

Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.


  1. Freckles and a simpler example

    We have a gene that determines whether or not we have freckles. There are two version of the “freckles” gene: “F”, which is for having freckles, and “f”, which is for not having freckles. Would that be “freckle-less”?

    Anyway, since everybody has two copies of the gene in their DNA, we have two alleles by definition. If both alleles are “F”, our genotype is FF, and our phenotype is that we have freckles. On the other hand, if both alleles are “f”, our genotype would be written ff, and our phenotype would be that we don’t have freckles.

    Definition of genotypes

    As mentioned above, we have two alleles for each of our genes. If those two alleles are the same, like FF or ff in the freckles case, the individual is said to be homozygous for that particular trait. The prefix “homo” means “same” and the root “zygous” refers to the zygote formed when the egg is fertilized by the sperm.

    If the two alleles in the zygote are different, as in Ff for the freckles case, the individual is said to be heterozygous for that trait. The prefix “hetero” means different.

    If one allele is dominant and one is recessive for a given trait, as in our brief freckles example, we need to further specify the homozygous possibilities. Therefore, if the genotype is FF, the individual is said to be homozygous dominant; if the genotype is ff, the individual is said to be homozygous recessive.

    There is no need to specify “dominant” or “recessive” with the heterozygous genotype, since that term alone tells you exactly which alleles are present. We only need to specify “dominant” or “recessive” for the homozygous genotypes because the term “homozygous” just tells us that the two alleles are the same as each other. It doesn’t tell us which allele they are.


    In this particular case, we know that even a single allele for freckles, F, will result in the person having freckles. That is, even if we have one F allele and one f allele, we will still have freckles. It is often said that the F allele “masks” the f allele. The F allele is dominant, by definition, and the f allele is recessive.

    Let’s take a closer look at this. If there are exactly two version of the gene, and each individual has two alleles for the gene, how many different genotypes are there? Well, the first allele can be the dominant allele and the second allele can be dominant. In the case of freckles, this would be written “FF” and represents the first possible combination.

    Or, both alleles could be the recessive allele, written as “ff”. That’s two so far.

    Finally, we could have one dominant allele and one recessive allele, which would be written “Ff”. That makes three different genotypes for an individual with regard to freckles.

    (Note that the Ff genotype could also be written as fF, but by convention, when both types of allele, the dominant and the recessive, are present, we write the dominant allele first.)

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