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.