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The term "allele" refers to specific forms of genes. Often, a gene in one person can differ in sequence to another person's gene. When the sequence of a gene differs noticeably, the various forms of the gene are called alleles. Allele frequency of a gene is the proportion of a population having one particular type of gene.
Each organism has a unique genome, a sequence of genes specific to that organism. Individual genes in any given genome hold the information necessary for the body to build appropriate cell products. Specific genes have specific, fixed locations on the genome.
When a geneticist looks at a human genome, he or she can locate a gene that has a specific purpose. While the sequence of a gene dictates what product the cell makes from it, the sequence does not have to be exact. This is why different people can have a gene at the same location that does different things for each person.
One example of a gene that has two important alleles is the gene for sickle cell anemia. One allele is the "normal" allele that creates healthy red blood cells and the other is an allele that makes the red blood cells an unusual sickle shape. Each person has two copies of a gene, one contributed from the father and one from the mother. When a person only has one sickle cell gene, he or she is more resistant to malarial disease. When he or she has two sickle cell genes, this results in the presence of sickle cell anemia.
In terms of the sickle cell gene, the allele frequency of the abnormal gene takes into account the amount of people in a particular population that has at least one copy of the gene. A geneticist merely has to pick a population, such as all of the people in a particular African village, and test each person for the presence of the sickle cell gene. If the village was home to 100 people, and ten people carried one copy of the gene, then the allele frequency of the gene refers to 10 out of 200 copies of the gene at that location. Geneticists write this as a simple number of 0.05 which they calculate from 10 divided by 200. Allele frequency is useful information both for genetic research and for public health interventions such as genetic testing for couples who wish to have children.