Alleles are different forms of genes that can vary in sequence between individuals. The allele frequency of a gene is the proportion of a population that has a particular type of gene. Different people can have a gene in the same location that does different things. The sickle cell gene has two important alleles, one normal and one that causes sickle cell anemia. The allele frequency of the abnormal gene can be calculated by testing a population for the presence of the gene. This information is useful for genetic research and public health interventions.
The term “allele” refers to specific forms of genes. Often, a gene in one person may differ in sequence from another person’s gene. When the sequence of a gene differs significantly, the various forms of the gene are called alleles. The allele frequency of a gene is the proportion of a population that has a particular type of gene.
Every organism has a unique genome, a sequence of genes specific to that organism. The individual genes in a given genome contain the information the body needs to build appropriate cellular products. Specific genes have specific, fixed locations on the genome.
When a geneticist looks at a human genome, he or she can pinpoint a gene that has a specific purpose. While the sequence of a gene determines what product the cell makes from it, the sequence need not be exact. This is why different people can have a gene in the same location that does different things for each person.
An example of a gene that has two important alleles is the gene for sickle cell disease. One allele is the “normal” allele that creates healthy red blood cells and the other is an allele that makes red blood cells an unusual sickle shape. Each person has two copies of a gene, one contributed by the father and one by the mother. When a person has only one sickle cell gene, he is more resistant to the 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 who have at least one copy of the gene. A geneticist simply has to choose a population, such as all 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, the allele frequency of the gene refers to 10 out of 200 copies of the gene at that location. Geneticists write it as a simple number of 0.05 which they calculate from 10 divided by 200. The allele frequency is useful information both for genetic research and for public health interventions such as genetic testing for couples wishing to have children.
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