Gene Frequency
The gene frequency of an allele (a form of a gene) is equal to the number of alleles in a population compared to the total number (occurrence) of alleles of the gene that gene. The term gene dose refers to the number of copies of a gene that are present in any population's genome.
The frequency of alleles and the frequency of geneotypes are related within a mathematical equation known as the Hardy-Weinberg equation. The equation itself is written as p2+2pq+q2=1.
There are several different inheritance patterns associated with genetic disorders. Autosomal dominant and autosomal recessive are two of the most common. Chromosomes come in pairs, one from the ovum and one from the sperm. Autosomal dominant disorders require that a person inherit only one disease-causing allele in order to be affected. Even though the corresponding gene on the other chromosome in the pair may be the non-disease-causing allele, having one disease-causing allele is enough to cause the disorder to be present. Autosomal recessive disorders require that a person inherit two disease-causing alleles, one on each chromosome of the pair, for the individual to be affected. If a person inherits only one disease-causing allele of a recessive disorder, they are called a carrier. Carriers are not affected by disease, however, they carry the possibility of passing that disease on to a future child.
For autosomal recessive disorders, p2 represents the people within the population that have two non-disease-causing alleles (unaffected), 2pq represents the people within the population with one disease-causing allele and one non-disease-causing allele (carriers), and q2 represents the people within the population that have two disease-causing alleles (affected). Because the Hardy-Weinberg equation deals with allele frequencies, the equation p + q = 1 may also be used. In this case, p represents the frequency of the non-disease-causing allele within the population and q represents the frequency of the disease-causing allele within the population.
The Hardy-Weinberg equation suggests that there should exist an equilibrium, or balance, between different allele frequencies. They devised a list of conditions that must be true for this balance, known as the Hardy-Weinberg equilibrium, to occur. These conditions include the provision that there are no evolutionary forces acting upon the population; that the population in question is "infinitely" large; that all individuals receive two copies of each gene, and that there is random mating within the population. In addition, the frequencies of the alleles must be the same in both males and females and that there is no generational overlap.
The Hardy-Weinberg equation has several applications including use by population geneticists to study the characteristics of certain populations and use by genetic counselors to calculate recurrence risks for individual families affected by genetic disease. Although the true Hardy-Weinberg conditions do not exist in nature, the model provides a starting point for the determination of deviations related to imbalances in the conditions required for equilibrium
This is the complete article, containing 475 words
(approx. 2 pages at 300 words per page).
