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| Hardy-Weinberg Made Easy Learn the Principle Here Review of the Hardy-Weinberg Principle 1. What is allele frequency? Allele frequency is the percentage of appearances of an allele in the genotypes of a given population (compared to the other alleles of the studied gene). For example, in the ABO blood system there are three alleles (IA, IB and i). Considering a group of three persons, one with genotype IAi, other IAIB and other ii, the frequency of the allele IA in this “population” is 2/6, the frequency of the allele IB is 1/6 and the frequency of the allele i is 3/6. 2. What is genetic equilibrium? Genetic equilibrium is the result of the Hardy-Weinberg law, a principle that affirms that under specific conditions the frequencies of the alleles of a gene in a given population remain constant. (The Hardy-Weinberg principle is not valid in the following conditions: for populations too small, in the occurrence of noncasual (driven) crossings, for populations with many infertile members and in case of action of evolutionary factors, like natural selection, mutations and migrations.) Hardy-Weinberg - Image Diversity: Hardy-Weinberg rule 3. What is the mathematical expression of the genetic equilibrium for genes with two alleles? Is this statistical distribution the same as the statistical distribution of the respective phenotypes? Considering p the frequency of one of the alleles and q the frequency of the other allele of a given gene in a population, in this population individuals produce p gametes with the first allele for each q gamete containing the second allele. Therefore the probabilities of formation of homozygous genotype for the first allele is p2, of homozygous genotype for the second allele is q2 and of the heterozygous genotype is p.q + q.p, i.e., 2p.q. Since the sum of those probabilities necessarily is 1, the resulting mathematical expression is: p.p + 2p.q + q.q = 1. In general the number of genotypical forms is not identical to the number of phenotypical forms since there are dominance and other interactions between genes that affect the manifestation of the phenotype. 4. An hypothesis for the extinction of the dinosaurs is that the earth had been hit by a gigantic meteor that caused the death of those big reptiles. In that case the entire genetic pool of those animals has been destroyed, invalidating the Hardy-Weinberg equilibrium. In Genetics what is this type of gene frequency change called? The phenomenon in which a large number of genes is destroyed or introduced in a population is called genetic drift. When a genetic drift occurs the Hardy Weinberg principle is not applicable. Hardy Weinberg - Image Diversity: genetic drift 5. What are the penetrance and the expressivity of a gene? Individuals that carry a same genotype do not always manifest in an identical manner the correspondent phenotype. These manifestations may differ in intensity, from one individual to another, or even the phenotype may not manifest in some percentage of carriers. Gene penetrance is the percentage of phenotypical manifestation of a gene in a given population of carrier individuals (same genotype). Gene expressivity is the degree (intensity) of the phenotypical manifestation of a gene in each individual or group of individuals that carry the gene (same genotype). The gene penetrance and the gene expressiveness may be influenced by the environment. 6. Why is a balanced frequency of different alleles of a gene in a population more useful for the survival of that population facing environmental changes? For a trait conditioned by two alleles, for example, A and a, a balanced frequency between the alleles A and b is more advantageous for survival. For example, in an environmental situation in which the aa homozygous phenotype becomes incompatible with life the presence of a good number of individuals AA and Aa will result in better survival chance for the species. Another example: an environmental situation in which the dominant phenotype becomes incompatible with life; in this case the existence of heterozygous and recessive homozygous individuals in enough number may be fundamental for the survival of the species. |