Evolution and Genetic Variation 

The majority of populations have a certain degree of variation in their genetic pools. Scientists can predict the genetic variation happening over time by measuring the amount of genetic variation in a population and these predictions assist them in gaining important insights into the processes that allow organisms to adapt to the environment or to develop into new species over generations. This process is referred to as the process of evolution. 

Natural Selection: An Evolutionary Process 

Natural selection can induce microevolution, in other words, a change in allele frequencies over a given time or generations, as fitness-increasing alleles are more common in the population over generations. Microevolution is a change in the frequency of gene variants, alleles in a population, typically occurring over a relatively short period. Population genetics is the branch of biology that focuses on the allele frequencies in populations and how they change over time. 

Figure 1. Depicts different phenotypes of frogs observed with the different phenotypes (i.e., colors) and phenotypic frequencies in a population which ultimately forms a Gene pool. 

 “Allele frequency”

Example: Allele frequency depicts the frequency of occurrence of a particular allele in a population. For instance, if all the alleles in a frog population were with green color, G, the allele frequency of G allele would be 100%, or 1.0. However, if half the alleles were G and half were purple w, Each allele would have an allele frequency of 50%, or 0.5. 

In an interbreeding population, different genes are present. The total gene of the population is referred to as the gene pool. The gene pool concept refers to the sum of all the alleles present at different loci within the genes of a population of single species. The gene pool includes the expressed as well as non-expressed genes. A gene pool consists of the total genetic diversity observed in a population of a species. The large gene pool with extensive genetic variation has a greater ability to endure the challenges posed by environmental stresses.  

A variation or heterogeneity in gene frequencies in a population is referred to as genetic variation. Briefly, a relative frequency value represents the percentage of a given phenotype, genotype, or allele within a population. The basic law of Mendelian inheritance states that some alleles are dominant while others are recessive; an organism with at least one dominant allele will exhibit the effect of the dominant allele.  

Phenotype Frequency  

Phenotype frequency is the number of individuals in a population having a specific observable trait or phenotype. Briefly, it is a ratio depicting the number of times a specific phenotype occurs in a population at a given period in a single generation 

Relative phenotype frequency can be calculated as a ratio between the total number of times a particular phenotype appears in a population and the total number of individuals present. Relative frequency can further be used to compare different phenotype frequencies. Genetic variation is dependent on the relative genotype frequency  (percentage of individuals in a population with a specific genotype) and relative allele frequency (percentage of all copies of a specific gene in a population carrying a specific allele). It represents the distribution of genetic variation in a population. Allele frequency is an exact measurement of genetic variation in a population. 

Genotype frequencies—the fraction of individuals with a given genotype and phenotype frequencies—the fraction of individuals with a given phenotype. 

The Hardy-Weinberg Equation 

Allele frequency is determined as the ratio of the frequency of appearance of an allele in a population to the total number of copies of the alleles present at a specific genetic locus. Allele frequencies can be represented as a decimal, a percentage, or a fraction. The genetic variation of a population at equilibrium is calculated using The Hardy-Weinberg equation which is a mathematical equation. The Hardy-Weinberg equation was formulated by G. H. Hardy and Wilhelm Weinberg (1908) independently. This equation was based on the basic principle of population genetics. The equation expresses the principle known as Hardy-Weinberg equilibrium, which states that the amount of genetic variation in a population will remain constant from one generation to the next in the absence of factors such as mutation, nonrandom mating, genetic drift, natural selection and gene flow. 

Hardy-Weinberg equation can be understood by considering a simple genetic locus with two alleles, M  and m. The Hardy-Weinberg equation is expressed as: 

p2 + 2pq + q2 = 1 

p- frequency of the "M" allele;  

q - frequency of the "m" allele in the population,  

p2 - frequency of the homozygous (presence of identical allele) genotype MM,  

q2 -frequency of the homozygous genotype mm, and 

2pq represents the frequency of the heterozygous (presence of different alleles) genotype Mm.  

As it would be almost impossible to keep track of all the hidden alleles, it is easier to count the number of recessive phenotypes in a population. Two recessive alleles combine and express a recessive phenotype. Therefore, q2 can be easily observed as the ratio of the total number of recessive phenotypes and the total number of individuals.  

The equation mentioned above gives a fair estimate provided the conditions of equilibrium are met properly. The phenotypic ratio relates with the allelic ratio exactly the way the Mendelian phenotype relates with the Mendelian genotype. The factors involved here must not be affected by other genetic phenomena such as retroelements and their expression must not be affected by other factors. However, under normal genetic circumstances, phenotypic ratios in a population remain stable. 

When alleles from more than one locus (position on chromosome) are considered, it results in gamete frequency. Every new gamete being considered contains an allele from each locus.  

Several rare autosomal recessive disorders demonstrate a relatively high incidence in certain populations and communities, maybe because of the high allele frequency that has resulted from a combination of effects coupled with social, religious, or geographical isolation of the relevant group, referred to as genetic isolates. A serious autosomal recessive disorder resulting in reduced fitness in affected homozygotes has a high frequency in a large population. 

Understanding RBC antigen phenotype frequencies in a population help create a donor data bank for the preparation of indigenous cell panels and for providing antigen-negative compatible blood to patients with multiple alloantibodies. The International Society of Blood Transfusion (ISBT) has reported 287 antigens within the 33 blood group systems and 42 antigens in Collections.  

Factors causing Changes in Phenotypic Frequency 

Similarly, the characterization of single nucleotide polymorphisms (SNPs) has gained considerable interest in the discovery as it facilitates the analysis of the potential interaction between human genotype and phenotype. SNP affects the phenotypic expression of a trait by influencing the activity of the promoter (gene expression), messenger RNA, conformation, and subcellular localization of mRNAs or proteins. Along with SNPs, mutations are also one of the major causes of changes in genotypic and phenotypic frequencies. They can induce changes in the structure or the quantity of DNA (deoxyribonucleic acid). Unlike SNPs and mutations, meiosis can also change genotypic and phenotypic frequency. During the processes of meiosis, homologous chromosomes from each parent pair along their lengths, the chromosomes break and rejoin, trading some of their gene, and this process of recombination results in genetic variation.  

In modern science, the term Meta-analysis has demonstrated its usefulness as a tool in genetic association studies. Background differences in the populations being meta-analyzed give rise to allelic heterogeneity and multiple low frequencies and rare associated variants in the specific functional unit of interest. 

Common Mistakes  

1. Trying to find p First- One common mistake students commonly make calculating p by observing the population and calculating the square root later. This strategy does not work in typical recessive/dominant allele relationships, as the dominant allele can hide a recessive allele. 

2. Relating Allele Frequency to Fitness- another common misconception regarding allele frequency is that it is directly related to the evolutionary fitness of a particular allele. Many recessive deleterious traits do not get expressed in population which means that it appears at really low levels and does not show its presence in the hybrids of population. 

Context & Applications 

This topic will significantly help the students preparing for professional exams having science as a minor subject and undergraduate and graduate courses. 

  • Graduate and Postgraduate science students, especially with Biology, Biotechnology. 
  • Relative frequencies are used to study populations. 
  • Meta-analysis is useful as a tool in genetic association studies. 
  • The knowledge of RBC antigen phenotype frequencies helps create a donor data bank. 
  • Negative selection 
  • Models of DNA evolution 
  • Models of nucleotide substitution 
  • Gene conversion 
  • Gene duplication 
  • Population genetics 
  • Genetic genealogy 
  • Balancing selection 

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