The below mentioned article provides study notes on Population Genetics.
Study of the frequency of genes and genotypes in a mendelian population is known as population genetics. In other words, it is a branch of genetics which deals with the frequency of genes and genotypes in mendelian populations. Before dealing with population genetics, it is essential to define mendelian population, gene frequency and genotype frequency.
There are two important features of mendelian population, viz:
(1) Random mating, and
(2) Equal survival of all genotypes.
In case of random mating, each individual of one sex has equal chance of mating with every individual of opposite sex. In other words, there is no restriction on mating of one individual with other individuals. Such inter-mating populations are also known as panmictic populations.
Random mating populations maintain high level of variability and adaptability. Random mating individuals belong to the same species and same gene pool. The gene pool refers to the sum total of genes in a mendelian population.
Gene Frequency:
Gene frequency refers to the proportion of different alleles of a gene in a random mating population. It is also known as genetic frequency. In other words, the proportion of each type of allele at a particular locus in a random mating population is referred to as gene frequency. The composition of a population is described in terms of gene frequencies.
Estimation of gene frequencies in a population consists of three important steps as given below:
1. Sampling:
First a random sample of individual is drawn from the random mating population under study. The size of sample should be adequate to represent all the individuals of a population.
2. Classification:
After sampling, the individuals are grouped into different classes for a gene and their number is counted.
3. Calculation of gene frequency:
Suppose a random sample of 100 individuals was drawn from a random mating population of four ‘O’ clock plant (Mirabilis jalapa). Out of 100 plants, 30 were with red flower, 40 with pink flower and 30 with white flower.
Now, the allele frequency will be worked out as follows:
(a) In four o’clock plant, a cross between red and white flowered strains produces pink flower in Fi and red, pink and white flowered plants in 1 : 2 : 1 ratio in F2 generation. Thus, plants with red colour are homozygous for dominant allele (RR) and individuals with white flower colour are homozygous for recessive allele (rr).
(b) Each heterozygous individual with pink colour will have dominant (R) and recessive (r) alleles in equal number.
1. Number of R alleles in the Sample (30 individuals)
= 2 (No. of red individuals) + No. of pink individuals
= (2 x 30) + 40 = 100
2. Proportion of R alleles in the sample
= Number of RR Alleles/2 (Total plants in a sample)
= 100/(2 x 100) = 100/200
= 0.50
Similarly, the number of r alleles
= (2 x30) + 40 = 100
Proportion of r alleles
= 100/(2 x 100) = 100/200
= 0.50
Therefore, the frequency of RR and rr alleles is 0.50 each.
Genotype Frequency:
It refers to the ratio of different genotypes in a mendelian population. Genotypic frequency is also known as zygotic frequency. The estimation of genotypic frequency for a gene in a population also consists of three important steps mentioned above.
Thus, the genotypic frequency of three types of individuals from the above sample will be calculated as ratio of each individual, class or genotypes to the total individuals in a sample. Thus,
1. Frequency of Red (RR) individuals = 30/100 = 0.30
2. Frequency of Pink (Rr) individuals = 40/100 = 0.40
3. Frequency of White (rr) individuals = 30/100 = 0.30
Hardy-Weinberg Law:
Foundation of population genetics was laid by G.H. Hardy, an English mathematician and W. Weinberg, a German physician in 1908. They independently discovered a principle concerned with the frequency of genes (alleles) in a population. Their principle is commonly known as Hardy-Weinberg Law.
The Hardy-Weinberg Law states that:
1. In a random mating population, the frequency of genes and genotypes remains constant generation after generation, if there is no selection, mutation, migration and random genetic drift.
2. They also developed a mathematical relationship to describe the equilibrium between alleles. According to this relationship, the frequencies of three genotypes for a single locus with two alleles (A and a) are in the ratio of P2AA : 2PqAa : q aa. where P and q are the frequencies of allele A and ‘a’ respectively. P + q are always equal to 1 or P = q = 0.50.
P + q = 1 or P= 1 -q and q = 1 – P
Effect of Random Mating:
Random mating results in maintaining the equilibrium of gene frequency in a population. For example, if the frequency of allele A is P and that of ‘a’ is q. If we make a cross between AA and aa, it will produces Aa. If individuals with Aa genotype are allowed to mate randomly, the gene frequency of three genotypes will be in the ratio of P2AA + 2PqAa +q2aa (Fig. 30.1).
When gene frequencies are in equilibrium, it indicates absence of mutation, selection, migration and genetic drift in a population.
Factors Affecting Gene Frequency:
Hardy-Weinberg principle is based on three main assumptions, viz:
(1) Random mating,
(2) Equal survival of all genotypes, and
(3) Absence of evolutionary forces like selection, mutation, migration and random genetic drift. Non fulfillment of these assumptions will lead to alteration in gene and genotype frequencies in a population.
However, the last assumption is seldom fulfilled. Mutation, migration and genetic drift change gene frequencies in a population. These factors are also known as forces of evolution because they play a key role in natural evolution.
These are briefly discussed below:
Selection:
Selection refers to a process which favours the survival and reproduction of some individuals in a population. The process of evolution in nature in which the fittest individuals survive and restore wiped out is known as natural selection. Natural selection favours those characters which are advantageous for survival.
The selection by human efforts is known as artificial selection. Such selection favours those plant characters which are useful for mankind like productivity. Before discussing the effect of various types of selection, it is necessary to give brief account of fitness and selection coefficient.
Fitness:
The relative reproductive success of different genotypes of a population in the same environment under natural selection is known as fitness or selective value or adaptive value or selective advantage. It is denoted by W. If the value of W is unity (W = 1), there is 100 per cent survival and if this value is 0 (W = 0), the genotype is completely lethal.
Survival depends on two main factors, viz:
(i) The number of seeds produced by each genotype, and
(ii) The proportion of seeds of each genotype which reaches maturity and produces offspring.
The reproductive rate of different genotypes is estimated in relation to the most fit genotype. If the reproductive rate of the most fit genotype is X] and that of other genotypes is X2 and X3 then Fitness W = X1/X1, X2/X1, X3/X1, etc. The value of W varies between 0 and 1.
Selection Coefficient:
Selection coefficient is a measure of the rate of elimination of different genotypes from a population under natural selection in a particular environment. In other words, it is the measure of the rate of reduction in the adaptive value of a genotype in relation to standard or the most favoured genotype. It is also known as selective disadvantage and is represented by S.
The greater the value of selection coefficient, lesser the survival rate and lesser the value of S greater the survival value. The value of S varies between 0 and 1. If S = 1 there is no survival at all, if S = 0 there is 100 per cent survival.
There is a close relationship between fitness (W) and selection coefficient (S) as given below:
W = 1 – S and S = 1 – W. Thus, selection coefficient is estimated with the help of fitness value or for the estimation of selection coefficient first the value of fitness (W) is estimated. Selection coefficient differs from selection differential in three ways (Table 30.1).
Thus, selection coefficient measures the rate of elimination of different genotypes from a population under natural selection, whereas the selection differential is a measure of difference between the mean phenotypic value of selected plants and mean phenotypic value of parental population under human selection.
Selection may operate at any stage of life (gametic or zygotic) cycle of an individual. Sometimes, selection acts at gametic stage which is referred to as gametic selection. Such selection acts mostly in haploid organisms and in some higher organisms. The tendency of higher organisms to exhibit differential survival rate of gametes is termed as segregation distortion or meiotic drive.
Meiotic drive is generally restricted to either male or female sex in a species. The zygotic selection operates generally in higher organisms. When certain genotypes are favoured by selection, the Hardy-Weinberg equilibrium will be disturbed. In such situation, frequency of some alleles in the population will increase while those of others will decrease.
Zygotic selection may act in three ways, viz:
(i) Against dominant phenotypes,
(ii) Against recessive phenotypes, and
(iii) In favour of heterozygotes.
(i) Selection against Dominant Phenotypes:
When selection acts against dominant phenotypes, it will eliminate both AA and Aa individuals from a population and favour only recessive phenotypes (aa). The elimination process will continue till the entire population is converted into homozygous recessive (aa) phenotypes.
Such selection, leads to fixation of recessive genes and elimination of dominant genes in a population. Since the phenotypes of both homozygous dominant (AA) and heterozygous dominant (Aa) are same, the allele A cannot be protected from elimination even in the heterozygous condition. In such situation, the value of S is 1 for AA and Aa genotypes.
(ii) Selection against Recessive Phenotypes:
Such type of selection leads to elimination of homozygous recessive phenotypes (aa) from a population. Under such type of selection, the value of coefficient of selection (S) is 1 for aa phenotypes. Such selection will lead to increase of AA and Aa genotypes in a population. However, Aa genotypes will continuously produce aa phenotypes due to segregation.
(iii) Selection in Favour of Heterozygotes:
Such type of selection leads to elimination of both dominant and recessive homozygotes (AA and aa). The value of S in such situation is 1 for AA and aa genotypes.
The excess of heterozygotes in a population is an indication of selection in favour of heterozygotes or against both the homozygotes; the frequency of homozygotes decreases sharply and the population is dominated by heterozygotes. Such heterozygotes are available in Oenothera.
Genetic Polymorphism:
The regular occurrence of several phenotypes in a genetic population is known as genetic polymorphism. The genetic polymorphism is usually maintained due to superiority of heterozygotes over both homozygotes. When polymorphism is maintained as a result of heterozygote advantage, it is known as balanced polymorphism.
Sometimes it is difficult to identify the polymorphic allelic forms by visual observations. The best way of detecting the polymorphic alleles is the isozyme studies or gel electrophoretic studies. It has been reported that two third of the loci in a population exhibit polymorphism.
Genetic polymorphism increases the adaptive value or buffering capacity of a population by providing increased diversity of genotypes in a population. Thus, genetic polymorphism enhances the adaptability of a population, because heterozygotes are more adaptable than homozygotes.
Mutation:
Mutation refers to a sudden heritable change in the features of an organism. Mutations differ from segregants in terms of their extremely low frequency. Gene mutations are ultimate sources of new alleles and thus of genetic variability.
The new mutation which we observe today would have originated long ago. Mutations lead to alteration of gene frequencies in a population. Alleles change from one form to another by way of mutation. Mutations may occur in both forward and reverse directions, but the frequency of forward mutations is much higher than reverse mutations.
When there is mutation in both the directions the equilibrium condition can be expressed as follows:
The equilibrium is attained very slowly.
Joint Effects of Mutation and Selection:
The rate of change in gene frequency will increase, if mutation and selection are in the same direction. However, if they are in opposite direction which is the usual case, a stable equilibrium may be observed. If a dominant allele arises by mutation at the rate u per generation and is opposed by selection at the rate S, the equilibrium frequency of mutant q will be as follows:
q = u/s. ifs= 1 then q = u
If s equals the selection pressure against the heterozygote and u is the mutation rate from A —>a, then equilibrium value for harmful recessive would be:
Migration:
Gene flow or migration can also change frequencies of alleles in populations. Migration includes both immigration (in coming) and emigration (outgoing) of alleles in a population. Mass immigration and emigration have tremendous potential in changing allelic frequencies in populations.
Migration generally refers to the movement of individuals into a population from a different populations. Migration may introduce new alleles into the population. These new alleles after mating with the individuals of original population may alter gene and genotype frequencies in a population.
The rate of change in gene frequency, through migration depends on the number of migrants. If the number of migrants is high, the rate of change will be rapid and vice versa. Emigration of some individuals from a population results in decrease in the frequency of alleles migrated to another population.
Genetic Drift:
Random drift or genetic drift refers to random change in gene frequency due to sampling error. Random drift is generally more in case of small sample size. Large sample size provides true representative value of a population or value which is nearer to the population mean.
Therefore, sample size should be adequate to avoid sampling error. Three forces of evolution viz., selection, mutation and migration alter gene and genotype frequency in a particular direction and are called as directional factors. However random genetic drift is a non-directional factor because it does not change the gene frequency in a particular direction.
The direction of change in the gene frequency may differ from generation to generation. In one generation, the change of gene frequency may be in one direction, which may change to opposite direction in the next generation.
Founder Effect:
Sometimes a new population is established by a single or few individuals in the main population. Such individuals are referred to as founders and effect of such individuals on the gene frequency of a population is known as founder effect. Founder effect is an important factor which sometimes results in the formation of new species.
Significance of Population Genetics:
1. Knowledge of gene and genotype frequency in a population is useful for a plant breeder in the assessment of competitive ability of various genotypes in varietal mixtures. Such studies help in identification of genotypes with high adaptive value.
If such studies are conducted over multiplications, the varietal flexibility or stability can also be assessed in varietal blends. Hardy-Weinberg Law operates in random mating or panmictic species.
2. Study of gene frequency in a population also reveals significance of various factors in natural evolution. In cross pollinated crops, development of composite and synthetic varieties is based on Hardy-Weinberg principle.