- Slides: 18
• Population genetics is the genetics of a population dealing principally with gene and genotype frequencies through time and space. • Population is a collection of individuals or data.
• It may be static or dynamic. • A static population is one that existed at a particular time only. • A dynamic population is one that existed over time and is maintained from generation to generation through procreation of new individuals, i. e. progeny.
GENETIC PROPERTIES OF POPULATION i) Gene frequency • A character in an individual that passes on from generation to generation due to breeding of the parents is controlled by genes. • The distribution of genes in different individual of the population is one of the properties of a population and is expressed as ‘Gene frequency’.
ESTIMATION OF ALLELE FREQUENCIES • The frequency of an allele is given by: 2 Ho + He p = -----------2 N Where Ho = number of homozygotes for that allele, He = number of heterozygotes for the allele and N = number individuals scored at the locus.
HARDY - WEINBERG LAW • The gene and genotype frequencies remain constant from generation to generation in the absence of mutation, migration and selection. • The population then is said to be in Hardy. Weinberg equilibrium.
DERIVATIVES OF LAW A large population that is not in equilibrium attains equilibrium after one generation of random mating, provided mutation, migration and selection are absent. The genotype frequency of the progeny is fully determined by the gene frequency of the parents in an equilibrium population and the gene frequency is obtained by taking square-root of the homozygote frequency. The frequency of the heterozygotes in an equilibrium population is never more than 50 per cent.
FORCES CHANGING GENE FREQUENCY IN POPULATION The magnitudes of change in the gene and genotype frequency are as follows. • Non-random mating: 1)Inbreeding 2)Mutation 3)Migration 4)Selection 5)Small populations 6)Genetic drift
INBREEDING • Mating among close relatives will lead to a surplus of homozygotes. • Assortative mating – If individuals prefer to mate with other individuals with similar genotypes this may lead to a surplus of homozygotes.
• Disassortative mating - If individuals prefer to mate with other individuals with different genotypes, then this may lead to a surplus of heterozygotes. • Wahlund effect – This denotes a surplus of homozygotes due to the presence of individuals that represent different populations and do not inter breed.
MUTATION • It is a rare event in nature. • It can occur in both the directions, i. e. forward and backward. • If it is recurrent, it can change the gene and genotype frequency of the population. • If mutation occurs continuously in a gene it is called recurrent mutation.
• Some mutations increase fitness or lead to the evolution of new species by creating new or improved phenotypes. • Some mutations lower fitness. • Many mutant alleles produce phenotypes that are so abnormal that they either cause death or reduce viability severely. • Other cause only small reductions in viability.
MIGRATION • The change in gene frequency due to migration depends on the preparation of individuals that enter into the population and their gene frequency. • The change in gene frequency due to migration q = m (qm-q) q- initial gene frequency of the population qm-gene frequency in the immigrants and ‘m’ is the proportion of immigrants entering into the population
SELECTION • It is defined as the non-random differential propagation of the genotypes. In other words. • Selection acts on the phenotype of the next generation and is considered to select in favor by nature. • One who contributes the highest number of progeny is called as the fittest individual. • Has highest fitness (F=1). • The change in gene frequency due to selection depends on the coefficient of selection or the degree of disadvantage of the phenotype and its frequency in the population.
SMALL POPULATIONS • In a large population the gene frequencies are inherently stable in the absence of mutation, migration and selection. • The gametes that transmit genes to the next generation carry a sample of genes in the parent generation and, if the sample is not large, the frequencies are liable to change can be predicted, the direction of change cannot be predicted. • This results in dispersion of gene frequencies and is called the dispersive process (random genetic drift).
GENETIC DRIFT • It is a phenomenon that leads to a random changes in the gene frequency in a founder population, which may not carry some alleles due to sampling error. • Genetic drift leads to loss or fixation of alleles within populations. • Genetic drift can irreversibly alter gene frequencies and eliminates alleles, which can decrease a populations ability to survive or to adapt to an altered environment, and it can preclude future selection.
CONSEQUENCES OF RANDOM GENETIC DRIFT • The random change in gene frequency found in small populations from generation to generation is called as ‘random genetic drift’. • Random drift occurring independently in different subpopulations possess significantly different gene and genotype frequencies. Uniformity within sub-populations. • Increased homozygosity