factor affecting H.W equilibrium..pptx

ppt about factor affecting gene equilibrium

1. By: Syeda Laraib Zahra Roll # 1238(563480) FACTORS AFFECTING GENE EQUILIBRIUM

2. FACTORS AFFECTING GENE EQUILIBRIUM Some of the major factors which affect the genetic equilibrium and induce the variability in population are as follows: 1.Mutations 2.Recombinations during Sexual Reproduction 3.Genetic Drift 4.Gene Migration (Gene Flow) 5.Natural Selection According to Hardy-Weinberg Equilibrium law, the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when : (i) The population is large enough so that accident of sampling may be ignored. (ii) Mating takes place at random. (iii) Mutation does not take place or if it does, the rate is same in both directions.

3. (iv) All the members of the population survive and have equal reproduction rates. Factors affecting the Hardy-Weinberg equilibrium : There are five factors which affect the genetic equilibrium and induce the variability in the population. These factors are called evolutionary agents. 1.Mutation: “ These are sudden, large and inheritable changes in the genetic material .” These are characterized by : (1)Mutations are random (indiscriminate) and occur in all directions . (2) Most mutations are harmful or neutral. It is estimated that only one out of 1,000 mutations is useful. (3) Rate of mutation is very low, i.e. one per million or one per several million genie loci. But rate of mutation is sufficient to produce considerable genetic variability.

4. (4) Certain mutations are preadaptive and appear even without exposure to a specific environment. These express and become advantageous only when after exposure to new environment which only selects the preadaptive mutations that occurred earlier. Existence of preadaptive mutations in Escherichia coli was experimentally demonstrated by Esther Lederberg (1952) in replica plating experiment (Explained in Neo-Darwinism). On the basis of amount of genetic material involved, mutations are of three types:

5. Three types of mutation Gene mutations (change in sequence of nucleotide in DNA) 3 types 1.Deletion 2.Addition or insertion 3.Subsitution or replacement (a)Transition (b) Transversion (c) Tautomerism Chromosomal aberrations (change in chromosomes structure) 4 types 1.Deletion (a)Terminal (b)Intercalary 2.Duplication (a)Tandem (b)Reverse 3.Translocation (a)Simple (b)Reciprocal 4.Inversion (a) Paracentric (b) Pericentric Chromosomal numerical mutation ( change in number of chromosomes) Aneuploidy (Gain or loss of one or two chromosomes) 1.Hypoploidy (a) Monosomy (b) Nullisomy 2.Hyperploidy (a)Trisomy (b) Tetrasomy Euploidy ( Gain or loss of one or more genome) 1.Haploidy 2.Polyploidy (a)Auto (b) Allo

6. On the basis of their origin, mutations are of two types : Differences between Spontaneous and Induced mutations Characters Spontaneous mutation Induced mutation 1.Caused by 2.Frequency of mutations 3 . Causes By natural agents,so also called natural mutations or background mutations . Very low (about one per million genes or even more ). Not certain, many cellular products e.g. formaldehyde, nitrous acid, peroxides, etc. act as mutagens. By man Faster Certain physical {e.g. radiations temperature, etc.) and chemical agents called mutagens

7. Significance of mutations: 1.Mutations create and maintain variations within a population . 2.These also introduce new genes and alleles in a gene pool

8. 2 . Recombinations during Sexual Reproduction : “Recombination involves reshuffling of genes of chromosomes. Chances of recombination are more in those organisms which undergo sexual reproduction which involves gametogenesis followed by fertilization” Sexual reproduction involves recombinations during three stages : 1.Crossing over 2.By independent assortment of chromosomes 3.By random fertilization Crossing Over: “It involves the exchange of genetic material between the non-sister chromatids of homologous chromosomes” Mechanism of crossing over involves following stages:

9. (a)Synapsis involves pairing of homologous chromosomes during zygotene stage of Prophase 1 of meiosis to form bivalents . ( b) Tetrad formation as each bivalent is formed of four chromatids during pachytene stage of meiosis I . (c) Chiasma formation due to coiling of non-sister chromatids of homologous chromosomes to form X-shaped points, called chiasma . (d) Crossing over involves exchange of genetic material . (ii) By independent assortment of chromosomes : During metaphase-I, the bivalents arrange at the equator of the spindle in two equatorial or metaphase plates. During anaphase I, homologous chromosomes move towards the opposite poles. This is called disjunction and results in reduction of chromosome number. Variations occur during the chance arrangement of bivalents during metaphase I of meiosis. Number of recombination’s produced depends upon the number of bivalents in the organism and is given by the formula 2n (where n represents the number of bivalents) e.g. in human being, number of possible combinations of the gametes will be 8.6 x 106(223).

10. (iii) By random fertilization : It involves the chance fusion of gametes e.g., in human being, number of possible types of zygote formed is 70 x 1012. It is so as any sperm with any combination of genes can fuse with any ovum with any combination of genes. Significance: Due to recombination’s, though only reshuffling of already existing characters takes place and no new genes are produced but it leads to redistribution of different traits to different individuals of a population. Different combinations bring diversity in genotype and phenotype of different organisms. So recombination is an agent of evolution. 3.Genetic Drift : “It is the random change in the frequency of alleles occurring by chance fluctuations” It is characterized by : (i) It is a binomial sampling error of the gene pool, i.e. that alleles which form the gene pool of the next generation are a sample of the alleles of present population.

11. (ii) Genetic drift always influences frequencies of alleles and is inversely proportional to the size of population. So genetic drift is most important in very small populations in which there are increased chances of inbreeding which increases the frequency of individuals homozygous for recessive alleles, many of which maybe deleterious . (iii) Genetic drift occurs when a small group separates from a larger population and may not have all the alleles or may differ from the parental population in the frequencies of certain genes. This explains for the difference between island populations and mainland population . (iv) In a small population, a chance event (e.g. snow storm) may increase the frequency of a character having little adaptive value. (v) Genetic drift can also operate through founder effect. In this, genetic drift can cause dramatic changes in the allele frequencies in a population derived from small groups of colonisers , called founders, to a new habitat. These founders do not have all of the alleles found in their source population. These founders become quickly different from the parental population and may form a new species, e.g. evolution of Darwin finches on Galapagos Islands which were probably derived from a few initial founders.

12. Population Bottleneck It is reduction in allele frequencies caused by drastic reduction in population size called population crash e.g. decrease in cheetah population in Africa due to over-hunting. As the given gene pool is limited, population bottleneck often prevents the species to reestablish its former richness so new population has a much restricted gene pool than the larger parent population.

13. Founder Effect : A founder effect, as related to genetics, refers to the reduction in genomic variability that occurs when a small group of individuals becomes separated from a larger population. Over time, the resulting new subpopulation will have genotypes and physical traits resembling the initial small, separated group, and these may be very different from the original larger population. A founder effect can also explain why certain inherited diseases are found more frequently in some limited population groups. In some cases, a founder effect can play a role in the emergence of new species.

14. 4.Gene Migration (Gene Flow): Most populations are only partially isolated from other populations of same species. Usually some migration-emigration (moving out of some individuals out of a population) or immigration (entry of some members of a population into another population of same species ) occurs between the populations . This addition or removal of alleles when individuals enter or leave a population from another locality is called gene flow. 5.Natural Selection : Definition: “ Process through which populations of living organisms adapt and change”.Individual in population are naturally variable,meaning that they are all different in some ways.This variation means that some organisms have traits better suited to environment than others. Example: A classical example of natural selection at work is the origion of giraffes’long necks.The ancestors of natural giraffes were animals similar to deer with neck of ordinary length.

15. References http://www.biologydicussion.com A book of Genetics-I by P.S Verma & V.K Agarwal from page number 975-992 .

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