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*Population

A group of individuals of the same species occupying a given area that can freely interbreed and produce fertile offspring in nature.

*Population genetics emphasizes the extensive genetic variation within population and recognizes the importance of quantitative characters.Population genetics is concerned with determining the relative properties of the various genotypes present in a population (genotype frequency), from which can be calculated the relative proportions of alleles in the population (allele frequency).

*There are 3 different types of population;Natural populationUsually involves evolution. The composition of the gene pool changes in the gene frequency.Controlled populationPopulation set up in the laboratory by selective breeding under experimental condition.Mathematical populationHypothetical population based on the assumption that the population shows genetic equilibrium, that is frequencies of genes are constant from generation to generation.

*TERMSGene poolThe total aggregate of genes in a population at any one time is called the populations gene pool.

AllelesEach kind of gene in the pool usually exists in two or more slightly different molecular forms called allelesThe gene pool consists of all alleles at all gene loci in all individuals of the population.For a diploid species, each locus is represented twice in the genome of an individual who may be either homozygous or heterozygous for those homologous loci.Homozygous individuals have two identical alleles for a given character, whereas heterozygous individuals have two different alleles for that character.Different combinations of alleles leads to variations in phenotype.

*TERMSAlleles

Each kind of gene in the pool usually exists in two or more slightly different molecular forms called alleles

The gene pool consists of all alleles at all gene loci in all individuals of the population.

For a diploid species, each locus is represented twice in the genome of an individual who may be either homozygous or heterozygous for those homologous loci.

Homozygous individuals have two identical alleles for a given character, whereas heterozygous individuals have two different alleles for that character.

Different combinations of alleles leads to variations in phenotype.

*TERMPhenotypes

Phenotypes are the outcome of the genes expression.

A pool of genetic resources is shared by all members of a population and passed on to the next generation.

A deviation from the stability of a gene pool results in evolution, so individuals do not evolve but populations do.

*The Hardy-Weinberg equilibriumIn 1908, G. H. Hardy (an English mathemathician) and W. Weinberg (a German physician) independently identified a mathematical relationship between alleles and genotypes in populations. This relationship has been called the Hardy-Weinberg equilibrium and it concerns allele frequency

*Allele frequencyAllele frequency is the percentage of organisms in a population carrying a particular allele.

The relationship states that : The frequency of alleles and genotypes in a population will remain constant from generation to generation provided certain conditions are met or in other words it means that the population is in genetic equilibrium.

If a population is not evolving, it is in genetic equilibrium and the allele frequency do not change.

When a population evolves, the allele frequency in the population will change

*Hardy-Weinberg Law For a population , it must satisfy five main conditions:

very large population size so that genetic drift can be avoided (chance fluctuation in the gene that can cause phenotype frequencies to change over time).

no migration that cause gene flow due to immigration into or emigration out from the population

no net mutations because by changing one allele into another, mutation alter the gene pool

random mating because if individuals pick mates with certain genotypes, the random mixing of gametes required for Hardy-Weinberg equilibrium does not occur

all genotypes are equally fertile so that no natural selection is taking place

*Hardy-Weinberg Equation. Genotype frequencythe ratio of number of individuals with certain genotype in a population.

Phenotype frequencythe ratio of number of individuals that have certain phenotype in a population

For a gene locus of diploid species where only two alleles occur in a population, population geneticists usep - the frequency of the dominant allele q - the frequency of the recessive allele

*Example 1 A wildflower population with two varieties contrasting in flower colour. An allele for pink flowers which will be symbolize by A are completely dominant to an allele for white flowers, symbolized by a.

If the frequency of allele A is 0.8 or 80%, the frequency of allele a must be 0.2 or 20%. Note that p + q = 1; the combined frequencies of all possible alleles must account for 100% of the genes for that locus in the population.

*If p + q = 1, then p = 1 - q or q = 1 - p

When gametes combine their alleles to form zygotes, the probability of generating an AA genotype or individuals of homozygous dominant is p2 and for aa genotype or individuals of homozygous recessive is q2.

There are two ways in which an Aa genotype can arise, depending on which parent contributes the dominant allele. Therefore, the frequency of heterozygous individuals in the population is 2pq.

*If we have calculated the frequencies of all possible genotypes correctly, they should add up to 1:p2 + 2pq + q2 = 1frequency frequency of frequency of AA Aa + aAof aa

In mathematical terms p + q = 1 is the mathematical equation of probability and p2 + 2pq + q2 = 1 is the binomial expansion of that equation, in this case:p + q = 1(p + q)2 = 12p2 + 2pq + q2 = 1

*To summarize

p=dominant allele frequencyq=recessive allele frequencyp2=homozygous dominant genotype2pq=heterozygous genotypeq2=homozygous recessive genotype

*It is possible to calculate all allele, genotype, dominant and recessive phenotype frequencies using the expressions :

allele frequency, p + q = 1 genotype frequency, p2 + 2pq + q2 = 1dominant phenotype frequency = p2 + 2pq recessive phenotype frequency = q2

For each individual of dominant phenotype, it must have at least one dominant allele (AA or Aa ) and for each individual of recessive phenotype, all of it alleles must be recessive or aa.

*EXAMPLE 2In a population of 500 wildflowers, 20 are white flowers or having recessive phenotype and the rest have dominant phenotype. If the alleles involved are A and a and there are 320 AA plants and 160 Aa plants, what are the frequencies of Aa genotype individuals, dominant and recessive phenotype individuals ?

*Heterozygous individual showing normal phenotypic characteristics but possessing a recessive gene capable of producing some form of metabolic disorder when present in homozygous recessives are described as carriers.

For a population to be in Hardy-Weinberg equilibrium, it must obey the principle of p2 + 2pq + q2 = 1

*Since these are diploid organisms, there are a total of 1000 copies of genes for flower colour in the population

gene pool consists of=2 ( 20 + 160 + 320 )=1000 allelestotal of allele a= 2 x 20 (for aa plants) + 160 x 1 (for Aa plants)=40 + 160=200frequency of allele a=200 / 1000=0.2 or q=0.2

*total of allele A=2 x 320 (for AA plants) + 160 x 1 (for Aa plants)=640 + 160=800

frequency of allele A=800 / 1000=0.8or p=0.8

frequency of AA genotype = 320 / 500=0.64or p2=0.82

*frequency of Aa genotype=160 / 500=0.32 or 2pq=2 x 0.8 x 0.2=0.32

frequency of aa genotype=20 / 500=0.04or q2=0.22=0.04

Frequency of aa individuals=q2q2=20 / 500q2=0.04q=0.2

*using the equation p + q =1p=1 qp=1 0.2p=0.8

frequency of Aa individuals=2pq=2 x 0.8 x 0.2=0.32

frequency of dominant phenotype= p2 + 2pq= 0.64 + 0.32= 0.96

*frequency of recessive phenotype= q2= 0.04 If this population is in Hardy-Weinberg equilibrium,p2 + 2pq + q2 = 10.64 + 0.32 + 0.04 = 1

So, this population obey the Hardy-Weinberg principle

*EXAMPLE 3The colour of soya beans is controlled by a pair of codominant alleles. CGCG genotype produces gold colour, CGCD produces light green and CDCD produces dark green. A sample of soya beans consists of 8 gold beans, 32 light green beans and 160 dark green beans. What is the frequency of allele CG?

*CGCG =8CGCD=32CDCD=160Total of soya beans=200

Total of all alleles=2 x 200 =400

total of allele CG=2 x 8 + 32 x 1=48

frequency of allele CG=48 / 400=0.12

*In this case, we cannot use CG or CD as a dominant allele because both are codominant alleles. Therefore the usage of CDCD as dominant genotype or as p2 and CGCG as recessive genotype or as q2 is incorrect and we cannot get the answer because it doesnt obey the Hardy-Weinberg principle or p2 + 2pq + q2 1. So as a reminder, only in the case of dominant and recessive alleles we can use Hardy-Weinberg calculation.

*END OF THIS TOPIC