CHAPTER POPULATION GENETICS Principal points • A population is a localized group of individuals belonging to the same species • A species as a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature • Gene pool is the complete set of genetic information contained within the individuals in a population The gene pool includes all alleles present in the population • Evolution is a process in which genetic variation in a population changes over time • Genetic variation originates when one allele mutates to another • Population genetics studies the origin of variation, the transmission of variants from parents to offspring generation after generation i.e., population genetics is the branch of genetics that deals with frequencies of alleles and genotypes in breeding populations Frequencies of Alleles & Genotypes - Genotype frequency in a population is the proportion of organisms that have the particular genotype (eg RR, Rr or rr ) - Allele frequency is the proportion of all alleles that are of the specified type (eg R or r ) Suppose, these are diploid organisms: + There are only two allelic forms of the gene in the population + There are a total of 1,000 copies of gene for flower color in the population of 500 individuals A gene has two alleles (R and r), their frequencies are represented: p = 0.8 (frequency of R allele) q = 0.2 (frequency of r allele) p + q = 0.8 + 0.2 = Results of gene pool in parent population: - The frequency of homozygous individuals for dominant allele (RR) is 320/500 = 0.64 = 64 %; and for recessive allele (rr) is 0.04 (4%) in the population of 500 individuals + The frequency of heterozygous individuals (Rr) in the population is 0.32 = 32% - The frequency of the R allele in the gene pool of this population is p = 800/1,000 = 0.8 = 80% And the r allele must have a frequency of q = 0.2 = 20% + The chance that the gamete will bear an R allele is 80%, and the chance that the gamete will have an r allele is 20% - The relationship between p and q : p+q=1 p and q are the proportions of the two alleles of a gene in a population Gene pool of next generation Results of gene pool in next generation: - The probability of picking two R alleles: p2 = 0.8x0.8 = 0.64, i.e., about 64% of the plants have the genotype RR + The frequency of rr individuals: q2 = 0.2 x 0.2 = 0.04 (4%) + And the frequency of heterozygous plants (Rr + rR): 2pq = x 0.8 x 0.2 = 0.32 (32%) The genotype frequencies add up to 1: 0.64+0.32+0.04=1 This is: (p + q)2 = p2 + 2pq + q2 = - The allele frequencies are the same as the allele frequencies in the parent population The chance that the gamete will bear an R allele is 0.8, and the chance that the gamete will have an r allele is 0.2 This is p + q = The frequencies of alleles and genotypes remain the same between one generation (a) and the next generation (b) Hardy – Weinberg law - The Hardy-Weinberg law states that “allele and genotype frequencies in a population remain constant generation after generation if there is no selection, mutation, migration or random drift” i.e.: the frequencies of three genotypes RR, Rr and rr for a locus with two alleles (R and r) will remain constant at p2, 2pq and q2 where p and q are the frequencies of alleles R and r, respectively Such populations are said to be at an equilibrium - Hardy-Weinberg equilibrium since their allele and genotype frequencies are constant or unchanging from one generation to the next - The Hardy-Weinberg allele frequencies: p+q=1 (p and q are the proportions of the two alleles of a gene in a population) - The Hardy-Weinberg genotype frequencies: (p + q)2 = p2 + 2pq + q2 = p2 : frequency of homozygous dominant genotype 2pq : frequency of heterozygous individuals in the population q2 : frequency of homozygous recessive genotype Hardy – Weinberg principles For a population to be in Hardy-Weinberg equilibrium, it must satisfy five main conditions: Very large population size; If that size is small, genetic drift, which can cause genotype frequencies to change over time No migration; If gene flow, the transfer of alleles between populations due to the movement of individuals or gametes, can increase the frequency of any genotype that is in high frequency among the immigrants 3 No net mutations; If mutations can alter the gene pool by changing one allele into another Random mating; If individuals pick mates with certain genotypes, then the random mixing of gametes required for Hardy-Weinberg equilibrium does not occur No natural selection; If selection, differential survival and reproductive success of genotypes will alter their frequencies and may cause a detectable deviation from frequencies predicted by the HardyWeinberg equation Factors affecting gene frequencies • Small population size (Genetic drift) - A change in a population’s allele frequencies due to chance, is called genetic drift e.g., the frequencies of the alleles for red (R ) and white (r ) flowers to change over the generations This small wildflower population has a stable size of only ten plants + For generation 1, only the five boxed plants produce fertile offspring + In generation 2, only two plants manage to leave fertile offspring Small population size can cause allele and genotype frequencies to change over time Over the generations, genetic drift can completely eliminate some alleles, as is the case for the r allele in generation of this imaginary population • Migration (Gene flow) - A population may gain or lose alleles by gene flow, genetic exchange due to the migration of fertile individuals or gametes between populations e.g A population consists entirely of white-flowered individuals (rr) to grow near our wildflower population + A windstorm may blow pollen from the rr population to our wildflower population + Result, the allele frequencies may change in the next generation • Mutation - A mutation is a change in an organism’s DNA - A new mutation that is transmitted in gametes can immediately change the gene pool of a population by substituting one allele for another - Mutation is, very important to evolution because it is the original source of the genetic variation that serves as raw material for natural selection • Natural selection - Hardy-Weinberg equilibrium requires that all individuals in a population be equal in their ability to survive and produce viable, fertile offspring - Populations consist of varied individuals, with some variants leaving more offspring than others, which Darwin meant by natural selection - Selection results in alleles being passed along to the next generation in numbers disproportionate to their relative frequencies in the present generation eg In wildflower population, (i) White flowers (rr) are more visible to herbivorous insects, so that more white flowers are eaten Therefore, plants with red flowers (RR or Rr) would have more opportunity to produce offspring (ii) Red flowers may be more effective than white ones in attracting the pollinators required for seed production Result: - The frequency of the R allele would increase in the gene pool, and the frequency of the r allele would decline This difference would disturb Hardy-Weinberg equilibrium Natural selection and Genetic drift cause most of the changes in allele frequencies that we observe in evolving populations However, allele frequencies can also be changed by migration between populations or by mutation Application of the Hardy-Weinberg law eg One out of approximately 10,000 babies in the U.S is born with Phenyl ketonuria (PKU) The disease is caused by a recessive allele; calculation of frequencies of recessive allele and heterozygous carriers in the population - Applying the Hardy-Weinberg law, the frequency of individuals in the U.S population born with PKU corresponds to q2 (q2 : frequency of the homozygous recessive genotype) - Given one PKU occurrence per 10,000 births, q2 = 0.0001 + The frequency of the recessive allele for PKU in the population is: q = √0.0001 = 0.01 = 1% + The frequency of the dominant allele is: p = - q = - 0.01 = 0.99 + The frequency of carriers, heterozygous people who not have PKU but may pass the PKU allele on to offspring, is: 2pq = x 0.99 x 0.01 = 0.0198 (≈ 2%) Thus, - There are 1% of the recessive allele in the population - And about 2% of the U.S population carries the PKU allele