Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Quantitative trait locus wikipedia , lookup
Group selection wikipedia , lookup
Genetics and archaeogenetics of South Asia wikipedia , lookup
Koinophilia wikipedia , lookup
Human genetic variation wikipedia , lookup
Polymorphism (biology) wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Population genetics wikipedia , lookup
Hardy–Weinberg principle wikipedia , lookup
Evolution of Populations Chapter 23 Populations Evolve individuals do not evolve, populations do a population is a localized group of individuals that are capable of interbreeding & producing fertile offspring microevolution – a change in the genetic makeup of a population from generation to generation population genetics – the study of how populations genetically change over time Gene Pool the total collection of genes in a population at any one time; consists of all the alleles in all the individuals of that population each allele has a frequency in the population for a trait controlled by two alleles, p & q are used to represent the frequency of the alleles usually… p represents the frequency of the dominant allele q represents the frequency of the recessive allele Calculating Allele Frequencies EXAMPLE – the incomplete dominant trait for flower color is controlled by two alleles: in a population of 500 flowers… total R = RR + R from RW = (320 x 2) + 160 = 800 q (the frequency of W) = 200/1000 = 0.2 320 are red (RR), 160 are pink (RW), & 20 are white (WW) because each flower has two alleles for the trait, there are 1000 alleles in the population p (the frequency of R) = 800/1000 = 0.8 R = red W = white total W = W from RW + WW = 160 + (20 x 2) = 200 notice that p + q = 1 Hardy-Weinberg Theorem the proportion of genotypes (AA, Aa, aa) in a population will remain constant from generation to generation if… the population is very large there is no gene flow (immigration or emigration) there are no mutations random mating is occurring there is no natural selection …because under these conditions, the frequency of the alleles (A & a) does not change populations exhibiting these conditions are not evolving & are said to be in Hardy-Weinberg equilibrium Hardy-Weinberg Equation p2 + 2pq + q2 = 1 where… p = the frequency of the dominant allele (A) q = the frequency of the recessive allele (a) p2 = the frequency of the homozygous dominant genotype (AA) 2pq = the frequency of the heterozygous genotype (Aa) q2 = the frequency of the homozygous recessive genotype (aa) uses: to calculate the frequencies of alleles & genotypes in a population to determine if a population is in Hardy-Weinberg EQ to estimate what % of a population is carrying the allele for a recessively inherited disease Example #1 What are the allele & genotypic frequencies for a population of 500 mice in which 245 are black (BB), 210 are brown (Bb), & 45 are white (bb)? (BB) = 245/500 = 0.49 (Bb) = 210/500 = 0.42 (bb) = 45/500 = 0.09 (B) = p2 = 0.49 = 0.7 (b) = q2 = 0.09 = 0.3 Example #2 What are the allele & genotypic frequencies for a population of 1000 pea plants in which 750 have purple flowers (PP) & 250 have white flowers (pp)? (pp) = 250/1000 = 0.25 (p) = q2 = 0.25 = 0.5 recall p + q = 1, (P) = 1 – q = 1 – 0.5 = 0.5 (PP) = p2 = (0.5)2 = 0.25 (Pp) = 2pq = 2(0.5)(0.5) = 0.5 Example #3 What % of the US population carries the allele for PKU, a homozygous recessive disease? (NOTE: 1 in 10,000 babies born in the US have PKU) (aa) = 1/10,000 = 0.0001 (a) = q2 = 0.0001 = 0.01 (P) = 1 – q = 1 – 0.01 = 0.99 (Aa) = 2pq = 2(0.01)(0.99) = 0.0198 percent carriers in US population is ~2% Factors that disrupt HW-EQ natural selection genetic drift – changes in allele frequencies due to chance results in alleles being passed to the next generation in different proportions bottleneck effect – occurs when a population is drastically reduced in size due to an environmental disaster (ie: fire, flood) & the gene pool of the survivors no longer represents that of the original population founder effect – occurs when a small group of individuals is isolated from the larger population & the gene pool of this splinter population does not reflect the source population gene flow – the loss/gain of alleles due to emigration/immigration Sources of Variation mutations point mutations, chromosome rearrangements, duplication rate if low in animals & plants rate is higher in viruses & bacteria b/c they have short life spans sexual recombination reshuffling of alleles can lead to phenotypic variations 3 Modes of Natural Selection directional selection – favors variants at one extreme disruptive selection – favors variants at both extremes stabilizing selection – favors intermediate variants Final Question… Why doesn’t natural selection result in the culling of all unfavorable genotypes? recessive alleles are carried (hidden) in heterozygotes heterozygote advantage (ex: sickle-cell trait) frequency-dependent selection when the fitness of any one variation declines if it becomes too common neutral variation genetic variation that has no impact on reproductive success