Genetic Variation in Populations PDF

3/14/2024 Chapter 18 Genetic Variation in Populations 1 Mutation Mutation is an important attribute of populations. New mutations that arise, if not neutral in effect, will rarely be better, an...

3/14/2024 Chapter 18 Genetic Variation in Populations 1 Mutation Mutation is an important attribute of populations. New mutations that arise, if not neutral in effect, will rarely be better, and likely will be worse, than the genes already present. Change in environmental conditions can elicit a genetic response based on the available genetic variation. 3 1  3/14/2024 Mutation Rapid genetic changes in many insect populations exposed to pesticides such as dieldrin and DDT Resistance to DDT in common houseflies 4 →Variety of adaptive mechanisms e.g. DDT resistance Increase in lipid content. Enzymes to breakdown DDT. Reduction in nervous toxic response. Cuticle permeability changes. Behavior → contact reduction with DDT. 5 2  3/14/2024 RANDOM MUTATIONS? Pre-adaptive Post-adaptive random non-random prior to exposure to after exposure selective agent 6 “Replica-plating” (performed by J. & E. Lederberg) Master plate (on non- phage medium) Wooden block *** * with velvet cloth * ** * * *** * *** * ** * * *** ** ** * * ** * ** * ** ** ** ** Replica plates on phage medium 7 3  3/14/2024 10 GENETIC POLYMORPHISM Mutations harmful Beneficial Neutral Recessives Accumulation Inversions, translocation (duplications) & extra chromosomes polymorphism 11 4  3/14/2024 Neo-Darwinism and Genetic Polymorphism Neutral mutations or deleterious but recessive mutations provide a reservoir of potential genetic variation. Genetic polymorphism provides a much greater source of genetic variation than do the relatively few new mutations that arise each generation. 21 Genetic Polymorphisms In the fruit fly, Drosophila pseudoobscura, populations in different localities are polymorphic for a wide variety of gene arrangements. Figure 03B: Chromosomal inversions found at different months during the year in one locality, Mount Figure 03A: Third chromosome gene San Jacinto, California inversions in Drosophila pseudoobscura 22 5  3/14/2024 CONTINUOUS VARIATION could be due to polygenes (several genes each with a small phenotypic effect) and Quantitative Trait Loci (QTL). → independent assortment → normal distribution 23 Quantitative Trait Loci All genes (alleles) in a chromosome affecting quantitative aspects of phenotype e.g. size, shape or process affecting these. e.g. in some rats: 4 QTLs → tail growth and body weight. Environmental effects → quantitative influences. How much environmental and how much genetic? Heritability = the extent to which genetic differences affect a character → selection. 24 6  3/14/2024 CONTINUOUS VARIATION small heritable changes → variation → natural selection ==> Continuous variation e.g. body size distribution Figure 10.19: Distribution of the heights of 1,000 Harvard College students aged 18 to 25 25 e.g. crosses between 3 wheat strains 26 7  3/14/2024 e.g. rabbit color pattern due to 3 gene pairs, each with 2 alleles (1 white, 1 black) 27 a) complete set of dermal plates; complete pelvic girdle b and c: reduction of the pelvic girdle and in the number of dermal plates. Figure 06: Three spine sticklebacks 28 8  3/14/2024 Basic concepts Gene frequency (allele freq.) = proportion of an allele of a gene in population. Total gene frequency for all alleles = 1 Gene pool = sum of all genes in reproductive gametes in a pop. 29 Gene Frequencies Given two alleles – T, dominant, for tasting phenylthiocarbamide – t, recessive, no tasting. A population of 200 individuals – 90 TT (tasters) – 60 Tt (tasters) – 50 tt (non-tasters) Diploid ==> 400 genes at locus. T = 90 x 2 (in TT) + 60 (in Tt) = 240/400 = 0.6 t = 100 (in tt) + 60 (in Tt) = 160/400 = 0.4 30 9  3/14/2024 Gene Frequencies Also obtained from frequencies of genotypes: T =.45 TT + 1/2 (.30 Tt) =.45 +.15 =.60 t =.25 tt + 1/2 (.30Tt) =.25 +.15 31 Gene Frequencies if p = frequency of allele T q = frequency of allele t =1-p p+q=1 Genotype frequencies: p2 (TT) + 2pq (Tt) + q2 (tt) By binomial expansion (p + q)2 = p2 + 2pq + q2 32 10  3/14/2024 Gene Frequencies P q p P2 pq q pq q2 33 Gene Frequencies P q 0.6 0.4 p P2 pq 0.6 0.36 0.24 q pq q2 0.4 0.24 0.16 34 11  3/14/2024 35 Equilibrium with 3 alleles If 3 alleles on the same locus: trinomial expansion (p + q + r)2 → P2 + 2pq + 2pr + q2 + 2qr + r2 36 12  3/14/2024 Hardy-Weinberg Equilibrium Under certain conditions, frequencies of such alleles will reach an equilibrium. i.e. frequencies become stable from one generation to next. (= Conservation of gene frequencies) These conditions are: - Panmixia: random mating between individ. in pop. - gametes of all genotypes have the same viability - Population is static. - Turns out rarely to be seen in nature 37 Hardy–Weinberg principle In a population of randomly mating individuals, allele frequencies are conserved and in equilibrium unless external forces act on them. 38 13  3/14/2024 Equilibria in natural populations Some populations have been shown to have reached the Hardy- Weinberg Equilibrium. random mating has also been demonstrated. Table 21.08: Comparison of observed acid phosphatase phenotypes and those expected according to Hardy-Weinberg equilibrium 39 Alleles at 2 or more Loci → frequencies p, q, r, s expansion of : (pr + ps + qr + qs)2 i.e. frequencies of gametes AB, Ab, aB, ab Equilibrium complicated by linkage between the two loci: gametes in repulsion (recombinant): Ab, aB gametes in coupling (non-recombinant): AB, ab 40 14  3/14/2024 Linkage Equilibrium if the frequencies of the various gametes are as expected that indicates linkage equilibrium If not as expected then ==> linkage disequlibrium. ==> lower recombination frequency. 41 Linkage disequilibrium Combinations of several alleles at various loci → more reproductive success (more fitness) Linkage between some genes can be preserved for some time (restricted recombination) → Coadapted gene complexes. 42 15  3/14/2024 Sex Linkage If females =XX then frequencies in females is the same as for autosomal genes in males: may reach equilibrium but after several generations. At equilibrium, sex-linked allele frequencies are the same in both sexes, but the genotypes differ 43 Equilibria in natural populations In some populations, esp. when dominant-recessive situations → similar phenotype e.g. Aa and AA, when recessive phenotypes (aa) are rare, carrier heterozygotes are present in relatively high frequencies. 44 16  3/14/2024 INBREEDING H-W Equilibr. not maintained when non-random mating. e.g. inbreeding = mating occuring within a very small group. Self-fertilization: extreme case of inbreeding. Inbreeding quantified by inbreeding coefficient, F. F= probability that 2 alleles in a diploid zygote are identical. Range: from 1 (complete homozygosity) to 0 (complete heterozygosity) 45 INBREEDING 46 17  3/14/2024 INBREEDING The frequencies expected for the Hardy-Weinberg equilibrium are modified most if F=1. Inbreeding depression = appearance of rare deleterious alleles in an inbred population. 47 18