Shailendra-Population Genetics

POPULATION GENETICS :

POPULATION GENETICS Guided By: Dr. JYOTI BHOJWANI Presented By: Shailendra Tiwari

Population genetics:

Population genetics genetic structure of a population

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genetic structure of a population group of individuals of the same species that can interbreed Population genetics

Population Genetics::

Population Genetics: The study of allele frequencies within a population. Changes in allele frequencies are caused by the following: Natural Selection Genetic Drift Mutation Gene Flow

Natural Selection:

Natural Selection A process whereby creatures with good traits survive and reproduce and where creatures with negative traits, die off.

Genetic Drift:

Genetic Drift Changes in allele frequencies due to the fact that alleles in offspring are a random sample of those in parents. Not a result of which traits are beneficial, genetic drift is random.

Causes of Mutation:

Causes of Mutation Spontaneous Radiation Chemicals/pollution Crossing over during meiosis Meiosis makes sperm and egg

Gene Flow:

Gene Flow Transfer of alleles from one population to another. Caused by the migration of organisms

Genetics:

Genetics Let’s say “B” = Brown and “bb” = white. Geneticists can predict babies B b x bb = 50% Brown and 50% White

Genetics:

Genetics Let’s say “B” = Brown and “bb” = white. Let’s say that 4% of a population is white. Geneticists can now predict for the population b B B 4% bb b B B b BB b 4% = 0.04 “bb”= 0.04 “b” x “b” = 0.04 The square root of 0.04 = 0.2 “b” = 0.2 Therefore “b” = 20% of all alleles 4% of the population = 0.2 = 0.2

Genetics:

Genetics If “b” = 20% of all alleles, then… “B” must equal 80%. 4% bb b B B b BB b = 0.2 b = 0.2 B = 0.8 B = 0.8 4% of the population

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What percent of the bunny population will be homozygous dominant ? b = 0.2 b = 0.2 B = 0.8 B = 0.8 4% bb BB 64% will be BB . 32% will be hybrids 64% 16% bB 16% Bb 4% of the population

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A new recessive mutation has resulted in 9% of the USA becoming vampires. What percent of the population will be carriers? n = 0.3 n = 0.3 42% will be carriers. N = 0.7 N = 0.7 49% 9% 21% 21%

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“b” = 0.6. Do the rest. b = 0.6 b = 0.6 What are the percentages of the different genotypes? Fill out the punnett. B = 0.4 B = 0.4 16% 24% 24% 36%

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Equilibrium = frequencies of alleles remain constant if… No mutation: No allelic changes occur. No gene flow: Animals do not enter or leave the population. Random mating: Individuals pair by chance and not according to the genotypes or phenotypes. Large population: The population is large so changes in allele frequencies due to chance are insignificant. Large populations decrease chances of genetic drift. No selection: All genotypes are healthy and there is no selective force that favors one genotype over another.

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Within a population of moths, the color dark (D) is dominant over the color white (d). Before the industrial revolution, nearly 81% of all moths were white. What percentage of the population was DD before the environment was polluted? d = 0.9 d = 0.9 1% of the butterflies will be DD. D = 0.1 D = 0.1 1% 9% 9% 81% dd D _

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Evolution = frequencies of alleles change. Evolution occurs if… Mutations occur Gene Flow. Animals enter or leave the population, thereby introducing or eliminating alleles. Non-random mating. Animals breed according to phenotypes or genotypes. Small population Selection. One genotype is selected over another. Which moth was selected for during the industrial revolution?

Allele Frequencies:

Allele Frequencies Allele frequencies (gene frequencies) = proportion of all alleles in an all individuals in the group in question which are a particular type Allele frequencies: p + q = 1 Expected genotype frequencies: p 2 + 2pq + q 2

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If only random mating occurs, then allele frequencies remain unchanged over time. After one generation of random-mating, genotype frequencies are given by AA Aa aa p 2 2pq q 2 p = freq (A) q = freq (a) Hardy-Weinberg Theorem

Hardy-Weinberg Principle:

Hardy-Weinberg Principle Hardy-Weinberg principle measures change in allele frequencies in a gene pool. Therefore it can be used to demonstrate the evolution of a single trait. Genetic Equilibrium – The frequency of a given allele remains stable, one generation after the next, thus no evolution. Five conditions must be met to maintain genetic equilibrium (No Evolution) No mutations Large population (probability) – no genetic drift Isolated populations – no gene flow No natural selection Random mating

Calculating H-W Principle:

Calculating H-W Principle p represents the frequency of allele A q represents the frequency of allele a p + q = 1 To determine genotypic frequencies (AA, Aa, aa) from allelic frequency (A and a) The genotypic frequencies must add up to 1, therefore: p 2 + 2pq + q 2 = 1 (Hardy-Weinberg equation). Used to determine genotypic frequencies from allelic frequencies. A(p) a(q) A(p) AA (p 2 ) Aa (pq) a(q) Aa (pa) Aa (q 2 )

Hardy-Weinberg Notation:

Hardy-Weinberg Notation p – frequency of the dominant allele (A) q – frequency of the recessive allele (a) p 2 – frequency of the homozygous dominant genotype (AA) q 2 – frequency of the homozygous recessive genotype (aa) 2pq – frequency of the heterozygous genotype (Aa)

H-W Principle – Example 1:

H-W Principle – Example 1 About 70 % of North Americans can taste the chemical PTC, and the remainder cannot. The ability to taste is determined by the dominant allele T, and the inability to taste is determined by the recessive allele t. If the population is assumed to be in Hardy-Weinberg equilibrium, what are the genotypic and allelic frequencies in the population?

Example 1 Solution:

Example 1 Solution Since 0.70 are tasters (TT) then 0.30 must be not tasters (tt). Since q 2 is represents (tt) we can determine q by taking the square root. The square root of 0.30 is 0.55 (q). Since p + q = 1 then p must equal 0.45. To calculate the genotypic frequencies: TT = p 2 = (0.45)2 = 0.20 Tt = 2pq = 2(0.45)(0.55) = 0.50 tt = q 2 = (0.55)2 = 0.30

Hardy-Weinberg Equilibrium:

Hardy-Weinberg Equilibrium Yule thought that equilibrium meant that the allele freq. must be 0.50 and 0.50 Hardy demonstrated that they don’t have to be 0.50/0.50, but simply must remain the same between generations if allele freq. are given by p and q, then the genotypes can be calculated by p 2 +2pq+q 2= 1 Also,p+q=1

Gregor Mendel (1822 - 1884) :

Gregor Mendel (1822 - 1884)

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Population genetics genetic structure of a population group of individuals of the same species that can interbreed alleles genotypes Patterns of genetic variation in populations Changes in genetic structure through time

Describing genetic structure:

Describing genetic structure genotype frequencies allele frequencies rr = white Rr = pink RR = red

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200 white 500 pink 300 red genotype frequencies allele frequencies 200/1000 = 0.2 rr 500/1000 = 0.5 Rr 300/1000 = 0.3 RR total = 1000 flowers genotype frequencies: Describing genetic structure

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200 rr 500 Rr 300 RR genotype frequencies allele frequencies 900/2000 = 0.45 r 1100/2000 = 0.55 R total = 2000 alleles allele frequencies: = 400 r = 500 r = 500 R = 600 R Describing genetic structure

for a population with genotypes::

for a population with genotypes: 100 GG 160 Gg 140 gg Genotype frequencies Phenotype frequencies Allele frequencies calculate:

for a population with genotypes::

for a population with genotypes: 100 GG 160 Gg 140 gg Genotype frequencies Phenotype frequencies Allele frequencies 100/400 = 0.25 GG 160/400 = 0.40 Gg 140/400 = 0.35 gg 260/400 = 0.65 green 140/400 = 0.35 brown 360/800 = 0.45 G 440/800 = 0.55 g 0.65 260 calculate:

another way to calculate allele frequencies::

another way to calculate allele frequencies: 100 GG 160 Gg 140 gg Genotype frequencies Allele frequencies 0.25 GG 0.40 Gg 0.35 gg 360/800 = 0.45 G 440/800 = 0.55 g OR [0.25 + (0.40)/2] = 0.45 [0.35 + (0.40)/2] = 0.65 G g G g 0.25 0.40/2 = 0.20 0.40/2 = 0.20 0.35

Population genetics – Outline:

Population genetics – Outline What is population genetics? Calculate Why is genetic variation important? genotype frequencies allele frequencies How does genetic structure change?

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changes in allele frequencies and/or genotype frequencies through time How does genetic structure change?

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mutation migration natural selection genetic drift non-random mating How does genetic structure change? changes in allele frequencies and/or genotype frequencies through time

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mutation migration natural selection genetic drift non-random mating spontaneous change in DNA creates new alleles ultimate source of all genetic variation How does genetic structure change?

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introduces new alleles individuals move into population How does genetic structure change? mutation migration natural selection genetic drift non-random mating “gene flow”

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differences in survival or reproduction certain genotypes produce more offspring leads to adaptation differences in“fitness” How does genetic structure change? mutation migration natural selection genetic drift non-random mating

Natural selection:

Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant

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Natural selection Generation 1: 1.00 not resistant 0.00 resistant Resistance to antibacterial soap

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Natural selection Resistance to antibacterial soap mutation! Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant

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Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant

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Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24 resistant Generation 4: 0.12 not resistant 0.88 resistant

Selection on “sickle-cell anaemia” allele in Africa:

Selection on “sickle-cell anaemia ” allele in Africa aa – abnormal ß hemoglobin sickle-cell anemia very low fitness intermed. fitness high fitness Selection favors heterozygotes ( Aa ). Both alleles maintained in population ( a at low level). Aa – both ß hemoglobins resistant to malaria AA – normal ß hemoglobin vulnerable to malaria

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sampling error genetic change by chance alone misrepresentation small populations How does genetic structure change? mutation migration natural selection genetic drift non-random mating

Genetic drift:

Genetic drift 8 RR 8 rr Before: After: 2 RR 6 rr 0.50 R 0.50 r 0.25 R 0.75 r

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mutation migration natural selection genetic drift non-random mating cause changes in allele frequencies How does genetic structure change?

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mutation migration natural selection genetic drift non-random mating non-random mating non-random allele combinations mating combines alleles into genotypes How does genetic structure change?

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Thank You All 