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Population Genetics History • Monk at the Brno monastery • Mathmatician – statistics – probability – physics • Assigned to tend the gardens Plant Molecular Genetics Lecture 1 courtesy Dr. Michael Neff http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html X X http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html (self fertilized) X Mendel’s work went unnoticed for over 30 years until other scientists started to observe the same things. All were European except one. William Spillman – In 1894 came to Washington Agricultural College. It was while Spillman was here that he experimented with wheat, attempting to hybridize the crop for the growing conditions here in the Palouse. He also independently rediscovered Mendel’s Law of Heredity and is credited with being a major role in the acceptance of Mendel’s Law by scientists and agriculturists. Plant Molecular Genetics Lecture 1 courtesy Dr. Michael Neff http://www.wsulibs.wsu.edu/Holland/masc/finders/cg250.htm Vocabulary • Alleles – different forms of the same gene • Genotype – forms of genes an individual possesses • Meiosis – happens during sexual reproduction when only one copy of the genes are passed to the gamete • Gamete – sexual reproductive cell • Homozygous – when both copies (alleles) of the gene are the same • Heterozygous – when the alleles of the same gene are different • Phenotype – the expressed type of the gene(s) • Fixation – when a population becomes limited to just one allele of a gene – loss of genetic diversity • Directional selection – shift in a trait that confers fitness (higher survival) or greater reproduction in individuals • Stabilizing selection – maintaining a range of traits Reginald Punnett – 1875 - 1967 1st Generation ♀ 0.50 B maternal gametes 0.50 b • Alleles – B & b • Meiosis produces the haploid gametes that merge to form the diploid zygote ♂ paternal gametes 0.50 B 0.50 b 0.25 BB 0.25 Bb 0.25 Bb 0.25 bb 1:2:1 ratio of genotypes 3:1 ratio of phenotypes 25% homozygous dominant 25% homozygous recessive 50% heterozygous dominant http://thehandiestone.typepad.com/blog/2010/03/sixyearold-chinese-boy-with-31-fingers-and-toes.html There was debate about how if Mendel’s Laws apply to people, and disorders such as brachydactyly is a dominant trait, why doesn’t this phenotype affect 3 out of 4 people? It is said that Punnett played cricket with G.H. Hardy. Both G.H. Hardy and Wilhelm Weinberg, independent of each other, pointed out that it was the percentage of the alleles in the population that had to be taken into consideration. Due to the research efforts of both we now have a theorem that we know of as : The Hardy – Weinberg equilibrium This equilibrium is only valid if certain conditions are met. 1. The population is very large – small populations have a tendency to cause genetic drift 2. There is random mating 3. No mutation – from B to b or b to B 4. No migration – movement of individuals out of population or in to the population 5. No selection – opposite is what we think of as natural selection http://www.geocities.com/haplogroupb/mtdnamigrations.jpg http://aquarium.nefsc.noaa.gov/FAQs/GeneralFAQ/im ages/bluelob2.jpg http://www.angelo.edu/faculty/kboudrea/lagnia ppe/pictures_san_antonio_zoo_4/3_Albino_Allig ator_01.JPG The Hardy – Weinberg equilibrium (remember in this instance 1 equals a population) (B + b) or (p + q) = 1 (100%) Where p = the percentage (frequency) of the dominant allele and q = the percentage (frequency) of the recessive allele & Diploids have a 2 alleles that make up their genotype (BB, Bb, bb) so: p2 + 2pq + q2 = 1 *Not technically squared BB = p2 where Bb = 2pq bb = q2 (p2 + 2pq + q2) or [(B)(B) + 2(B)(b) + (b)(b)] = 1 The Hardy – Weinberg equilibrium BB = 25% (0.25) Bb = 50% (0.50) bb = 25% (0.25) B+b=1 Allelic Frequencies Genotypic Frequencies B = (25% + ½ 50%) = 50% b = (25% + ½ 50%) = 50% 1st Generation ♂ paternal gametes ♀ 0.50 B maternal gametes 0.50 b 0.50 B 0.50 b 0.25 BB 0.25 Bb 0.25 Bb 0.25 bb (p2 + 2pq + q2) or [(B)(B) + 2(B)(b) + (b)(b)] = 1 The Hardy – Weinberg equilibrium How to figure out the allelic frequency based on phenotype observation 9 black cats (TT) 23 black & white cats (Tt) 18 white cats (tt) 1st figure out where the phenotype fits - ( TT is homozygous dominant or p2 ) - ( Tt is heterozygous dominant or 2pq ) - ( tt is homozygous recessive or q2 ) 2nd figure out what the percentage of the total number of alleles the selected phenotype represents –18 white cats = 36 t alleles, 9 black = 18 T alleles, 23 B&W = 23 T alleles and 23 t alleles 3rd add up all the t’s and T’s 36 + 23 = 59 t’s, 18 + 23 = 41 T’s 4th is to do the math - There are 100 total alleles so the allelic frequency is – t’s = 59/100 or 59% - T’s = 41/100 or 41% p + q = 1 check .41 + .59 = 1 (.41)(.41) + 2(.41)(.59) + (.59)(.59) = 1 0.1681 + 0.4838 + 0.3481 = 1 And you can see from the calculation that the homozygous populations are not the square root of the frequencies Lab Exercises Exercise 1 – Testing the Hardy - Weinberg Equilibrium : • using a bead model • calculation to estimate heterozygote frequency Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine due to an enzyme called phenylalanine hydroxylase is missing. This can be represented in our genotype as aa. Exercise 2 – Effect of Small Population Size: Simulation of Genetic Drift • Bottleneck effect • Founder effect Exercise 3 – Simulation of Natural Selection Summary at the end of this exercise is what your group will present at the end of lab to the rest of the groups Next Week Quiz Microscope skills/Protists & Fungi