Life Cycles, Meiosis and Genetic Variability24/02/2015 2:26 PM

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1 Life Cycles, Meiosis and Genetic Variability iclicker: 1. A chromosome just before mitosis contains two double stranded DNA molecules. 2. This replicated chromosome contains DNA from only one of your parents 3.One homologous pair of chromosomes contains DNA from one parent on one chromosome and DNA from the other parent on the other chromosome. Human cells: Contain 46 chromosomes, or 23 pairs Homologous chromosomes are about the same size and have the same gene loci Homologous chromosomes are different from sister chromatids because sister chromatids are identical n meosis (2n) diploid haploid gamete sperm fertilization 2n! mitosis! 2n " meiosis " n " mitosis " n (normal) (plants only: multicellular haploid- mitosis of haploid structure) Sexual in a diploid: meiosis Asexual in a diploid: mitosis Sexual self fertilizing diploid: meiosis Asexual in a haploid: mitosis Sexual reproduction must involve meiosis even if it is self fertilizing. Meiosis Produces 4 daughter haploid cells N number of chromosomes ( half of the original amount)

2 Mendelian and Chromosomal Genetics iclicker if human life cycles were like plant life cycles which structure would be equivalent? Multicellular sperm Sexual reproduction: meiosis and mitosis Asexual reproduction: mitosis " mitosis " genetic variability " mutations Haploid " mitosis " multicellular organism If a cell with 40 chromosomes undergoes meiosis, but there is nondisjunction in meiosis II, what will the daughter cells look like? ^ non disjunction ^ Genetic variability mitosis " none meiosis " independent assortment! (AAbb, aabb) crossing over! Segregation: a gamete is formed, the members of a homologous pair are separated and go into different gametes Aa " A a

3 Mendelian and Chromosomal Genetics Mendelian Inheritance and Punnet Squares How do alleles get transmitted from parent to offspring? Meiosis! How do we predict the ratios of offspring phenotype with Punnet Squares? One gene locus and 2 possible alleles Types of dominance Simple: normal Incomplete: heterozygote has a different phenotype from the dominant and recessive phenotype (flowers!) Codominance: both dominant traits are present without one overshadowing the other (blood type) Homozygous: AA or aa Heterozygous: Aa Heterozygous not simple: A A Convention (wild type): +/a Test cross: use to predict an unknown genotype, mate the individual with the homozygous recessive trait Recessive traits: defined at the molecular level as loss of function mutations aka like hypercholesterolemia- lack of LDL receptor protein, cystic fibrosis

4 Hardy-Weinburg Population Genetics What causes variation in a population? Mutations! H-W equation is a qualitative expression relating allele frequencies to genotype frequencies Allele Frequency p + q = 1 p = A (dominant allele) q = a (recessive allele) Genotype Frequency p^2 + 2pq + q^2 = 1 p^2 = homozygous dominant 2pq = heterozygous q^2 = homozygous recessive Assumptions of the H-W Theorem 1. Large population 2. Random mating 3. No gene flow into/between populations 4. No mutations 5. No natural selection Non random mating assertive mating (positive or negative); sexual selection add to non random mating Inbreeding decreases variation (selective for one allele) Self fertilization = pretty much cloning Genetic Drift: changes in the gene frequencies caused by stochastic factors (by chance), operates strongly in small populations Bottle neck effect: natural disaster causing huge mortality, survivors mate" more variety usually in a normal population, but there is no variety of a certain trait

5 Natural Selection 1. Genetic Drift Genetic bottle neck effect: natural disaster Founder effect: one group leaves the population and starts a new population creating their own gene pool 2. Gene Flow new alleles are introduced to a gene pool new mating mating between populations 3. Mutation gene turned off or on causes favorable or unfavorable can be neutral or no change 4. Non Random Mating inbreeding leads to fixation of alleles sexual selection SURVIVAL OF THE FITTEST Example of Natural Selection: Winter comes and kills off all brown individuals, survival of white individuals Artificial selection: selection on genetic variation leads to new phenotypes Evolution by natural selection Fitness: individual s contribution to the next generation, the individual must survive and reproduce Natural selection: individuals with a higher fitness in a particular environment that allows them to survive and reproduce more than individuals with a lower fitness Adaptation: product of natural selection, an aspect of the phenotype that gives an individual a fitness advantage in a certain environment