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Lecture 16

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Biological Sciences
Mark Fitzpatrick

th BIOA01H3 – Lecture 16 Week of October 7 : Peas, Probability, and Pedigree Chapter 10 10.1 The Beginnings of Genetics: Mendel’s Garden Peas  Until about 1900, scientists and general public believed in blending theory of inheritance, which suggested that hereditary traits blend evenly in offspring through mixing of parent’s blood  Mendel studied variety of heritable characteristics called characters, i.e. flower colour or seed shape  Variation in character i.e. purple or white flowers called traits  Established that characters passed to offspring in form of discrete hereditary factors now known as genes 10.1a Mendel Chose True-Breeding Garden Peas for His Experiment  Chose garden pea b/c plant could be grown easily  Gametes produced in structures of flower  Male gametes are sperm nuclei contained in the pollen, produced by anthers of flower  Female gametes are egg cells, produced in carpel of flowers  Normally, pea plants self-fertilize (or self-pollinate): sperm nuclei in pollen fertilize egg cells housed in carpel of same flower  Mendel prevented self-fertilization by cutting off anthers & cross-pollinated with other plants  Mendel chose pea-plants that were true-breeding; when self-fertilized, passed traits w/o change from one generation to the next 10.1b Mendel First Worked with Single-Character Crosses  Flower colour among seven characters Mendel selected for study  one true-breeding variety of peas w/ purple flowers, other one with white  Would traits blend evenly if plants with purple flowers were cross-pollinated with white flowers?  Answer question  Mendel took pollen from anthers of plants w/ purple flowers & placed in the flowers of white-flowered plants  Placed pollen on stigma, part of carpel that receives pollen in flowers  Also performed reciprocal experiment  Sees were result of crosses; each seed contains zygote/embryo that’ll develop into new pea plant  Plants that grew from F1 seeds all formed purple flowers  Mendle allowed F1 plants to self-pollinate, producing seeds of F2 generation  When planted F2 generation, white-flowered trait appeared along w/ purple flowered  Mendel counted 705 plants w/ purple flowers & 224 w/ white, roughly 3:1  Crossed rest of the other 6 single-character & all had same ratio in F2 generation 10.1c Mendel’s Single-Character Crosses Led Him to Propose the Principle of Segregation  Mendel used knowledge of mathematics to develop set of hypotheses 1) The adult plants carry a pair of factors that govern the inheritance of each character o Correctly deduced that for each character, an organism inherits one factor from each parent o Mendel’s factors called genes, variations in genes called alleles o Mendel deduced that trait that disappeared in F1 generation was actually still present but was masked by “stronger” allele o Called masking effect dominance, leading to Mendel’s second hypothesis 2) If an individual’s pair of genes consists of different allele, one allele is dominant over the other recessive allele o Purple allele declared to be dominant simply b/c when both alleles present, it is expressed rather than white 3) Pairs of alleles that control character segregates as gametes formed; half the gametes carry one allele, and other half carry other allele o Now known as Principle of Segregation o During fertilization, fusion of maternal and paternal haploid gametes produces diploid nucleus called zygote  Hypotheses explain how individuals may differ genetically but still look the same Genotype refers to genetic constitution of an organism. Phenotype refers to its outward appearance. 10.1d Mendel Could Predict Both Classes and Proportions of Offspring from His Hypotheses  In mathematics, likelihood of outcome predicted on scale of 0 to 1  Outcome that is certain to happen has probability of 1 & vice versa Product Rule in Probability  Product rule – individual probabilities are multiplied  Probability of events A and B both will occur equally because results of event A has no effect on result of event B  i.e. rolling a die twice to get 6. First time, rolling a 6 is a 1/6 chance. Second time, rolling a 6 is still 1/6. Therefore rolling a 6 in two rolls: 1/6 x 1/6 = 1/36 The Sum Rule in Probability  Sum rule – several different events all give the same outcome; probability of either event A or event B or event C will occur equals the probability of event A + event B + event C  i.e. 2 dice, what is the probability of rolling a 7? Several different events can all give same total  roll a 1 on one die, and a 6 on another die; 5 on first, 2 on second; 4 on first, 3 on second  three of these combinations would be expected to occur at a frequency of 1/6 x 1/6 = 1/36  total six ways of obtaining a 7 (other three combos are the reciprocals of the original three) so sum individual probabilities: 1/36 + 1/36 + 1/36 + 1/36 + 1/36 + 1/36 = 1/6 Probability in Mendel’s Crosses - super easy, don’t worry about it (10.1e also easy) 10.1f Mendel Tested the Independence of different Genes in Crosses  Dihybrid cross produced a 9:3:3:1 ratio  consistent with Mendel’s previous findings if he added one further hypothesis: The alleles of the genes that govern the two characters segregate independently during formation of gametes.  I.e. allele for seed shape that gamete receives (R or r) has no influence on allele for seed colour it receives (Y or y) & vice versa  assumption termed independent assortment; now known as Principle of Independent Assortment 10.1h Sutton’s Chromosome Theory of Inheritance Related Mendel’s Genes to Chromosome  Walter Sutton recognized similarities between inheritance of genes discovered by Mendel and behavior of chromosomes in meiosis and fertilization  Sutton drew all necessary parallels between genes and chromosomes - Chromosomes occur in pairs in sexually reproducing, diploid organisms, as do the alleles of each gene - The chromosomes of each pair are separated and delivered singly to gametes, as are the alleles of a gene - The separation of any pair of chromosomes in meiosis and gamete
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