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Unit 3: Genetics

Unit 3: Genetics. The Cell Cycle + DNA structure/function Mitosis and Meiosis Mendelian Genetics (aka - fun with Punnett squares) DNA replication. Yesterday’s Exit Ticket. Today’s Agenda. Where does variation come from? Mendelian Genetics, Part One. Sources of genetic variation.

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Unit 3: Genetics

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  1. Unit 3: Genetics • The Cell Cycle + DNA structure/function • Mitosis and Meiosis • Mendelian Genetics (aka - fun with Punnett squares) • DNA replication

  2. Yesterday’s Exit Ticket

  3. Today’s Agenda • Where does variation come from? • Mendelian Genetics, Part One

  4. Sources of genetic variation • Mutations (changes in an organism’s DNA) are the originalsource of all genetic variation • Mutations create different versions of genes called alleles

  5. Clarity check: homologous chromosomes Gene for hair color; allele for brown hair SAME gene, different ALLELES Gene for hair color; Allele for blonde hair

  6. Sources of genetic variation • The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation • Three mechanisms contribute to genetic variation: • Independent assortment of chromosomes • Crossing over • Random fertilization

  7. Sources of genetic variation Fig. 13-8b a) Independent assortment • Homologous pairs of chromosomes orient randomly during Meiosis I  maternal and paternal homologs assort into daughter cells independently of the other pairs Blue can be on top or bottom Metaphase Iof meiosis I

  8. Sources of genetic variation Fig. 13-11-2 a) Independent assortment Possibility 2 Possibility 1 with n = 2 there are 4 possibilities for the lineupduring Meiosis II • 4 possible assortments of chromosomes in the gametes

  9. Sources of genetic variation Fig. 13-11-3 a) Independent assortment Possibility 2 Possibility 1 Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

  10. Sources of genetic variation a) Independent assortment • “2n rule”: the number of possible chromosome sorting combinations = 2n  For humans (n = 23), there are 223 = 8,388,608 possible combinations of chromosomes based on independent assortment alone!

  11. Sources of genetic variation b) Crossing over (Prophase of Meiosis I) • homologous chromosomes pair up gene by gene and exchange homologous segments • This combines alleles that originated fromtwo (grand)parentsinto a single chromosome blond hair from G’pa red hair from G’ma red hair from G’ma red hair from G’pa brown eyes from G’ma brown eyes from G’ma blue eyes from G’pa blue eyes from G’pa Mom’s ovary cell

  12. Sources of genetic variation b) crossing over Early inMeiosis I Nonsister chromatids held together during synapsis Pair of homologs A single crossing over event leads to 4 genetically unique daughter cells! during Meiosis I(at anaphase I) during Meiosis II(at anaphase II) Daughter cells Recombinant chromosomes

  13. Human cells → n = 23 What is n for the cells shown here? • 1 • 2 • 3 • 4 • 5

  14. Which cells in this picture are haploid? • all • none • those above line #1 • those below line #1 • only those below line #2 1 2

  15. A detailed look at meiosis FIRST CELL DIVISION = “MEIOSIS I” 2nd CELL DIVISION = “MEIOSIS II”

  16. Sources of genetic variation > 70 trillion possible offspring!!! c) Random fertilization 8.4 million possible gametes 8.4 million possible gametes

  17. Today’s Agenda • Where does variation come from? • Mendelian Genetics, Part One

  18. Foundations of Genetics Chapter 14

  19. Outline The work of Gregor Mendel Probability and genetic outcomes Ah, if only it were so simple: complications on genes and traits

  20. 1. Mendel Fig. 14-2a a) The scientific method 1 TECHNIQUE: “crossing” or “hybridizing” true-breeding varieties 2 Parental generation (P) Stamens Carpel 3 4

  21. 1. Mendel Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation 224 white-flowered plants 705 purple-flowered plants

  22. 1. Mendel Making sense of the data: Why were ALL the F1 flowers purple? Why were some F2 flowers white? Why was the ratio in the F2 generation 3:1? To explain the data, Mendel developed a model

  23. 1. Mendel Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses (remember, DNA had not yet been discovered!) • Alternative versions of heritable “particles” (i.e., different alleles of the same gene)

  24. 1. Mendel Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent

  25. 1. Mendel Fig. 14-4 Mendel’s explanatory framework Diploid organisms Allelefor purple flowers Homologous pair of chromosomes Location of lower color gene Allele for white flowers

  26. 1. Mendel Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent all F1 purple some F2 white, F2 purple:white ratio 3:1

  27. 1. Mendel Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent 3. If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance all F1 purple some F2 white, F2 purple:white ratio 3:1

  28. 1. Mendel Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent 3. Some alleles are “dominant”, others “recessive” 4. “Law of segregation” = the two alleles for a character are separated (segregated) during gamete formation and end up in different gametes

  29. 1. Mendel b) Mendel’s explanatory framework Mendel’s Model: 4 related hypotheses Alternative versions of heritable “factors” (i.e., alleles) account for variations in inherited characters 2. For each character an organism inherits two alleles, one from each parent 3. Some alleles are “dominant”, others “recessive” 4. “Law of segregation” • all F1 purple • some F2 white, • F2 purple:white ratio 3:1

  30. Outline The work of Gregor Mendel Probability and genetic outcomes Ah, if only it were so simple: complications on genes and traits

  31. 2. Probability and genetic outcomes EXPERIMENT P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F1 individuals and their gametes RR rr homozygous

  32. 2. Probability and genetic outcomes F1 individuals and their gametes F1 Generation (hybrids) All plants had purple flowers Possible gamete types (with respect to flower color)?

  33. Fig. 14-5-3 R R P Generation Rr Rr Purple flowers Appearance: White flowers r Genetic makeup: RR rr r Gametes: R Rr Rr r F1 Generation Appearance: Purple flowers Genetic makeup: Rr r Gametes: 1/2 1/2 R heterozygous Sperm F2 Generation r R R RR Rr Eggs r rr Rr 3 1

  34. Fig. 14-5-3 Mendel’s “Law” of segregation is used to construct a “Punnett square”  this simple square tells you the expected frequencies of genotypes and phenotypesfrom a particular cross

  35. Fig. 14-5-3 P Generation Purple flowers Appearance: White flowers Genetic makeup: RR rr Reviewing the numbers with respect to this flower color gene:  2 alleles x 2 alleles = 4 outcomes • only 3 distinct genetic types, or genotypes, 1:2:1 • only two distinct traits, or phenotypes, 3:1 r Gametes: R F1 Generation Appearance: Purple flowers Genetic makeup: Rr r Gametes: 1/2 1/2 R Sperm F2 Generation r R R RR Rr Eggs r rr Rr 3 1

  36. Testcross: a useful tool How can we figure out the GENOTYPE of a purple flower?  could be PP or Pp

  37. Testcross: a useful tool How can we figure out the GENOTYPE of a purple flower? (A) PP x (B) Pp PP or Pp? What do we cross the purple flower with? (C) pp

  38. Today’s Exit Ticket • Create and complete two Punnet squares: • A testcross of a heterozygote (rr x Rr) • A testcross of a homozygous dominant individual (rr x RR) • Explain why using a homozygous recessive individual is useful for distinguishing between Rr and RR.

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