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Chapter 10 Mendelian Genetics

Chapter 10 Mendelian Genetics. Gregor Mendel. Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used experimental method used quantitative analysis collected data & counted them excellent example of scientific method.

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Chapter 10 Mendelian Genetics

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  1. Chapter 10Mendelian Genetics

  2. Gregor Mendel • Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas • used experimental method • usedquantitative analysis • collected data & counted them • excellent example of scientific method

  3. Mendel’s work • Bred pea plants • cross-pollinated true breeding parents (P) • raised seed & then observed traits (F1) • filial • allowed offspring to cross-pollinate & observed next generation (F2)

  4. Mendel collected data for 7 different pea traits, each with two different versions… and roughly got the same 3:1 ratio every time! What did this mean?

  5. true-breeding purple-flower peas true-breeding white-flower peas 100% purple-flower peas 1st generation (F1 hybrids) 100% 75% purple-flower peas 25% white-flower peas 3:1 2nd generation (F2) Looking closer at Mendel’s work Parents generation (P) X phenotype self-pollinate

  6. What did Mendel’s findings mean? • Traits come in alternative versions • purple vs. white flower color • alleles • different alleles vary in the sequence of nucleotides at the specific locus of a gene purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus different versions of gene on homologous chromosomes

  7. What are theadvantages ofbeing diploid? Traits are inherited as discrete units • For each characteristic, a diploid organism inherits 2 alleles, 1 from each parent • diploid organism • inherits 2 sets of chromosomes, 1 from each parent • homologous chromosomes • like having 2 editions of encyclopedia • Encyclopedia Britannica • Encyclopedia Americana

  8. What did Mendel’s findings mean? • Some traits mask others • purple & white flower colors are separate traits that do not blend • purple x white ≠ light purple • purple masked white • dominant allele • fully expressed • recessive allele • no noticeable effect • the gene makes a non-functional protein

  9. Prevalence of Dominance • Because an allele is dominant does not mean… • it is better • it is more common Polydactyly: dominant allele

  10. Polydactyly Individuals are born with ‘extra’ fingers or toes! Is dominantto the recessive allele for 5 digits… Recessive allele far more common than dominant.  399 individuals out of 400 have only 5 digits  most people are homozygous recessive (aa)

  11. P F1 Genotype vs. phenotype • difference between how an organism “looks” & its genetics • phenotype • description of an organism’s trait • genotype • description of an organism’s genetic makeup Explain Mendel’s results using dominant&recessive phenotype&genotype

  12. PP pp x Making crosses • using representative letters • flower color alleles  P or p • true-breeding purple-flower peas  PP • true-breeding white-flower peas  pp Pp

  13. self-pollinate Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X P PP pp phenotype F1 generation (hybrids) 100% purple-flower peas 100% purple-flower peas 100% 100% Pp Pp Pp Pp 75% purple-flower peas 75% purple-flower peas 25% white-flower peas 3:1 3:1 F2 generation ? ? ? ?

  14. PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Punnett squares % genotype % phenotype Pp x Pp PP Pp Pp pp 1:2:1 3:1

  15. Genotypes • Homozygous = same alleles = PP, pp • Heterozygous = different alleles = Pp homozygousdominant heterozygote (hybrid) homozygousrecessive

  16. purple PP homozygous dominant purple Pp heterozygous Phenotype vs. genotype • 2 organisms can have the same phenotype but have different genotypes

  17. So howdo you figure outthe genotype? Dominant phenotypes • It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it. PP? Pp?

  18. Test cross • Cross-breed the dominant phenotype — and unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x is itPP or Pp? pp

  19. x x Test cross PP pp Pp pp p p p p Pp Pp Pp Pp P P P Pp Pp p pp pp 100% 50%:50%1:1

  20. P P P p p PP Pp pp p Mendel’s laws of heredity (#1) • Law of segregation • when gametes are produced during meiosis, homologous chromosomes separate from each other • each allele for a trait is packaged into a separate gamete

  21. Meiosis 1 And Mendeldidn’t even knowDNA or genesexisted! Law of Segregation • What meiotic event creates the law of segregation?

  22. Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters • flower color • seed color • monohybrid crosses

  23. Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characteristics • dihybrid crosses • seed color andseed shape

  24. F1 generation (hybrids) yellow, round peas 100% self-pollinate 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas 9:3:3:1 F2 generation Dihybrid cross P true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr

  25. YyRr YyRr YR YR yR Yr yr yr Which systemexplains the data? WHY? What’s going on here? • How are the alleles on different chromosomes handed out? • together or separately?

  26. 9/16 yellow round 3/16 green round YR Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

  27. Mendel’s laws of heredity (#2) • Law of independent assortment • each pair of alleles segregates into gametes independently • 4 classes of gametes are produced in equal amounts • YR, Yr, yR, yr • only true for genes on separate chromosomes YyRr Yr Yr yR yR YR YR yr yr

  28. Remember…Mendel didn’t even know DNA —or genes—existed! Meiosis 1 Law of Independent Assortment • What meiotic event creates the law of independent assortment?

  29. The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring!

  30. Review: Mendel’s laws of heredity • Law of segregation • monohybrid cross • single trait • each allele segregates into separate gametes • established by Meiosis 1 • Law of independent assortment • dihybrid (or more) cross • 2 or more traits • each pair of alleles for genes on separate chromosomes segregates into gametes independently • established by Meiosis 1

  31. Mendel chose peas wisely • Pea plants are good for genetic research • available in many varieties with distinct heritable features with different variations • flower color, seed color, seed shape, etc. • Mendel had strict control over which plants mated with which • each pea plant has male & female structures • pea plants can self-fertilize • Mendel could also cross-pollinate plants: moving pollen from one plant to another

  32. Mendel chose peas luckily… • Pea plants are good for genetic research • relatively simple genetically • most characters are controlled by a single gene • each gene has only 2 alleles, one of which is completely dominant over the other

  33. Nerds are good to have too! Any Questions??

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