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Today:

Today:. Microbial Genetics Wrap-up Mendelian Genetics Adding Chromosomes to the Mix??. Tomorrow: UW Fieldtrip!. Back to Eukaryotes: Bringing in Mendel…. If DNA replication and cell division are both so precise, and so accurate, why are we all so unique??.

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Today:

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  1. Today: • Microbial Genetics Wrap-up • Mendelian Genetics • Adding Chromosomes to the Mix?? Tomorrow: UW Fieldtrip!

  2. Back to Eukaryotes:Bringing in Mendel… If DNA replication and cell division are both so precise, and so accurate, why are we all so unique??

  3. Meiosis Creates Genetic Diversity:1. Independent Assortment Homologous Chromosomes are INDEPENDENTLY (randomly) parceled out during Meiosis I

  4. INDEPENDENT ASSORTMENT contributes to GENETIC DIVERSITY

  5. 2. CROSS-OVER produces RECOMBINANT CHROMOSOMES, contributing to GENETIC DIVERSITY Cross-over occurs as duplicated chromosomes pair with their homologues in SYNAPSIS. During this process, nonsister chromosomes cross at CHIASMATA.

  6. #3: Random Fertilization 8 million possible chromosome combinations in each egg, and each sperm… = >70 trillion possibilities! How are we able to predict ANYTHING about inheritance??

  7. Looking forward to Genetics: The Paradox Gregor Mendel, 1822-1884 Charles Darwin, 1809-1882

  8. Setting the Stage for Mendel Leading theory at the time is Blended Inheritance What makes a good model?? Mendel will need a good model organism!

  9. Mendel’s Technique: • Studies peas: • Typically Self- Fertilizing • Multiple distinct CHARACTERS, with easy to identify TRAITS • Several TRUE-BREEDING varieties available

  10. What Mendel Observes, Part 1: What does this data suggest about “blended inheritance”?

  11. What Mendel Observes, Part 2: What does this data suggest about “blended inheritance”?

  12. How would you explain Mendel’s results? (Can you reconcile what he observed with what we know about chromosomes and meiosis??) Create a hypothesis to explain his new results!

  13. Mendel’s Hypothesis- Part 1 Different genes account for the variation in inherited characters

  14. Mendel’s Hypothesis- Part 2 For each character, an organism inherits two alleles, one from each parent.

  15. Mendel’s Hypothesis- Part 3 If the alleles are different, than one will control the organism’s appearance (the dominant allele) while the other will have no noticeable effect (the recessive allele)

  16. Mendel’s Hypothesis- Part 4 The two alleles are separated during gamete production (At what stage??)

  17. Testing the Law of Segregation: The Punnett Square

  18. The Punnett Square for Mendel’s Experiments: What will the F1 Generation look like? The F2 Generation?

  19. The Punnett Square for Mendel’s Experiments:

  20. vs

  21. Understanding the predicted results of a PUNNETT SQUARE, allows for a TESTCROSS What’s my phenotype? My genotype?

  22. Try a Test Cross! Part 1: In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog?

  23. Using Simple Mendelian Genetics Sickle Cell Disease

  24. Sickle Cell Disease Questions: Part 2A: Two individuals who are heterozygous at the Sickle Cell locus have four children together. One of the children is affected with the disorder. Based on this information, is the sickle cell trait dominant or recessive?

  25. Sickle Cell Disease Questions: 2B. If the affected offspring has a child with an unaffected individual (who does not carry the sickle allele), what is the probability that any given child will be unaffected? Be a carrier? Be affected?

  26. An Aside: Unusual Gene Frequencies!? What do you notice? What does this suggest?

  27. Mendelian Genetics- Example 3: Cystic Fibrosis is also an Autosomal Recessive Trait with Unusual Gene Frequencies A. If two carriers of the cystic fibrosis trait have children, what is the probability that their first child will be affected? B. If they eventually have three children, what is the probability that all three will be affected?

  28. Calculating Probabilities

  29. Huntington’s Disease Figure 1. Samples of coronal and sagittal magnetic resonance imaging from a patient with Huntington's disease (top row) and a normal control (bottom row) showing the outlines of caudate and putamen (left), cerebral (center) and cerebellar volumes (right). H.H. Ruocco, I. Lopes-Cendes, L.M. Li, M. Santos-Silva and F. Cendes. 1129 Striatal and extrastriatal atrophy in Huntington’s disease and its relationship with length of the CAG repeat. Braz J Med Biol Res 2006; 39: 1129-1136 

  30. Dependent Assortment? Mendel’s Next Question: What happens in a dihybrid cross? What would the outcome look like if it’s dependent assortment??

  31. What Mendel Sees: So is it dependent assortment??

  32. Try a Messy Dihybrid Cross! 5A. What fraction (or number) of the offspring of the couple described would be homozygous tongue-rollers who are non-tasters (RRtt)??

  33. Mendel’s Contributions Law #1: Segregation Law #2: Independent Assortment

  34. Complication #1: (Mendel was lucky!) INCOMPLETE DOMINANCE Heterozygotes have a unique phenotype, between that of the homozygous dominant or recessive parents. Note: This is not blended inheritance! Why?

  35. Complication #1: (Mendel was lucky!) INCOMPLETE DOMINANCE

  36. Another Exception:Codominance • In codominance, both alleles affect the phenotype in separate, distinguishable ways. • Example: • Human blood groups M, N, and MN • Group MN produce both antigens on the surface of blood cells

  37. Another Exception:Codominance Example: Tay-Sachs disease- Heterozygous individuals produce both functional, and dysfunctional enzymes. A section of the brain of a Tay Sachs child. The empty vacuoles are lysosomes that had been filled with glycolipid until extracted with alcohol in preparing the tissue. organismal level = recessive biological level = codominant

  38. Three Important Points about Dominant/Recessive Traits: • They range from complete dominance  incomplete dominance  codominance. (can be a subtle distinction!) • They reflect mechanisms through which specific alleles are expressed in the phenotype (i.e. this is not one allele subduing another at the DNA level) • They’re not related to the abundance of an allele within a population!

  39. Further Complications: Multiple Alleles

  40. Further Complications: Multiple Alleles

  41. Practice Question 6: Paternity Testing Scenario : Suppose mother is Type A, baby is Type B. Consider these three putative fathers: can any be the biological father? #1 (Type A): Yes or No? #2 (Type B): Yes or No? #3 (Type O): Yes or No?

  42. Further Complications: Pleiotropy Most genes have multiple phenotypic effects!

  43. Further Complications: Pleiotropy No production of melanocytes during development causes: 1. White fur color and 2. Inability to transmit electrical signals to brain from hair cells in the ear.

  44. More Complications: EPISTASIS Example: The “color gene”, C, allows pigment to be deposited in hair. When lacking, a mouse is albino, regardless of its genotype at the other locus.

  45. Epistasis and Lab Pups Black is dominant to Brown, so Heterozygotes (Bb) are black. The delivery gene is also dominant, so EE or Ee individuals both express their pigments. Only ee individuals are yellow. Coat color in labradors is determined by 2 genes, a pigment gene (B), and a pigment delivery gene (E).

  46. Epistasis and Lab Pups Your Question (7): If I cross a Brown Lab (bbEe) with a Black Lab (BbEe), can I expect any yellow puppies? If so, what proportion of the pups would I expect to be yellow?

  47. There’s more… Polygenic Inheritance This results in a broad norm of reaction

  48. Other Issues: Environmental Effects on Phenotype Many factors, both genetic and environmental, influence the phenotype.

  49. Similarities between the behavior of chromosomes and Mendel’s “factors”: ?

  50. Similarities between the behavior of chromosomes and Mendel’s “factors”: • Chromosomes and genes are both present in paired in diploid cells • Homologous chromosomes separate and alleles segregate during meiosis • Fertilization restores the paired conditions for both chromosomes and genes

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