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Announcements

Announcements. Start looking over chapter 6 problems 10, 14, 15, 28. Quiz next Friday 9/27 on linkage Please print powerpoint notes 6/pg if in Brooks Hall.

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Announcements

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  1. Announcements Start looking over chapter 6 problems 10, 14, 15, 28. Quiz next Friday 9/27 on linkage Please print powerpoint notes 6/pg if in Brooks Hall. Reminder- papers on “Monk in the garden” due in lab section 10/1, 10/2. I encourage you to see me to discuss your outlines. I am also willing to look over first drafts prior to the due date. When you cite examples from the book, cite specific pages. I will not discuss the exam until everyone has taken it; ie. Monday after class at the earliest. Goal is to have written portion graded by Friday, 9/27.

  2. Outline of Lecture 10 I. Discussion of “Monk in the garden” II. Genes linked on the same chromosome segregate together III. Crossing over

  3. Writing Assignment Topic 1 Discuss several of the reasons that we now acknowledge Mendel as the father of genetics. You should mention some of the practical issues that resulted in Mendel’s experiments successfully unlocking the secrets of heredity. You should also discuss some of the reasons his “genius” was not widely known until 35 years after he presented his findings on the “ laws of inheritance”. Lastly, present an argument on whether or not Mendel was truly a “genius”. Support your stance with specific examples form the book

  4. Topic 2 - Writing assignment Which of the three rediscoverers do you most respect and why? Cite examples from the book to explain your answer. What aspects of “being a scientist” surprised or interested you as you learned about the personalities of Mendel and his rediscoverers? Cite specific examples from the book and explain why you were surprised or interested

  5. Topic 3 - Writing assignment How could Darwin’s theories of evolution and Mendel’s theories of inheritance have influenced one another? IF they had crossed paths, who might have benefited more from knowing the other’s work? Explain why. How did theories of evolutionary change influence each of the three rediscoverers

  6. Darwinian Selection All natural selection results in evolution, but not all evolution is the product of natural selection. • What is evolution? • Evolution is the change in allele frequencies (or traits) over time. • What is natural selection? • Natural selection is the differential reproductive success resulting from an adaptation. • What is an adaptation? • An allele (or trait) which provides superior reproductive success to an individual possessing it relative to an individual with a different allele (or trait).

  7. EVOLUTIONARY PROCESSES AND PATTERNS • Theories of Evolution • Darwin and Wallace • Species are related to one another, and they change over time, thus species existing today have descended, with modifications, from other preexisting species. • Natural selection acts on individuals; individuals with certain favorable characteristics will produce more offspring.

  8. Darwinian Selection • The Modern Synthesis • Combines genetics, geology, and natural selection • Gradual evolution results from small genetic changes acted upon by natural selection • Speciation and macroevolution can be explained by microevolution given the age of the earth

  9. Darwinian Selection • Modern Synthesis - Darwin’s postulates restated • Mutation is the ultimate source of all variation, segregation and independent assortment create novel combinations of alleles • Alleles are passed between generations • More offspring are produced than can survive • Individuals with the most advantageous allelic combination for the current situation produce more offspring.

  10. Topic 3 - Writing assignment (A) How could Darwin’s theories of evolution and Mendel’s theories of inheritance have influenced one another? (B) IF they had crossed paths, who might have benefited more from knowing the other’s work? Explain why. (C) How did theories of evolutionary change influence each of the three rediscoverers? OR How did Willilam Bateson’s interactions with Mendel’s paper and with the rediscoverers affect the spread of Mendelism? How did Bateson’s ideas about evolution affect the spread of Mendelism and the creation of genetics as a scientific discipline?

  11. II. Linkage and Crossing Over • Linkage happens when genes don’t assort independently. • Genes on the same chromosome are linked. • Genes linked on the same chromosome segregate together. • Crossing Over involves reciprocal exchange of chromosome segments between homologs; increases genetic variation (recombination).

  12. chiasma Chiasmata Between SynapsedHomologs During Meiosis Prophase I

  13. Crossing Over and Mapping • Linkage without crossing over creates only parental (noncrossover) gametes. • Linkage with crossing over creates parental gametes and recombinant (crossover) gametes. • Interlocus distance is proportional to the degree of crossing over between. • Little or no crossing over in close genes. • Frequent, even multiple crossovers between distant genes. • Chromosome map, determined from recombination rates, indicates relative locations of genes on a chromosome.

  14. No Linkage: Independent Assortment

  15. Linkage without Recombination

  16. Linkage with Recombination

  17. Complete Linkage: P1 Cross In complete linkage only parental gametes form

  18. Complete Linkage: F1 Cross Not 1:1:1:1 testcross ratio! 1:1 testcross ratio w/linkage Not 9:3:3:1 phenotypic ratio! 1:2:1  (complete) linkage ratio

  19. Linkage Ratio • The F2 phenotypic ratio unique to two linked genes in cross of double heterozygotes. • If completely linked, should be • 1:2:1 for F1 X F1 • 1:1 for F1 X test cross parent • Linkage group - group of genes which show linkage; in theory = N (the haploid number).

  20. III. Morgan and Crossing Over • Morgan discovered crossing over when studying two genes on X chromosome in Drosophila. • Morgan proposed that the chiasmata visible on chromosomes were regions of crossing over. • Occurs between nonsister chromatids.

  21. Crosses of Two X-linked genes Expect only parental types if no crossing over occurs - Confirm this for yourself with a Punnett square.

  22. Morgan’s Interpretation • Recombination was caused by linear arrangement of genes and crossing over. • Frequency of recombination was determined by distance between genes: • y and w recombination rate = 1.3% • w an m recombination rate = 37.2% • Therefore y and w were closer together on the chromosome, while w an m are farther apart.

  23. Sturtevant and Mapping • Sturtevant, Morgan’s undergraduate student, discovered frequency of crossing over between all 3 genes: • yellow, white 0.5% • white, miniature 34.5% • yellow, miniature 35.4%

  24. Sturtevant’s Interpretation • Sturtevant reasoned that recombination frequencies were additive, so order of genes on chromosome was yellow-white-miniature.

  25. You only see recombination when it occurs between the genes you are watching!

  26. Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes

  27. 2-point Mapping Considers only Two Genes at a Time Drosophila chromosome I (X)

  28. Map Units • One map unit (centimorgan, cM) = 1% recombination between two genes • yellow and white are 0.5 cM apart • yellow and miniature are 35.4 cM apart • white and miniature are (35.4-0.5) = 34.9 cM apart • In Drosophila, crossing over occurs only in females, never in males.

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