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POST MENDELIAN GENETICS

POST MENDELIAN GENETICS. EXTENDING MENDEL. Thomas Hunt Morgan:. MORGAN & Drosophila. Morgan Revealed Extensions of Mendel’s Rules:. MORGAN’S WORK WITH Drosophila. Red Eyes In Flies Is Normal (wild). White Eyes Is Rare (mutation). MORGAN & Drosophila.

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POST MENDELIAN GENETICS

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  1. POST MENDELIAN GENETICS

  2. EXTENDING MENDEL • Thomas Hunt Morgan:

  3. MORGAN & Drosophila Morgan Revealed Extensions of Mendel’s Rules:

  4. MORGAN’S WORK WITH Drosophila Red Eyes In Flies Is Normal (wild) White Eyes Is Rare (mutation)

  5. MORGAN & Drosophila • To Explore How Eye Color is Inherited in Flies Morgan crossed: • Red-eyed Female White-eyed Male X

  6. MORGAN & Drosophila • Next, Morgan crossed F1: Red-eyed Female Red-eyed Male X

  7. THE DISCOVERY OF SEX CHROMOSOMES Nettie Stevens: Observed differences in chromosomes between male & female beetles • Chromosomes named X and Y: • Half male gametes contain Y, other half X • All female gametes contain X

  8. THE DISCOVERY OF SEX CHROMOSOMES Nettie Stevens: • Developed hypothesis about sex determination: • Male is formed when • Female is formed when X X Y

  9. NORMAL SEX CHROMOSOMES A Male

  10. BACK TO MORGAN… Gamete Formation In a Male Fly Morgan guessed that D. melanogaster (like T. molitor) had chromosomes that differ between sexes: sperm sperm

  11. MORGAN & Drosophila The X Linked Hypothesis: • Morgan hypothesized that the gene controlling eye color

  12. PRACTICE SEX LINKED PROBLEM Cross a heterozygous red-eyed female with a red-eyed male • R = red eyes, r = white eyes • Genotype of female parent = • Genotype of male parent =

  13. PRACTICE SEX LINKED PROBLEM PARENT: XR / Xr • Red eyed • female • crossed • with Red • eyed male • R = red • r = white PARENT: XR / Y ** hint: gametes along top, “whole people” on inside

  14. MORGAN & Drosophila Morgan’s work on Drosophila provided evidence that: • The X chromosome contains genes the Y doesn’t: • Inheritance patterns of sex-linked genes vary between sexes • Genes are located on chromosomes

  15. GENE LINKAGE Linkage: • Autosomal Genes: • Reside on the autosomal chromosomes • In humans: genes located on chromosome # • Sex-Linked Genes: • Found on sex chromosomes • In humans: genes found on #

  16. MORGAN & LINKED GENES • First examples of linked genes were found on X chromosome of Drosophila: • Morgan established that eye color & body color are linked traits • Both found on X chromosome of fruit fly

  17. MORGAN & LINKED GENES • Morgan re-evaluated Mendel’s Principle of Independent Assortment • Morgan predicted: • Linked genes should be transmitted together during gamete formation

  18. LINKAGE HYPOTHESIS White eyes A Female (2X chromo) Red eyes Body Color: Gray body = wild type (G) Yellow body = mutant (g) R R r r Hypothesis (Morgan): When two genes occur on ONE chromosome, INDEPENDENT ASSORTMENT DOES NOT OCCUR Gray body g g G G Yellow body Eye Color: Red = wild type (R) White = mutant (r) Meiosis I R R r r G G g g Meiosis II R R r r Gametes g G G g rG Rg ONLY 2 gamete types

  19. GENETIC RECOMBINATION: PRODUCTION OF NEW COMBINATION OF TRAITS Linked genes can become unlinkedthrough cross over r R G g G g Crossing over during meiosis I r r R R G g G g Meiosis II r r R R Gametes g G G g rg RG rG Rg Recombinant chromosomes

  20. RECOMBINATION PROBABILITY • The farther apart two genes, the higher the probability they will be separated during crossover:

  21. RECOMBINATION PROBABILITY • A & D are more likely than B & C to become separated (unlinked) • B & C more likely to be inherited together (stay linked)

  22. GENE MAPPING • Maps of genes can be constructed from recombination data Linkage map: genetic map based on recombination frequencies Crossing Over

  23. % recombinant gametes Reflects distance between 2 loci GENE MAPPING Gene 1 Crossing over rarely occurs between adjacent loci, recombinations are rare. 0 : Yellow body 0 : Yellow body 0 : Yellow body Gene 2 1.4 : White eyes 1.4 : White eyes Gene 3 Gene 4 Map units Gene 5 Gene 6 Gene 7 20 : Cut wings Crossing over almost always occurs between distant loci, recombinations are frequent. Gene 8 Gene 9 Gene 10 Gene 11 Gene 12 Linkage map Chromosomes are composed of genes The physical distance between loci determines the frequency of crossing over Frequency of cross over can be used to map physical between loci

  24. GENE MAPPING • If % ofrecombinant gametes is high, 2 genes are assumed to be far apart on a chromosome • Map Unit refers to distance between 2 loci • Function of % recombinants

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