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Beyond Mendel

Beyond Mendel. Rediscovery of Mendel’s Work. Carl Correns Erich von Tschermak Hugo De Vries. Chromosomal theory of inheritance. Walter Sutton. Theodor Boveri. Chromosomal theory of inheritance.

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Beyond Mendel

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  1. Beyond Mendel

  2. Rediscovery of Mendel’s Work Carl Correns Erich von Tschermak Hugo De Vries

  3. Chromosomal theory of inheritance Walter Sutton Theodor Boveri

  4. Chromosomal theory of inheritance • Mendelian genes have specific loci (positions) along chromosomes and it is the chromosomes that undergo segregation and independent assortment.

  5. Chromosomal theory of inheritance

  6. 1 2 1 3 3 2 P Generation Yellow-roundseeds (YYRR) Green-wrinkledseeds (yyrr) y Y r  R r R Y y Meiosis Fertilization r y Y R Gametes All F1 plants produceyellow-round seeds (YyRr). F1 Generation Figure 15.2 R R y y r r Y Y Meiosis LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation. LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologous chromosomesassort independently duringgamete formation. r R r R Metaphase I Y y Y y R R r r Anaphase I Y Y y y r R R r y Metaphase II Y y Y y Y Y Y y Y y y Gametes r R r R r r R R 1/4 1/4 1/4 1/4 yr yR YR Yr F2 Generation An F1 F1 cross-fertilization Fertilization recombinesthe R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratioin the F2 generation. : 3 : 3 : 1 9

  7. P Generation Yellow-roundseeds (YYRR) Green-wrinkledseeds (yyrr) Figure 15.2a y Y r  R r R Y y Meiosis Fertilization r y R Y Gametes

  8. 1 1 2 2 All F1 plants produceyellow-round seeds (YyRr). F1 Generation R R y y r r Y Y LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation. Figure 15.2b r R r R Metaphase I y Y Y y R r R r Anaphase I Y Y y y r R r R Metaphase II y Y y Y y Y Y y Y y Y y Gametes r R R r r r R R 1/4 1/4 1/4 1/4 yr yR YR Yr

  9. 3 3 LAW OF INDEPENDENTASSORTMENT LAW OF SEGREGATION Figure 15.2c F2 Generation An F1 F1 cross-fertilization Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. : 3 : 3 : 1 9

  10. Thomas Hunt Morgan

  11. The common fruit fly – Drosophila melanogaster

  12. Red eye – the “wild type” White eye – a mutant Drosophila melanogaster

  13. EXPERIMENT PGeneration F1Generation All offspringhad red eyes. RESULTS F2Generation CONCLUSION w w PGeneration X X Y X w w Sperm Eggs F1Generation w w w w w Sperm Eggs w w w F2Generation w w w w w

  14. EXPERIMENT PGeneration All offspringhad red eyes. F1Generation RESULTS F2Generation

  15. CONCLUSION w w PGeneration X X Y X w w Sperm Eggs F1Generation w w w w w Sperm Eggs w w w F2Generation w w w w w

  16. Human x and y chromosomes X Y

  17. 44 XY 44 XX Parents 22 X 22 Y 22 X or Sperm Egg 44 XY 44 XX or Zygotes (offspring) (a) The X-Y system 22 X 22 XX (b) The X-0 system 76 ZZ 76 ZW (c) The Z-W system 16 (Haploid) 32 (Diploid) (d) The haplo-diploid system

  18. Sex determination in Humans • In humans, the anatomical signs of sex first appear when the embryo is about two months old. • In individuals with the SRY gene (sex-determining region of the Y chromosome), the generic embryonic gonads are modified into testes. • Activity of the SRY gene triggers a cascade of biochemical, physiological, and anatomical features because it regulates many other genes. • In addition, other genes on the Y chromosome are necessary for the production of functional sperm. • In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.

  19. XNXn XNY XnY XNXn XnY XNXN Sperm XN Xn Sperm Sperm Y Y Xn Y XNXn XNXN XNY XNY XNXn XNY Eggs XN XN XN Eggs Eggs XnY XN XNXn XnY XnXn XNXn XNY Xn Xn (a) (b) (c) • Transmission of sex-linked recessive traits: • Father with trait passes trait to all daughters - carriers • Female carrier passes trait to half her sons and daughters • Female carrier mates with male with trait – half of offspring will have trait, half of daughters will be carriers, half of males will be free of trait

  20. Duchenne Muscular Dystrophy

  21. Duchenne muscular dystrophy • Duchenne muscular dystrophy affects one in 3,500 males born in the United States. • Affected individuals rarely live past their early 20s. • This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. • The disease is characterized by a progressive weakening of the muscles and a loss of coordination.

  22. Dystrophin muscle complex

  23. X Inactivation • Although female mammals inherit two X chromosomes, only one X chromosome is active. • Therefore, males and females have the same effective dose (one copy ) of genes on the X chromosome. • During female development, one X chromosome per cell condenses into a compact object, a Barr body. • This inactivates most of its genes. • The condensed Barr body chromosome is reactivated in ovarian cells that produce ova. • Mary Lyon, a British geneticist, has demonstrated that the selection of which X chromosome to form the Barr body occurs randomly and independently in embryonic cells at the time of X inactivation. • As a consequence, females consist of a mosaic of cells, some with an active paternal X, others with an active maternal X.

  24. Mary Lyon

  25. X Inactivation Mosaic

  26. X inactivation and coat color in tortoiseshell cats

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