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Genes and Chromosomes

Genes and Chromosomes. Gene Linkage, Crossing Over, Codominance and Incomplete Dominance. Mendel and Chromosomes. Mendel never asked “Where in the cell are the factors that control heredity?” i.e. where are the genes He couldn’t have answered the question though

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Genes and Chromosomes

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  1. Genes and Chromosomes Gene Linkage, Crossing Over, Codominance and Incomplete Dominance

  2. Mendel and Chromosomes • Mendel never asked “Where in the cell are the factors that control heredity?” • i.e. where are the genes • He couldn’t have answered the question though •  His promotion to abbot kept him too busy

  3. Where are they? • By 1900 cell biologists had discovered most organelles and the process of mitosis and meiosis • Nucleus was logical place

  4. Why the Nucleus? • Central Location • Large size • Lots of activity during mitosis and meiosis

  5. Chromosome Theory of InheritanceWalter Sutton – Columbia University • Genes are located on chromosomes • Each gene occupies a specific place on a chromosome • A gene may exist in several forms, or alleles • Each chromosome contains one allele for each gene

  6. Thomas Morgan • Embryologist at Columbia University • Worked with fruit flies

  7. Fruit Flies?! …Yuck!

  8. Drosophila melanogaster • Prolific reproducers: a single cross will produce 100s of offspring • 4 pairs of chromosomes • Larva hatch within two weeks

  9. Genes on a Chromosome • Each chromosome has hundreds or thousands of genes • Genes on the same chromosome tend to be inherited together • Remember: a chromosome is passed along as a single unit during meiosis

  10. Thomas Morgan • First to associate a specific gene with a specific chromosome • Crossed gray, normal winged homozygous parents with black, vestigial wing parents. • GGNN x ggnn = GgNn • Then test crossed GgNn x ggnn • Let’s do the Punnet Square:

  11. Expected Punnet Square

  12. Expected: • 25% Gray, Normal wing • 25% Gray, Vestigial wing • 25% Black, Normal wing • 25% Black, Vestigial wing

  13. Actual Phenotypes: • 41.5 % Gray, Normal • 41.5% Black, Vestigial • 8.5% Gray, Vestigial • 8.5% Black, Normal

  14. If they are linked… • Let’s let G = G and N • Let’s let g = g and n • So GG would be GGNN and … • Gg would be GgNn and … • gg would be ggnn.

  15. If they are linked…

  16. If they are linked… • Don’t forget that Gg is GgNn and gg is ggnn • So we have 50% Gray bodied, Normal wing • And 50% Black bodied, Vestigial wing • This explains the variation from the original expected.

  17. Morgan’s Hypothesis • Body color (gray or black) and wing shape (normal or vestigial) are usually inherited together in a specific combination due to GENE LINKAGE

  18. Gene Linkage • Genes located on the same chromosome tend to be inherited together because they are part of a single chromosome that is passed along as a unit.

  19. Morgan’s Crosses • 83% of Morgan’s flies had characteristics of their parents. (gray/normal wings or black/ vestigial wings.) (41.5% gray/normal + 41.5% black/vestigial =83%)

  20. 17% of the flies had either a)Gray, vestigial wings b)Black, normal wings (8.5% gray, vestigial + 8.5% black, normal = 17%)

  21. Morgan and Sturtevant (his associate) hypothesized that linkages could sometimes be broken • Gray/normal and black/vestigial genes can occasionally be separated

  22. INTERESTING QUESTION: • If body color and wing shape genes are linked, how come all of the offspring were not like their parents (gray/normal and black/vestigial)?????????????

  23. CROSSING OVER IS TO BLAME!!!!!

  24. CROSSING OVER • DURING MEIOSIS, HOMOLOGOUS CHROMOSOMES EXCHANGE PORTIONS OF THEIR CHROMATIDS

  25. CROSSING OVER

  26. GENE MAPPING • Distance between chromosomes determines how often crossing over occurs • Further away = more crossing over • Frequency of crossing over allows mapping of position on chromosomes

  27. Gene Mapping Diagram • Genes A & B are very close together • C & D are farther apart • A random crossover event is much more likely to separate genes C & D than A and B (which are closer together).

  28. Sex Linkage • 1 exception to rule that every chromosome has a corresponding homologous chromosome … • 1905 American – Nettie Stevens – mealworms • Female contains 20 large chromosomes • male – 19 and one small

  29. These are sex chromosomes • Female: XX • Male: XY • Because X and Y determine sex, genes located on one of these chromosomes are called “Sex-Linked genes”

  30. HERE’S AN EXAMPLE: • Genes associated with vision in humans are located on the X chromosome. • Males only have ONE X chromosome!!!! This can be a problem if the X chromosome given to him by Mom has a genetic disorder!

  31. Red-Green Color Blind?

  32. Red-Green Color Blind?

  33. Colorblindness: • Your mother unknowingly has the genotype XCXc. Dad is not colorblind and therefore has the genotype XCY. • Will any of the children be colorblind??????

  34. XC Xc XC XCXC XCXc Y XCY XcY • Answer: Yes! One of the sons will be colorblind because he will have the genotype XcY.

  35. Revenge against Men! • Males have just one X chromosome. Therefore, if the mom gives them a recessive allele on her X chromosome, it will be expressed in the son EVEN though the mom’s phenotype did not express the allele!

  36. INCOMPLETE DOMINANCE • Josef Kolreuter – crossed white carnations (rr) with Red (RR) carnations. • Got all Pink!!! (Rr)

  37. INCOMPLETE DOMINANCE • Did red and pink blend together????

  38. DID THE COLORS BLEND???? • NO!!!!!!! How do we know???? • When crossed Rr x Rr (F1 x F1)……. • ¼ Red • ½ Pink • ¼ White • Let’s do the Punnett Square

  39. R r R RR Rr r Rr rr PUNNET SQUARE • 25% RR: Red • 50% Rr: Pink • 25% rr: White

  40. INCOMPLETE DOMINANCE • When mom makes a birthday cake and wants to frost it, how does she do it? • White frosting • Wants to make red frosting • no red food color makes white • little red food color makes pink • lots of red food color makes red

  41. INCOMPETE DOMINANCE • Same with genes • Genes code for a protein • If the protein is a pigment, like carnations: • no dose = white • single dose = pink • double dose = red

  42. CODOMINANCE • What does dominant mean? • In codominance, both alleles are expressed

  43. CODOMINANCE EXAMPLE • On sports teams, often have two captains. Do both captains have equal representation? • Sure they do!!!!

  44. CODOMINANCE • The same is true for codominant alleles…both are equally expressed within a phenotype. • Written as a capital letter with a superscript • HR or HW means the trait is hair color and red and white are both dominant.

  45. CATTLE: • Cattle – Red hair (HR) and White hair (HW) are codominant • HRHR= Red hair • HW HW= White hair • HR HW = roan or combination – rust colored (Red and white)

  46. Try This! • What would you get if you crossed a Roan with a White haired cow? • Do the punnett square:

  47. HR HW HW HR HW HWHW HW HR HW HWHW PUNNET SQUARE • 25% ROAN • 50% WHITE

  48. NONDISJUNCTION • Whole chromosomes or sets of chromosomes fail to separate normally during meiosis • Example: Down’s syndrome • When all chromosomes fail to separate, can cause 3N or 4N organisms.

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