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Chapter 15 The Chromosomal Basis of Inheritance

Chapter 15 The Chromosomal Basis of Inheritance. Timeline. 1866- Mendel's Paper 1875- Mitosis worked out 1890's- Meiosis worked out 1902- Sutton, Boveri et. al . connect chromosomes to Meiosis. Sutton. Developed the “Chromosome Theory of Inheritance”.

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Chapter 15 The Chromosomal Basis of Inheritance

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  1. Chapter 15 The Chromosomal Basis of Inheritance

  2. Timeline • 1866- Mendel's Paper • 1875- Mitosis worked out • 1890's- Meiosis worked out • 1902- Sutton, Boveri et. al. connect chromosomes to Meiosis.

  3. Sutton • Developed the “Chromosome Theory of Inheritance”. • Mendelian factors or alleles are located on chromosomes. • Chromosomes segregate and show independent assortment.

  4. Morgan • Chose to use fruit flies as a test organism in genetics. • Allowed the first tracing of traits to specific chromosomes.

  5. Fruit Fly • Drosophila melanogaster • Early test organism for genetic studies.

  6. Reasons • Small • Cheap to house and feed • Short generation time • Many offspring • Few chromosomes

  7. Genetic Symbols • Mendel - use of uppercase or lowercase letters. T = tall t = short • Morgan: symbol from the mutant phenotype. + = wild phenotype

  8. Examples • Recessive mutation: • w = white eyes • w+ = red eyes • Dominant Mutation • Cy = Curly wings • Cy+ = Normal wings

  9. Morgan Observed: • A male fly with a mutation for white eyes.

  10. Morgan crossed • The white eye male with a normal red eye female.

  11. The F1 offspring: • All had red eyes. • This suggests that white eyes is a genetic _________? • Recessive.

  12. F1 X F1 = F2 • Morgan expected the F2 to have a 3:1 ratio of red:white • He got this ratio, however, all of the white eyed flies were MALE. • Therefore, the eye color trait appeared to be linked to sex.

  13. Morgan discovered: • Sex linked traits. • Genetic traits whose expression are dependent on the sex of the individual.

  14. Fruit Fly Chromosomes

  15. Sex linked traits • Sex linked traits in humans will be covered in a few minutes.

  16. Morgan Discovered • There are many genes, but only a few chromosomes. • Therefore, each chromosome must carry a number of genes together as a “package”.

  17. Linked Genes • Traits that are located on the same chromosome. • Result: • Failure of Mendel's Law of Independent Assortment. • Ratios mimic monohybrid crosses.

  18. Body Color and Wing type

  19. Example b+bvg+vg X bb vgvg (b+ linked to vg+) (b linked to vg) If unlinked: 1:1:1:1 ratio. If linked: ratio will be altered.

  20. Crossing-Over • Breaks up linkages and creates new ones. • Recombinant offspring formed that doesn't match the parental types.

  21. If Genes are Linked: • Independent Assortment of traits fails. • Linkage may be “strong” or “weak”.

  22. Linkage Strength • Degree of strength related to how close the traits are on the chromosome. • Weak - farther apart • Strong - closer together

  23. Genetic Maps • Constructed from crossing-over frequencies. • 1 map unit = 1% recombination frequency.

  24. Comment - only good for genes that are within 50 map units of each other. Why?

  25. Genetic Maps • Have been constructed for many traits in fruit flies, humans and other organisms.

  26. Sex Linkage in Biology • Several systems are known: • Mammals – XY and XX • Diploid insects – X and XX • Birds – ZZ and ZW • Social insects – haploid and diploid

  27. Chromosomal Basis of Sex in Humans • X chromosome - medium sized chromosome with a large number of traits. • Y chromosome - much smaller chromosome with only a few traits.

  28. Human Chromosome Sex • Males - XYFemales - XX • Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

  29. SRY • Sex-determining Region Y chromosome gene. • If present - male • If absent - female • SRY codes for a cell receptor.

  30. Sex Linkage • Inheritance of traits on the sex chromosomes. • X- Linkage (common) • Y- Linkage (very rare if exists at all)

  31. Males • Hemizygous - 1 copy of X chromosome. • Show ALL X traits (dominant or recessive). • More likely to show X recessive gene problems than females.

  32. X-linked Disorders • Color blindness • Duchenne's Muscular Dystrophy • Hemophilia (types a and b) • Immune system defects

  33. Samples of X-linked patterns:

  34. X-linked Patterns • Trait is usually passed from a carrier mother to 1/2 of sons. • Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait.

  35. Comment • Watch how questions with sex linkage are phrased: • Chance of children? • Chance of males?

  36. Can Females be color-blind? • Yes, if their mother was a carrier and their father is affected.

  37. Y-linkage • Hairy ear pinnae. • Comment - new techniques have found a number of Y-linked markers that can be shown to run in the males of a family. • Ex: Jewish priests

  38. Sex Limited Traits • Traits that are only expressed in one sex. • Ex – prostate

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