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Chapter 15: The Chromosomal Basis of Inheritance

Chapter 15: The Chromosomal Basis of Inheritance. Background: Are Mendel’s “laws” accurate? What is their physical basis, if any? Early 1900s: data begin to accumulate. Flemming discovers chromosomes in 1875

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Chapter 15: The Chromosomal Basis of Inheritance

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  1. Chapter 15: The Chromosomal Basis of Inheritance Background: Are Mendel’s “laws” accurate? What is their physical basis, if any? Early 1900s: data begin to accumulate

  2. Flemming discovers chromosomes in 1875 Sutton, Boveri and others notice the similarity between meiosis and Mendelian genetics Stevens discovers sex chromosomes in 1905

  3. About 1910 Thomas Hunt Morgan discovers a white-eyed mutant fruit fly

  4. Phenotypic ratio different between sexes Correlates with sex chromosomes Sex linkage

  5. Bridges discovers “exceptional” fruit flies These flies showed nondisjunction of X chromosome Phenotype correlates with nondisjunction karyotype

  6. Chromosomal Sex Determination Many species use chrmosome complement to determine sex Different mechanisms in different organisms

  7. M and F mammals have different X chromosome content How to deal with the difference? Dosage compensation or X-inactivation or “lyonization” (Mary Lyon) Controlled by Xist RNA, which shuts down all but 1 X in each cell Occurs early in development Barr bodies “epigenetic”

  8. Summary Morgan’s original data show that phenotype correlates with chromosome composition (karyotype) Bridge’s data show that phenotype correlates with karyotype even in very unusual cases. And suggest that karyotype causes gender phenotype Convincing evidence The Chromosome Theory

  9. Nondisjunction occurs in humans Chromosomes fail to segregate during meiosis Zygote receives too many or too few chromosomes Leading cause of pregnancy loss and birth defects Nondisjunction almost always fatal to embryo with exceptions for sex chromosomes and very small chromosomes

  10. Some Human Nondisjunction Disorders

  11. Mechanism of Sex Determination in Mammals Embryo develops as female by default Unless functional Y chromosome is present SRY gene on Y chromosome is the “switch” Controls activity of other genes on X and autosomes These genes work together to activate the male developmental pathway Mutations in SRY can lead to sex-reversal (XY females)

  12. Other chromsome abnormalities can cause disease • Translocation between chromosome 9, 22 • leukemia

  13. Organelle Genetics Small, circular DNA molecule with few genes Does not undergo meiosis so does not obey Mendel’s laws Inheritance is through maternal line only

  14. Many Current Applications History Human origins Genealogy Forensic ID

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