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Chapter 15 Chromosomes

Chapter 15 Chromosomes. Chromosome theory of inheritance. Genes located on chromosomes = gene locus Thomas Hunt Morgan, Columbia Univ. “Fly room”. Drosophila 100s of offspring 2n = 8 3 prs autosomes X and Y sex chromosomes. Bithorax Wildtype White eye . Wildtype = normal

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Chapter 15 Chromosomes

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  1. Chapter 15 Chromosomes

  2. Chromosome theory of inheritance • Genes located on chromosomes = gene locus • Thomas Hunt Morgan, Columbia Univ. “Fly room”

  3. Drosophila • 100s of offspring • 2n = 8 • 3 prs autosomes • X and Y sex chromosomes

  4. BithoraxWildtype White eye Wildtype = normal Mutant = abnormal Wingless eyeless

  5. Drosophila genetics White eye allele = w Wildtype allele = w+ gene locus on X chromosome Sex-linked! Genotype? ww, w+ww+w+

  6. Sex-linked genes Cross a red female with a white male (pg. 288)

  7. Xw+Xw+ X Xw Y Punnett square results Cross f1 females with f1 males

  8. f1 Xw+Xw X Xw+ Y f2  Some genes are located on the X chromosome

  9. The chromosomal basis of sex In humans, Y chromosome determines sex XX X XY Gametes? p(son) p (daughter)

  10. Humans have an X/Y system

  11. Development <2 months gestation – embryo has rudimentary gonads

  12. 2 mos. SRY gene on Y chromosome active -> testes develop -> testosterone -> male If no SRY female default pathway  ovaries

  13. X-linked genes in humans Female genotypes Male genotypes XCXCXCXcXcXc XC Y XcY Terms: homozygous, heterozygous, hemizygous Concept check: • From whom do males obtain the Y chromosome? • From whom do females obtain the X chromosomes? • Why are X-linked disorders more prevalent in males?

  14. Example: colorblindness • 1/1O males • Colorblindness is an X-linked trait. A man is colorblind. What % of his sons and daughters are expected to be colorblind. His wife does not carry the colorblind allele on either X chromosome.

  15. No green photoreceptors

  16. X-chromosome inactivation in females • One X inactivated during embryonic development  Barr body (see nuclear envelope) • # in female cells? male cells?

  17. Genes on this X are not expressed • Lyon hypothesis • Females mosaics for X-linked traits • Allele key

  18. XCXc • Patches of colorblind cells in retina • Normal phenotype Blue colorblindness is rare

  19. Genotype of orange female? • Genotype of black female? • Genotype of orange male? • Genotype of black male?

  20. Linkage Unlinked genes -On different chromosomes -Independent assortment! Example in humans: Blood type (Chromosome 9) Lactose intolerance (Chromosome 2)

  21. Linked Genes • -on same chromosome • -do not assort independently • Example humans: • Freckles (Chromosome 16) • Red hair (Chromosome 16)

  22. Genetic recombination of linked genes Crossing over Meiosis Non-sister chromatids of homologous chromosomes 25% of each

  23. Crossing over results in recombinants

  24. Chromosomal abnormalities • Disjunction anaphase of meiosis

  25. If chromosome do not disjoin • Non-disjunction • Meiotic spindle error • Sister chromatids do not separate in Anaphase II • Gametes  ?

  26.  Fertilization (human) Aneuploidy – abnormal number of chromosomes

  27. 1. Monosomy (2n-1) Ex. Turner syndrome 45, X Only viable monosomy in humans

  28. 2. Trisomy (2n+1) Ex. Trisomy 21 (Down syndrome) Higher risk in women > 35 Abnormal meiosis

  29. Nondisjunction anaphase I • NondisjunctionanaphaseII • Fertilization animation at Learn Genetics

  30. Chromosome structure abnormalities • Deletion • Duplication • Translocation • Inversion Univ. Wisc. Genetics center tumors

  31. Inheritance of mitochondrial DNAand chloroplast DNA (plants) • Extranuclear genes

  32. Maternal inheritance (cytoplasm)

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