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THE CHROMOSOMAL BASIS OF INHERITANCE

THE CHROMOSOMAL BASIS OF INHERITANCE. CHAPTER 15. What you must know:. How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel ’ s laws of inheritance. The unique pattern of inheritance in sex-linked genes.

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THE CHROMOSOMAL BASIS OF INHERITANCE

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  1. THE CHROMOSOMAL BASIS OF INHERITANCE CHAPTER 15

  2. What you must know: • How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance. • The unique pattern of inheritance in sex-linked genes. • How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders. • How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance.

  3. Chromosome theory of inheritance: • Genes have specific locations (loci) on chromosomes • Chromosomes segregate and assort independently Chromosomes tagged to reveal a specific gene (yellow).

  4. Thomas Hunt Morgan • Drosophila melanogaster – fruit fly • Fast breeding, 4 prs. chromosomes (XX/XY) • Sex-linked gene: located on X or Y chromosome • Red-eyes = wild-type; white-eyes = mutant • Specific gene carried on specific chromosome

  5. Sex determination varies between animals In many species, it is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy (honey bees)

  6. Sex-linked genes • Sex-linked gene on X or Y • Females (XX), male (XY) • Eggs = X, sperm = X or Y • Fathers pass X-linked genes to daughters, but not sons • Males express recessive trait on the single X • Females can be affected or carrier

  7. Sex-linked disorders • Colorblindness • Duchenne muscular dystrophy- characterized by progressive muscle degeneration and weakness. It is one of nine types of muscular dystrophy. DMD is caused by an absence of dystrophin, a protein that helps keep muscle cells intact • Hemophilia

  8. X-Inactivation Barr body= inactive X chromosome; regulate gene dosage in females during embryonic development • Cats: allele for fur color is on X • Only female cats can be tortoiseshell or calico.

  9. Human development • Y chromosome required for development of testes • Embryo gonads indifferent at 2 months • SRY gene: sex-determining region of Y • Codes for protein that regulates other genes

  10. Genetic Recombination: production of offspring with new combo of genes from parents • If offspring look like parents  parental types • If different from parents  recombinants

  11. If results do not follow Mendel’s Law of Independent Assortment, then the genes are probably linked

  12. Linked genes: located on same chromosome and tend to be inherited together during cell division

  13. Crossing over: explains why some linked genes get separated during meiosis • the further apart 2 genes on same chromosome, the higher the probability of crossing over and the higher the recombination frequency

  14. Calculating recombination frequency

  15. Linkage Map: genetic map that is based on % of cross-over events • 1 map unit = 1% recombination frequency • Express relative distances along chromosome • 50% recombination = far apart on samechromosome or on 2 differentchromosomes

  16. Exceptions to Mendelian Inheritance

  17. Genomic Imprinting • Genomic imprinting: phenotypic effect of gene depends on whether from M or F parent • Methylation: silence genes by adding methyl groups to DNA

  18. Non-Nuclear DNA • Some genes located in organelles • Mitochondria, chloroplasts, plastids • Contain small circular DNA • Mitochondria = maternal inheritance (eggs) Variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from the maternal parent.

  19. Genetic Testing Reasons for Genetic Tests: • Diagnostic testing (genetic disorders) • Presymptomatic& predictive testing • Carrier testing (before having children) • Pharmacogenetics (medication & dosage) • Prenatal testing • Newborn screening • Preimplantation testing (embryos)

  20. Prenatal Testing May be used on a fetus to detect genetic disorders Amniocentesis: remove amniotic fluid around fetus to culture for karyotype Chorionic villus sampling: insert narrow tube in cervix to extract sample of placenta with fetal cells for karyotype

  21. Nondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II

  22. Karyotyping can detect nondisjunctions. Down Syndrome = Trisomy 21

  23. Nondisjunction Klinefelter Syndrome: 47XYY, 47XXY

  24. Nondisjunction Turner Syndrome = 45XO

  25. Chromosomal Mutations

  26. Chromosomal Mutations

  27. Nondisjunction • Aneuploidy: incorrect # chromosomes • Monosomy (1 copy) or Trisomy (3 copies) • Polyploidy: 2+ complete sets of chromosomes; 3n or 4n • Rare in animals, frequent in plants A tetraploid mammal. Scientists think this species may have arisen when an ancestor doubled its chromosome # by errors in mitosis or meiosis.

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