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Chapter 15

Chapter 15. Chromosomal Basis Of Inheritance. Chromosome Theory of Inheritance. Mendelian genes have specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment not the alleles. I. Morgan and Friends.

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Chapter 15

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  1. Chapter 15 Chromosomal Basis Of Inheritance

  2. Chromosome Theory of Inheritance Mendelian genes have specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment not the alleles.

  3. I. Morgan and Friends • First to associate a specific gene to a specific chromosome • Used drosophila / looked at wild types and mutants • He discovered sex-linked traits, linked genes, and crossing over

  4. I. Morgan and Friends • Sex-linked traits – genes located on sex chromosomes (usually X in humans) examples: color blindness and hemophilia

  5. I. Morgan and Friends • Linked Genes – genes located on the same chromosome and inherited together a. Started with wild and double mutant B-gray (wild) G-normal (wild) b-black (mutant) g-vestigial (mutant) P – BBGG x bbgg F1 - BbGg

  6. I. Morgan and Friends b. Crossed F1 with a double mutant F1 – BbGg x bbgg c. What did Morgan expect to get from this cross? Do the Punnett Square He expected a 1:1:1:1 ratio d. What did Morgan actually get? 965 – gray/normal 944 – black/vestigial 206 – gray/vestigial 185 – black/normal

  7. I. Morgan and Friends e. How did Morgan explain this? Maybe the traits were on the same chromosome (linked) f. If they are linked what results should Morgan have seen? He should have seen a 1 gray/normal : 1 black/vestigial ratio g. Why didn’t he get 1:1? Crossing Over

  8. I. Morgan and Friends • Crossing Over – sections of chromosomes switching places - genetic recombination – new traits in offspring - parent types – offspring like parents - recombinant – offspring w/ new traits - a 50% frequency of recombination is observed for 2 genes on different chromosomes - If Morgan’s flies were completely linked it should be 1:1:0:0 but there were some recombinant types because of crossing over

  9. II. Using Recombination to Map Loci • Sturtevant • Genetic Map – ordered list of loci on a chromosome • The farther apart 2 genes are the better chance of crossing over and a higher recombination frequency • Linkage Map – genetic map based on recombination frequency • Map Unit – unit used to distance genes, =1 recomb. Frequency (centimorgan) • Some genes are so far apart crossing over is inevitable resulting in over 50% rf. Can’t tell the difference between it and 2 chromosomes • Cytological Map – locate genes w/ respect to chromosomal structures, like centromere

  10. III. Sex Chromosomes • Not all organisms are X and Y based - insects are X – 0 / males XO, females XX - birds are Z – W / males ZZ, females ZW / ovum determines the gender not sperm / opposite of the XY system, that is why ZW is used - bees are haplo-diploid / males haploid and develop from unfertilized eggs (parthenogenesis), females diploid and develop from fertilized eggs / no sex chromosomes

  11. III. Sex Chromosomes

  12. III. Sex Chromosomes • XX=female XY=male / very little crossing over / SRY codes for proteins that regulate male characteristics • Sex-linked traits / Duchenne Muscular Dystrophy, Hemophilia, Color Blindness • X inactivation – one X in females is turned off during embryonic development • Barr body – what the inactive X condenses into • Inactivation is random after the embryo develops some so female is a mix of active mom and dad X’s • Methylation and XIST gene causes this

  13. IV. Errors in Chromosomal Inheritance

  14. 1. Non-disjunction • Uneven distribution of chromosomes • Aneuploidy – abnormal chromosome number • Trisomic – 2n + 1 • Monosomic – 2n – 1 • Polyploidy – more than 2 complete chromosome sets • Ex. Down’s Syndrome, Klinefelter’s Syndrome, Turner’s Syndrome, XXX, XYY

  15. 2. Altering Chromosome Structures • Deletion – piece of chromosome is lost • Duplication – when a fragment attaches to a sister chromatid • Inversion – fragment occurs and flips and reattaches Tall Oprah Boy • Translocation – fragment joins a non-homologous chromosome • Ex. Cri du chat, Chronic myelogenous leukemia

  16. V. Where’d You get that gene? Mom or Dad? • Sometimes phenotype depends on if the alleles were passed on by mom or dad • Prader-Willi – mental retardation, obesity, short, small hands and feet • Angelman – uncontrollable laughter, jerky movement • Both caused by deletion on 15 / P-W if it comes from Dad, Angelman if it comes from Mom

  17. V. Where’d You get that gene? Mom or Dad? • Genomic Imprinting – one gene is silenced while the homologous chromosome is expressed - These alleles have different effects depending on if they come from sperm or egg - Caused by methylation

  18. V. Where’d You get that gene? Mom or Dad? • Fragile X Syndrome – part of X hanging on by a DNA “thread” - methylation of abnormal allele by mom causes the syndrome in males. - it could come from mom or dad in females that have the condition.

  19. VI. Extranuclear Genes • Small circles of DNA found in mitochondria and chloroplasts • Mom passes down mitochondrial DNA • Mutations accumulate in mitochondrial DNA over a life time / some believe this is part of the aging process

  20. Problems!! • Do the following problems at the end of chapter 15. • 1, 2, 3, 4, 6, 7, 10, 15, 16

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