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AP Biology

AP Biology. Lecture #28 Sex-Linked Genetics. The Chromosomal Theory of Inheritance. Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment. Chromosomal Linkage. Morgan Drosophilia melanogaster XX (female) vs. XY (male)

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AP Biology

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  1. AP Biology Lecture #28 Sex-Linked Genetics

  2. The Chromosomal Theory of Inheritance • Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

  3. Chromosomal Linkage • Morgan • Drosophiliamelanogaster • XX (female) vs. XY (male) • Sex-linkage: genes located on a sex chromosome • Linked genes: genes located on the same chromosome that tend to be inherited together

  4. Classes of chromosomes autosomalchromosomes sexchromosomes

  5. Discovery of sex linkage true-breeding red-eye female true-breeding white-eye male X P Huh!Sex matters?! 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

  6. What’s up with Morgan’s flies? x x RR rr Rr Rr r r R r R Rr Rr R RR Rr Doesn’t workthat way! R r Rr Rr Rr rr 100% red eyes 3 red : 1 white

  7. Genetics of Sex • In humans & other mammals, there are 2 sex chromosomes: X & Y • 2X chromosomes • develop as a female: XX • gene redundancy,like autosomal chromosomes • an X & Y chromosome • develop as a male: XY • no redundancy X Y X XX XY XX XY X 50% female : 50% male

  8. Let’s reconsider Morgan’s flies… x x XRXR XrY XRXr XRY Xr Y XR Y  XR XR XRXr XRY XRXR XRY BINGO! XR Xr XRXr XRY XRXr XrY 100% red females 50% red males; 50% white males 100% red eyes

  9. Genetic recombination • Crossing over Genes that DO NOT assort independently of each other • Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency • Linkage maps Genetic map based on recombination frequencies

  10. Genes on sex chromosomes • Y chromosome • few genes other than SRY • sex-determining region • master regulator for maleness • turns on genes for production of male hormones • many effects = pleiotropy! • X chromosome • other genes/traits beyond sex determination • mutations: • hemophilia • Duchenne muscular dystrophy • color-blindness

  11. Human sex-linkage • SRY gene: gene on Y chromosome that triggers the development of testes • Fathers= pass X-linked alleles to all daughters only (but not to sons) • Mothers= pass X-linked alleles to both sons & daughters • Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia

  12. The transmission of sex linked recessive genes XnY XnY XNXN XNXn  XNY XNXn   Fig. 15-7 Sperm Sperm Sperm Xn Y XN Xn Y Y Eggs Eggs XNXN Eggs XNXn XNY XN XNY XN XNXn XNY XN XnXn XnY XNXn XNY XnXN XnY Xn Xn XN (a) (b) (c)

  13. XHXH XHXh XHXh XHXh XH XHY XHY XhY Xh male / sperm XH Y XHXH XHY XHY XH female / eggs Xh XhY XHXh Hemophilia sex-linked recessive Hh x HH XH Y carrier disease

  14. XH XHXh Xh X-inactivation • Female mammals inherit 2 X chromosomes • one X becomes inactivated during embryonic development • condenses into compact object = Barr body • which X becomes Barr body is random • patchwork trait = “mosaic” patches of black tricolor catscan only befemale patches of orange

  15. Human sex-linkage • X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

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