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Chapter 15~ The Chromosomal Basis of Inheritance

Chapter 15~ The Chromosomal Basis of Inheritance. 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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Chapter 15~ The Chromosomal Basis of Inheritance

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  1. Chapter 15~ The Chromosomal Basis of Inheritance

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

  3. Chromosomal Linkage • Morgan • Drosophilia melanogaster • 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. XHXH XHXh XHXh XHXh XH XHY XHY XhY Xh male / sperm XH Y XHXH XHY XHY XH female / eggs Xh XhY XHXh sex-linked recessive Hemophilia Hh x HH XH Y carrier disease

  13. 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

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

  15. Errors of Meiosis Chromosomal Abnormalities

  16. Chromosomal abnormalities • Incorrect number of chromosomes • nondisjunction • chromosomes don’t separate properly during meiosis • breakage of chromosomes • deletion • duplication • inversion • translocation

  17. n n Nondisjunction • Problems with meiotic spindle cause errors in daughter cells • homologous chromosomes do not separate properly during Meiosis 1 • sister chromatids fail to separate during Meiosis 2 • too many or too few chromosomes 2n n-1 n+1

  18. Alteration of chromosome number error in Meiosis 1 error in Meiosis 2 all with incorrect number 1/2 with incorrect number

  19. Nondisjunction • Baby has wrong chromosome number~ aneuploidy • trisomy • cells have 3 copies of a chromosome • monosomy • cells have only 1 copy of a chromosome n-1 n n+1 n monosomy 2n-1 trisomy 2n+1

  20. Human chromosome disorders • High frequency in humans • most embryos are spontaneously aborted • alterations are too disastrous • developmental problems result from biochemical imbalance • imbalance in regulatory molecules? • hormones? • transcription factors? • Certain conditions are tolerated • upset the balance less = survivable • but characteristic set of symptoms = syndrome

  21. Down syndrome • Trisomy 21 • 3 copies of chromosome 21 • 1 in 700 children born in U.S. • Chromosome 21 is the smallest human chromosome • but still severe effects • Frequency of Down syndrome correlates with the age of the mother

  22. Sex chromosomes abnormalities • Human development more tolerant of wrong numbers in sex chromosome • But produces a variety of distinct syndromes in humans • XXY = Klinefelter’s syndrome male • XXX = Trisomy X female • XYY = Jacob’s syndrome male • XO = Turner syndrome female

  23. Klinefelter’s syndrome • XXY male • one in every 2000 live births • have male sex organs, but are sterile • feminine characteristics • some breast development • lack of facial hair • tall • normal intelligence

  24. Klinefelter’s syndrome

  25. Jacob’s syndrome male • XYY Males • 1 in 1000 live male births • extra Y chromosome • slightly taller than average • more active • normal intelligence, slight learning disabilities • delayed emotional maturity • normal sexual development

  26. Trisomy X • XXX • 1 in every 2000 live births • produces healthy females • Why? • Barr bodies • all but one X chromosome is inactivated

  27. Turner syndrome • Monosomy X or X0 • 1 in every 5000 births • varied degree of effects • webbed neck • short stature • sterile

  28. error ofreplication error ofcrossing over Changes in chromosome structure • deletion • loss of a chromosomal segment • duplication • repeat a segment • inversion • reverses a segment • translocation • move segment from one chromosome to another

  29. Chromosomal errors VI Deletion Duplication Homologouschromosomes Inversion Reciprocaltranslocation Nonhomologouschromosomes

  30. Genomic imprinting • Def: a parental effect on gene expression • Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. • Fragile X syndrome: higher prevalence of disorder and retardation in males

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