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Chapter 3 - Genetics

Chapter 3 - Genetics. I. Structure Chromosomes in cell nuclei - 23 pairs/46 per cell 22 pairs = autosomes 1 pair = sex chromosomes. DNA - 1 long molecule - double-helix Gene - segment of DNA - gene pairs determine traits Allele - 1 member of gene pair

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Chapter 3 - Genetics

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  1. Chapter 3 - Genetics I. Structure • Chromosomes in cell nuclei - 23 pairs/46 per cell 22 pairs = autosomes 1 pair = sex chromosomes

  2. DNA - 1 long molecule - double-helix • Gene - segment of DNA - gene pairs determine traits • Allele - 1 member of gene pair - each parent gives 1 allele to child

  3. II. Genetic Transmission of Characteristics • 1 Gene/allele from each parent - 1 Brown or 1 blue from each parent • Traits = Dominant v. Recessive Brown = dominant blue = recessive

  4. Gene pairs = Homozygous or Heterozygous “same cell” “different cell” BB or bb Bb • Phenotype vs. Genotype Appearance vs. Genetic code

  5. III. Transmission Types • 1 Gene pair, Dominant trait - if even 1 allele for dominant trait, that trait will be exhibited (BB or Bb) - if parent is heterozygous, kids might not have trait

  6. 1 Gene pair, Recessive trait - to exhibit trait, must have both recessive alleles (bb) - both parents must contribute recessive trait - if both parents exhibit (bb), all kids will - if both parents carry (Bb) 1 child doesn’t have (BB), 2 carry (Bb), 1 exhibits (bb)

  7. Sex-linked traits - X determines more characteristics than Y - recessive x overridden by dominant X, not by Y - skews sex distribution of characteristics from recessive genes - so more boys exhibit, more girls carry - girl exhibits only if both parents have recessive x

  8. Note: • Sex-limited gene = on autosome, not sex chromosome - exerts effects more strongly in 1 sex than the other - hormones

  9. IV. Polygenetic Research • Polygenetic traits - most traits involve multiple genes + the environment - so most are on a continuum

  10. Three types of studies – all are consanguinity/concordance • Family • Twin • Adoption

  11. Family Study - the more closely related people are the more genes they share, the more similar they will be - 50% shared between siblings & parent/child - 25% between grandparents, aunts/uncles - 0% between unrelated people

  12. Problem - those who are more closely related are more likely to share environment

  13. Twin Study - identical vs. fraternal twins MZ DZ - special environment of twinness - difference = amount of genes shared

  14. MZ/identical = 100% shared genes • DZ/fraternal = 50% shared genes • If MZ = more similar than DZ, genetic evidence

  15. Problem - MZ also more likely to share environment& be treated the same

  16. MZ together vs. MZ apart

  17. Adoption Study - adopted children share only genes with biological parents - share only environment with adoptive parents - if more similar to biological parents, evidence for genetic influence

  18. Problem - Adoptive homes often similar to biological homes

  19. V. Chromosomal Abnormalities Types: Abnormal number vs. structure

  20. Abnormal number - problems with more or less than 46 • Down syndrome (Trisomy 21) - 1 in 600 births - additional 21st autosome - genetic but not inherited

  21. Turner’s syndrome - 1 in 3000 female births XO - females with only 1 X chromosome • Klinefelter’s syndrome - 1 in 500 male births - males with 1+ extra X chromosome(s) XXY

  22. XYY complement - Supermale Syndrome - 1 in 1000 male births - males with 1+ extra Y chromosome(s) XYY • Superfemale Syndrome - 1 in 1000 female births - females with 1+ extra X chromosome(s) XXX

  23. Abnormal structure • Fragile X syndrome - 1 in 2000 births - long arm of X breaks - recessive - males - MR - prevention: folic acid

  24. VI. Genetic Testing 5 Methods • Preimplantation - one cell from embryo

  25. Amniocentesis - extracts amniotic fluid - after 16th week • Chorionic Villus Sampling (CVS) - extract tissue from chorion (membrane surrounding fetus) - 7th/8th week

  26. Quadruple screen - test mother’s blood for 4 proteins/hormones (Alpha-feto protein, HCG, Estriol & Inhibin) - high = neural tube defect or multiple fetuses - low = Down syndrome • Targeted ultrasound - gross abnormalities - heart & neural tube abnormalities - proportions can indicate Down

  27. What can you screen for? • Diseases • Fragile X • Down Syndrome • Pharmacogenomics • Behavioral traits?

  28. Genetics and Ethics • Genetic determinism - polygenetic nature of most traits - role of environment - too complex to isolate single gene

  29. Cloning - identical twins - clones … original (brain connections determined by experience)

  30. Eugenics (“good genes”) - from evolution - in Britain and U.S. - laws preventing marriage of “feebleminded” - immigration restrictions - included forced sterilization - Carrie Buck (1928)

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