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Understanding Genetics: Traits, Inheritance, and Testing

Explore the intricate world of genetics - from chromosomes and gene pairs to hereditary traits and chromosomal abnormalities. Learn about genetic transmission and how traits are exhibited through alleles. Discover the different types of genetic studies and chromosomal abnormalities. Uncover the methods of genetic testing, including preimplantation and amniocentesis. Dive into the ethical considerations of genetics, including eugenics and cloning.

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Understanding Genetics: Traits, Inheritance, and Testing

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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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