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Chapter 14

Chapter 14. Mendel and the Gene Idea. Mendelian genetics. Gregor Mendel – father of genetics Austrian monk pioneer in the field gardener at monastery. Mendel’s Laws. 1) Law of Dominance and Recessiveness – when 2 different alleles are present, one

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Chapter 14

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  1. Chapter 14 Mendel and the Gene Idea

  2. Mendelian genetics • Gregor Mendel – father of genetics Austrian monk pioneer in the field gardener at monastery

  3. Mendel’s Laws 1) Law of Dominance and Recessiveness – when 2 different alleles are present, one masks, or covers up, another 2) Law of Segregation – alleles separate when gametes form 3) Law of Independent Assortment – one allele does not influence another ex: tall does not influence yellow

  4. Probability • Rule of Multiplication – probability that 2 or more independent events will occur simultaneously in some specific combination -take probability of each event and multiply together -ex: both coins landing heads up ½ x ½ = ¼ or Pp x Pp = pp

  5. Probability • Rule of Addition – probability of an event that can occur in 2 or more different ways - add separate probabilities - ex: heterozygote from Pp x Pp ¼ + ¼ = ½

  6. Law of Incomplete Dominance – (Intermediate inheritance) - When 2 different alleles are present (heterozygous), an intermediate trait is expressed ex: red flowers x white flowers = pink flowers

  7. Multiple Alleles - 3 or more alleles for 1 gene - ex: human blood groups phenotypegenotypeantigensantibodies A IAIA or IAIa A anti–B B IBIB or IBIb B anti–A AB IAIB A or B O ii neither A anti–A & nor B anti-B

  8. Test cross

  9. Pleitropy • One gene = many effects ex: sickle cell anemia

  10. Penetrance • Proportion of individuals who show expected phenotype from their genotype - ex: neuroblastomas

  11. Polygenic inheritance • Many genes = one trait -ex: human skin color

  12. Pedigree analysis

  13. Human Genetic Disorders • Cystic fibrosis • Tay-Sachs • Sickle-cell anemia • Huntington’s chorea • Duchenne’s Muscular Dystrophy • Down Syndrome • Achondroplasia

  14. PreventiveTesting for genetic disorders • usu. done when risk is high 1) pedigree determination 2) fetal testing: a) amniocentesis – 14th – 16th wk. of pregnancy; needle inserted into uterus; 10 ml fluid extracted & karyotype done b)chorionic villi sampling– sm. Amt. of fetal tissue is suctioned off from embryonic membrane villi (chorion) which forms part of placenta, then karyotype (results in 24 hrs.) advantages: 24 hr results vs. several weeks ; 8-10 wks of pregnancy

  15. Preventive c) ultrasound – soundwaves (noninvasive; no risk) d) fetoscopy – tube with viewing scope directly examines fetus 3) newborn screening – PKU test

  16. Chapter 15The Chromosomal Basis of Inheritance

  17. Genes are located on chromosomes, the structures that undergo segregation & independent assortment

  18. Thomas Hunt Morgan – 1st one to associate specific genes with specific chromosomes - studies with fruit flies, Drosophila melanogaster 1) grow rapidly 2) require small amt. of space 3) few chromosomes & these are large • 1st to discover a sex-linked gene (white eyes) X-linked

  19. Sex-linked traits • Carried on sex chromosomes • May be X-linked or Y-linked • No Y-linked found thus far • ex: red-green color blindness in humans

  20. Recombination • In unlinked genes, when 2 organisms produce offspring, the end result could be: parental types or recombinants (unlike either parent) Frequency of recombination – if ½ have different phenotype than the parent, we say there is a 50% frequency of recombination (maximum)

  21. Recombination frequency Frequency of = # of recombinants recombination total # of offspring x 100

  22. Gene mapping • map units - number assigned to show relative distance between genes on chromosomes • recombination frequency = # of map units ex: recombination frequency of 25% translates to 25 map units

  23. Sex determination systems a) X-Y system -humans, mammals, some insects -sperm (X or Y) determines sex b) X-O system -grasshoppers, crickets, roaches, some insects (only 1 sex chromosome) -female XX male XO -sperm either contains X or O

  24. Sex determination systems c) Z-W system -birds, some fishes, some insects (moths, butterflies) -Z & W used to avoid confusion with X-Y -female ZW male ZZ -egg determines sex d) haplo-diploid system – most bees, ants -no sex chromosome -females develop from fertilized eggs (2n) -males develop from unfertilized eggs (1n); fatherless

  25. Sex-linked traits • carried on sex chromosomes • may be X-linked or Y-linked • example: red-green colorblindness, hemophilia

  26. X-inactivation (in mammals) • fur color in calico cats determined by X chromosome 1X orange fur, 1X black fur • calico cats almost always female • 2 X chromosomes inherited, but in embryonic development, 1 is almost completely inactivated (inactive X condenses to Barr body) • selection of which X occurs ramdomly • ex: in humans dev. of sweat glands (mosaicism) heterozygous female have patches of normal skin & patches lacking sweat glands

  27. Aneuploidies • abnormal number of chromosomes • due to nondisjunction – failure of chromosomes to separate in anaphase 1) trisomy – 2n+1 having 3 chromosomes in a pair ex: Trisomy 21 (Down Syndrome) 2) monosomy – 2n-1 having only 1 chromosome in a pair ex: Turner Syndrome

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