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Gregor Mendel – 1822-1884

Mendelian Genetics. Gregor Mendel – 1822-1884. Asexual Reproduction. Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations in one day Thus capable of producing 6 x 10 35 offspring per day Bacteria often produce 1 mutation per 1000 replications of DNA

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Gregor Mendel – 1822-1884

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  1. Mendelian Genetics Gregor Mendel – 1822-1884

  2. Asexual Reproduction • Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations in one day • Thus capable of producing 6 x 1035 offspring per day • Bacteria often produce 1 mutation per 1000 replications of DNA • So for fast-growing species, mutation is a good way to respond to a changing environment

  3. Why Sex? John Maynard Smith

  4. Sexual reproduction leads to genetic variation via: • Independent assortment during meiosis • Crossing over during meiosis • Random mixing of gametes (sperm and egg)

  5. Independent Assortment

  6. Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughtercells Recombinant chromosomes

  7. The random nature of fertilization adds to the genetic variation arising from meiosis. • Any sperm can fuse with any egg. • A zygote produced by a mating of a woman and man has a unique genetic identity. • An ovum is one of approximately 8,388,608 possible chromosome combinations (223). • The successful sperm represents one of 8,388,608 different possibilities (223). • The resulting zygote is composed of 1 in 70 trillion (223 x 223) possible combinations of chromosomes. • Crossing over adds even more variation to this.

  8. Mendelian Genetics Gregor Mendel – 1822-1884

  9. Two possible types of inheritance • One possible explanation of heredity is a “blending” hypothesis • The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green • An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea • Parents pass on discrete heritable units, later known as genes

  10. Mendel’s time Today Mendel’s garden at Brunn (Brno) Monastery

  11. Some genetic vocabulary • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers Garden Pea

  12. Flower Structure

  13. 3 2 1 4 5 TECHNIQUE Parentalgeneration(P) Stamens Carpel RESULTS First filialgenerationoffspring(F1)

  14. In Mendel’s Experiments: • Mendel chose to track • Only those characters that varied in an “either-or” manner • Mendel also made sure that • He started his experiments with varieties that were “true-breeding” • In a typical breeding experiment • Mendel mated two contrasting, true-breeding varieties, a process called hybridization

  15. Breeding Terminology • The true-breeding parents • Are called the P (parental) generation • The hybrid offspring of the P generation • Are called the F1 (filial) generation • When F1 individuals self-pollinate • The F2 generation is produced

  16. EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers

  17. EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination

  18. EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants

  19. Mendel developed a hypothesis to explain his results that consisted of four ideas • Alternative versions of genes (different alleles) account for variations in inherited characters • For each character, an organism inherits two alleles, one from each parent • If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance. The other, recessive allele has no effect on a hybrid organism’s appearance • The two alleles for each character segregate (separate) during gamete formation

  20. P Generation Appearance: White flowers Purple flowers Genetic makeup: pp PP p Gametes: P Law of Segregation

  21. P Generation Appearance: White flowers Purple flowers Genetic makeup: pp PP p Gametes: P Law of Segregation F1 Generation Appearance: Purple flowers Genetic makeup: Pp p 1/2 1/2 P Gametes:

  22. P Generation Appearance: White flowers Purple flowers Genetic makeup: pp PP p Gametes: P Law of Segregation F1 Generation Appearance: Purple flowers Genetic makeup: Pp p 1/2 1/2 P Gametes: Sperm from F1 (Pp) plant F2 Generation p P P Pp PP Eggs from F1 (Pp) plant p pp Pp 3 : 1

  23. Genotype Phenotype PP(homozygous) Purple 1 Pp(heterozygous) 3 Purple 2 Pp(heterozygous) Purple pp(homozygous) White 1 1 Ratio 3:1 Ratio 1:2:1

  24. TECHNIQUE Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:pp Predictions Test cross If purple-floweredparent is PP or If purple-floweredparent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp pp Pp Pp RESULTS or All offspring purple 1/2 offspring purple and1/2 offspring white

  25. EXPERIMENT YYRR yyrr P Generation Gametes yr YR F1 Generation YyRr Hypothesis ofdependent assortment Predictions Hypothesis ofindependent assortment Sperm or Predictedoffspring ofF2 generation 1/4 1/4 1/4 1/4 yR yr Yr YR Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRr yyRr YyRR yyRR 3/4 1/4 yr 1/4 Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 3/16 1/16 9/16 Phenotypic ratio 9:3:3:1 RESULTS 108 101 315 Phenotypic ratio approximately 9:3:3:1 32

  26. Rr Rr  Segregation ofalleles into sperm Segregation ofalleles into eggs Sperm r 1/2 1/2 R R R r R R 1/2 1/4 1/4 Eggs r r r R 1/2 r 1/4 1/4

  27. 1/4 (probability of YY)   1/4 (RR) 1/16 Probability of YYRR   Probability of YyRR  1/4 (RR) 1/2 (Yy) 1/8

  28. 1/4 (probability of YY)   1/4 (RR) 1/16 Probability of YYRR   Probability of YyRR  1/4 (RR) 1/2 (Yy) 1/8 Probability of yyrr = ? A. 1/8 B. 1/16 C. 1/32

  29. 1/4 (probability of YY)   1/4 (RR) 1/16 Probability of YYRR   Probability of YyRR  1/4 (RR) 1/2 (Yy) 1/8 Probability of YYrr = ? A. ¼ B. 1/8 C. 1/16

  30. 1/4 (probability of YY)   1/4 (RR) 1/16 Probability of YYRR   Probability of YyRR  1/4 (RR) 1/2 (Yy) 1/8 Probability of YxRr = ? (x can be Y or y) A. ½ B. 3/4 C. 3/8 D. 1/16

  31. 1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppyyRr  1/16 ppYyrr 1/41/21/2  2/16 Ppyyrr 1/21/21/2  1/16 1/41/21/2 PPyyrr ppyyrr  1/16 1/41/21/2  6/16 or 3/8 Chance of at least two recessive traits

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