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Gregor Mendel – 1822-1884 Mendelian Genetics. Asexual Reproduction Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations.

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Presentation on theme: "Gregor Mendel – 1822-1884 Mendelian Genetics. Asexual Reproduction Bacteria can reproduce as often as every 12 minutes – and may go through 120 generations."— Presentation transcript:

1 Gregor Mendel – Mendelian Genetics

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 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 John Maynard Smith Why Sex?

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 I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells 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 (2 23 ). –The successful sperm represents one of 8,388,608 different possibilities (2 23 ). –The resulting zygote is composed of 1 in 70 trillion (2 23 x 2 23 ) possible combinations of chromosomes. – Crossing over adds even more variation to this.

8 Gregor Mendel – Mendelian Genetics

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 garden at Brunn (Brno) Monastery Mendel’s timeToday

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 Parental generation (P) Stamens Carpel First filial generation offspring (F 1 ) TECHNIQUE RESULTS 32145

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 F 1 (filial) generation When F 1 individuals self-pollinate –The F 2 generation is produced

16 P Generation EXPERIMENT (true-breeding parents) Purple flowers White flowers

17 P Generation EXPERIMENT (true-breeding parents) F 1 Generation (hybrids) Purple flowers White flowers All plants had purple flowers Self- or cross-pollination

18 P Generation EXPERIMENT (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers Self- or cross-pollination 705 purple- flowered plants 224 white flowered plants

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20 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

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

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

24 Law of Segregation P Generation F 1 Generation F 2 Generation Appearance: Genetic makeup: Gametes: Appearance: Genetic makeup: Gametes: Purple flowers White flowers Purple flowers Sperm from F 1 ( Pp ) plant Pp PP pp P P P P p p p p Eggs from F 1 ( Pp ) plant PP pp Pp 1/21/2 1/21/2 3 : 1

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

26 Test cross Dominant phenotype, unknown genotype: PP or Pp ? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP If purple-flowered parent is Pp or Sperm Eggs or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white Pp pp p ppp P P P p TECHNIQUE RESULTS

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28 P Generation F 1 Generation Predictions Gametes EXPERIMENT RESULTS YYRR yyrr yr YR YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predicted offspring of F 2 generation Sperm or Eggs Phenotypic ratio 3:1 Phenotypic ratio 9:3:3:1 Phenotypic ratio approximately 9:3:3: /21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 9 / 16 3 / 16 1 / 16 YR yr 1/41/4 3/43/4 Yr yR YYRR YyRr yyrr YYRRYYRrYyRR YyRr YYRrYYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr YyrryyRr yyrr

29 Segregation of alleles into eggs Segregation of alleles into sperm Sperm Eggs 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 Rr R R R R R R r r r r r  r

30 Probability of YYRR Probability of YyRR 1 / 4 (probability of YY) 1 / 2 (Yy) 1 / 4 (RR) 1 / 16 1/81/8      

31 Probability of YYRR Probability of YyRR 1 / 4 (probability of YY) 1 / 2 (Yy) 1 / 4 (RR) 1 / 16 1/81/8       Probability of yyrr = ? A. 1/8B. 1/16C. 1/32

32 Probability of YYRR Probability of YyRR 1 / 4 (probability of YY) 1 / 2 (Yy) 1 / 4 (RR) 1 / 16 1/81/8       Probability of YYrr = ? A. ¼B. 1/8C. 1/16

33 Probability of YYRR Probability of YyRR 1 / 4 (probability of YY) 1 / 2 (Yy) 1 / 4 (RR) 1 / 16 1/81/8       Probability of YxRr = ? (x can be Y or y) A. ½B. 3/4C. 3/8D. 1/16

34 Chance of at least two recessive traits ppyyRr ppYyrr Ppyyrr PPyyrr ppyyrr 1 / 4 (probability of pp)  1 / 2 (yy)  1 / 2 (Rr) 1/4  1/2  1/21/4  1/2  1/2 1/2  1/2  1/21/2  1/2  1/2 1/4  1/2  1/21/4  1/2  1/2 1/4  1/2  1/21/4  1/2  1/2  1 / 16  2 / 16  1 / 16  6 / 16 or 3 / 8


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