Mendel

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Mendel Meiosis

• Chapter 10

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Meiosis

• Process of haploid (n) gamete formation for
  sexual reproduction ? 4 haploid gamete cells that
  are genetically different
• Review steps in Ch 10 if needed
• Pre-Meiosis 1 round of interphase what
  happens here?
• Phases - 2 sets of PMAT-C
• Meiosis I PMAT-C I
• Meiosis II PMAT-C II

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Meiosis Increasing genetic variation in
offspring

• Genetic Recombination
• Crossing over
• Occurs late Prophase 1 Metaphase 1
• Provides new variation

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Spermatogenesis vs. Oogenesis

• Sperm Formation
• Millions survive

• Egg Formation
• Only 1 survives

Why the difference?
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Spermatogenesis vs. Oogenesis
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Mistakes in Meiosis

• Nondisjunction
• Failure of homologues to separate properly

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Mistakes in Meiosis

• Trisomy an extra chromosome (3) per gamete
• Ex Downs Syndrome (TRISOMY 21), XYY males
• Monosomy ONE LESS CHROMOSOME PER GAMETE
• Ex Turners Syndrome

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Turners Syndrome

• Monosomy X (Turners Syndrome)
• Missing 1 chromosome missing 1,000s of
  genes!!!
• Short neck
• short stature
• sterile,
• Heart, kidney, bone, thyroid problems

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Summary - Meiosis

• Sexual reproduction
• Gametes (n) produced by both male and female
  organisms recombine in fertilization
• Fertilization ? 2n zygote genetically DIFFERENT
  to parents
• WHY must gametes be haploid (n) not diploid
  like produced in mitosis?

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GeneticsPre-Mendel Theory BLENDING

• Pre Mendel, theory of inheritance qualities of
  the parents blended to form the qualities of the
  child
• Ex tall and short parent medium height child
• Theory did NOT explain examples like
• two brown-eyed parents giving birth to a
  blue-eyed baby
• Because of Mendel's work there became a
  consistent theory of heredity GENETICS

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The Origins of Genetics

• Gregor Mendel (1822-1884)
• Father of Genetics
• Czechoslovakian
• cross pollinated pea plants to see the outcomes
• Noticed atypical characteristics
• Figure 10.3

Gregor's life story
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Actual garden sweet pea flowers
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Cross Pollination
Mendel mixed the pollen from a white flowered pea
plant to a purple flowered pea plant pistil, and
the results were..
Mendels Experiment
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• Generation 1
• Purple x White Purple (no
  white)
• (P1 Parents) (F1 offspring)

What does this mean?
Next, he mixed F1 plants with other F1 plants,
and the results were
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• Generation 2
• F1 x F1 Mostly purple, some white
• (P2 Parents) (F2)

Out of 929 flowers, 705 purple, 224 white
Thats a 31 RATIO!
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• Mendel called these new plants as hybrids

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Genotype vs. PhenotypeWhat do these terms mean?
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Mendels Conclusion

• Each organism has 2 factors that control each of
  its traits (now factors called alleles types
  of a gene)
• Dominant Trait
• A trait that dominates (masks/covers up) a
  recessive one written w/ Capital letter
• Recessive Trait
• Cant be expressed (seen) if a dominant trait is
  present written w/ lower case letter
• Ex Purple Flower P
• White Flower p

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For every trait

• There are 2 alleles (types of genes)
• One from female parent, one from male parent
• Process of meiosis allows for this
• Thus
• Purple flower PP
• HOMOZYGOUS DOMINANT pure dominant
• White flower pp
• HOMOZYGOUS RECESSIVE pure recessive
• What about Pp?
• HETEROZYGOUS
• Hybrid carrier

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What Mendel Saw

• How did he know which were dominant which were
  recessive?

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Mendels Laws (Principles)

• The Principle of Unit Characters states that
  individuals pass information on as individual
  traits.
• The Principle of Dominance states that some unit
  characters (genes) can mask the expression of
  others.
• The Principle of Segregation states that each
  unit character (gene) separates into a different
  sex cell.
• The Principle of Independent Assortment states
  that genes segregate according to chance
  different genes separate INDEPENDENTLY of each
  other.

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Alleles alternative versions of a gene
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Lets go back and look at the genes (alleles) in
Mendels Flowers

• Parent Generation (P1) PP x pp
• Each parent donate only 1 gene to F1
• Thus, all offspring here must be
• Pp
• Next (P2) he crossed F1 x F1 Pp x Pp
• In the F2 generation he received a ratio of 31
  purple phenotype to white phenotype flowers
• A punnett square predicts this ratio also

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A Test Cross

• Used to determine an unknown genotype of parents
• (Works backwards)

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• Punnett Squares using Mendels Work
• Gametes from the parents are written on the
  outside
• Each allele gets a separate box

• This is a MONOHYBRID cross
• It looks at only ONE trait

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• Possible genotypes are placed inside the boxes by
  mixing alleles from the parents
• All POSSIBLE combinations are shown here

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Green-podded plant
Yellow-podded plant
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• Genotypic ratio - boxes in cross/ total boxes
  with certain genotype
• 2/4(50) probability of creating a homozygous
  dominate genotype (GG)
• 2/4 (50) probability of creating a heterozygous
  genotype (Gg) plants
• Phenotypic ratio - specific type of physical
  trait observed in offspring/all offspring traits
  observed
• 4/4 (100) probability of creating a phenotype of
  green-podded plants