Gregor Mendels experiments

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Gregor Mendels experiments

• Who was he? A monk, who lived in what is now
  Czech Republic around 1860
• Interested in mathematics of probability and in
  inheritance in animals and plants
• Studied inheritance in peas, because they were
  easy to grow and had some very visible inherited
  traits.

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Mendels experiments

• Peas are usually self pollinating (fertilize
  their own eggs with their own sperm)
• It was easy to obtain pure-breeding peas which
  had contrasting traits (for example, purple
  flowers or white flowers)
• Mendel could cross (mate) pure breeding purple
  flowered plants with pure breeding white flowered
  plants.
• Here is what he found in one experiment

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Mendels experiments

• Mendel discovered all the new peas had purple
  flowers!
• He performed similar crosses for 6 other traits
  in peas.
• In each case, he discovered that all the
  offspring of two pure breeding parents showed the
  trait of only one of the parents.
• He called this trait the DOMINANT trait. The
  trait that did not show up he called RECESSIVE.

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Mendels experiments

• He didnt stop there!
• He then let the peas self pollinate (inbreed) to
  get a third generation.
• He found that the next generation showed both
  traits, and always in about the same ratio! The
  ratio was always 3 dominant for every one
  recessive.

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Mendels experiments

• Definitions the pure breeding peas he started
  with, he called the P generation.
• Their offspring, which showed the trait of only
  one of the two parents, he called the F1
  generation.
• The offspring of the F1 generation were called
  the F2 generation.
• These offspring always showed both traits , and
  always in a specific ratio, 3 to 1.
• Here are some of his results

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Trait Studied
Dominant Form
Recessive Form
F2 Dominant-to- Recessive Ratio
SEED SHAPE
2.961
5,474 round
1,850 wrinkled
SEED COLOR
3.011
6,022 yellow
2,001 green
POD SHAPE
2.951
299 wrinkled
882 inflated
POD COLOR
2.821
152 yellow
428 green
FLOWER COLOR
3.151
705 purple
224 white
FLOWER POSITION
3.141
651 long stem
207 at tip
STEM LENGTH
2.841
787 tall
277 dwarf
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Mendels experiments

• The ratio of 3 1 could also be represented as
  3/4 to 1/4.
• Does this sound familiar?
• Think about your coin tossing, when you tossed 2
  coins at once.
• Suppose the 1/4 was the column labelled T/T
• Suppose the 3/4 was the other 3 columns, H/H,
  H/T, andT/H.

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Imagine that A stands for heads and a for tails
POSSIBLE EVENT
PROBABLE OUTCOME
sperm A meets egg A sperm A meets egg a sperm
a meets egg A sperm a meets egg a
1/4 AA offspring 1/4 Aa 1/4 Aa 1/4 aa
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Mendels experiments

• Think back to chapter 6 and 7 Organisms are
  diploid, gametes are haploid.
• If a pure breeding plant is AA (diploid) then its
  gametes will be As (haploid)
• If a plant is aa (diploid), the gametes will be
  as (haploid)
• If an A gamete fertilizes an a gamete, the new
  organism will be Aa (diploid)

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True-breeding homozygous recessive parent plant
F1 PHENOTYPES
aa
True-breeding homozygous dominant parent plant
Aa
Aa
a
a
Aa
Aa
A
AA
A
Aa
Aa
Aa
Aa
An F1 plant self-fertilizes and produces gametes
F2 PHENOTYPES
Aa
Aa
AA
A
a
AA
A
Aa
a
Aa
aa
aa
Aa