Mendel and His Discoveries

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Mendel and His Discoveries

• Chapter 11

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Gregor Mendel

• Gregor Mendel (1822-1884) Experimented with pea
  plants and developed fundamental rules of
  genetics and patterns of inheritance.

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Punnett Squares

• Crosses between parents that differ in only one
  trait are called MONOHYBRID CROSSES.
• P Parents
• F1 First Filial generation (kids)
• F2 Second Filial generation (grandkids)

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Mendel and Pea Plants

• For his plant experiment he crossed a tall plant
  with a short plant.
• All of the offspring looked tall.
• Why?
• Because the tall plants had a dominant trait and
  the short plants had a recessive trait.

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Alleles

• Alleles are simply versions of genes

THE ALLELES FOR COW FUR COLOR ARE BLACK. BROWN,
AND WHITE
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Prediction of Genetic Crosses

• The alleles for tallness of a plant
• TT Dominant (Tall)
• tt Recessive (Short)
• How do we know for sure that they would all be
  tall?
• The Punnet Square

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Punnett Squares

• Tool to predict outcomes of genetic crosses
• Make a tic-tac-toe board
• Place the parent alleles like below

TALL PARENT PLANT
SHORT PARENT PLANT
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Lets try another one

• If the trait is for feather color of parrots,
  then GG Dominant and is green.
• gg recessive and is gold.
• GG allele is homozygous dominant (Homo- same
  zygous-sex cell dominant- dominant)

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• gg allele is homozygous recessive (Homo- same
  zygous- sex cell recessive- recessive)
• So from the cross above we get all Gg. So are
  they Green or gold or a mix of both?

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• They are all green. Gg is called heterozygous.
  Heterozygous means different.
• If a G and a g are together to make a
  heterozygous trait, the trait looks like the
  dominant gene or G. So, ALL the parrots look
  green.

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But what about the KIDS of these new, green
parrots?

• Genetics of the new, GREEN (F1) parrots
• Gg x Gg

G g
G GG Gg
g Gg gg
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Ratios?

• Phenotype the look of the genes
• Genotype the letters or alleles of the organism

PHENOTYPE RATIOS 31
GENOTYPE RATIOS 1 GG, 2Gg
1gg
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Why we needed the math The Dihybrid Cross

• Lets say that in peas, We cross a Homozygous
  dominant Smooth, yellow seed with a Homozygous
  recessive wrinkled, green seed. What would be
  the genotype and phenotype of the F1 generation?

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Setting it up

• SS Smooth YY yellow
• ss wrinkled yy green
• So we are crossing a SSYY x ssyy

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Ready, set, go!

• Most important question What are all of the
  possible sperm and egg that each plant can
  produce with those alleles? (huh?)
• SSYY Plant ssyy Plant

sy
SY
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• The product of this cross will all be
• SsYy, or all Smooth, Yellow peas (F1)
• But what about the F2 generation?

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Ask the Important Question again!

• Most important question What are all of the
  possible sperm and egg that each plant can
  produce with those alleles?

SY
Sy
SsYy
sY
sy
PEA SEED
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Incomplete Dominance

• What do you get when you cross a homozygous
  dominant red rose with a homozygous white rose?

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Incomplete Dominance

• A Pink Rose!
• Since all of the F1 are Rr pink, what would be
  the F2 generation of all of the F1 pink roses?

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1 Red 2 Pink 1 White
R r
R RR Rr
r Rr rr
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Codominance

• Codominance- when both alleles contribute to the
  phenotype of the organism

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Multiple Alleles

• When three or more alleles of the same gene exist
  in a population
• i.e. blood types (A, B, O)

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Continuous Variation

• - The range of small differences of a single
  trait in a population.
• It is usually where several genes effect a single
  trait.