mendelian genetics the basics. gregor mendel mendel was an austrian monk who published his research...
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Mendelian Genetics
The Basics
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Gregor Mendel
• Mendel was an Austrian monk who published his research on the inheritance of pea plant characteristics in 1866.
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• Mendel studied the inheritance of several traits by crossing true-breeding plants.
• He studied– Flower color– Plant height– Seed color– Seed shape– Pod shape– Pod color– Flower position– And many others.
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• He carefully controlled each cross by manipulating the reproductive structures of flowers
• And carefully observed the results.
• In this case, all of the F1 generation were purple, which contributed to the development of the rule of dominance.
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Dominant and recessive alleles
• Different forms of a gene are called alleles.
• In some cases, one form is dominant to the other, and will be expressed in the organisms appearance.
• In the example here, purple flowers are dominant to white flowers.
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• When two F1 plants are crossed, we get a ratio of individuals that look like either one of the original parent plants.
• This led to the development of Mendel’s Law of Segregation
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Mendel’s Law of Segregation
• This is the idea that every organism has two copies of each trait.
• When reproducing, the offspring receives one copy from each parent, but not both.
• We now know that this is the result of meiosis and production of haploid gametes.
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Genotype and Phenotype
• Based on the laws of dominance and segregation, it is possible for different combinations of genes (genotypes) to produce the same physical appearance (phenotype).
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• In this example, plants with the genotype PP or Pp have purple flowers, while the genotype pp produces plants with white flowers.
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• Organisms with two of the same alleles are called homozygous, those with different alleles are called heterozygous.
• Homozygotes can be Homozygous dominant (PP) or homozygous recessive (pp).
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Punnett Squares• Punnett squares can
be used to predict the outcome of genetic crosses.
• The possible gametes from each parent are placed on adjacent sides of the grid.
• The intersecting grids will then show possible genotypic results of the cross.
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Probable results• In this cross we get a
probable result of 3 purple flowers to 1 white flower.
– The phenotype ratio is 3:1
• However, we have one that is homozygous dominant, two that are heterozygous, and one that is homozygous recessive.
– The genotype ratio is 1:2:1
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Punnet Square Practice
• In Pea plants, tall (T) is dominant to short (t).
• Use a punnett square to predict the genotype and phenotype ratios for a cross between a pure breeding tall plant (TT) and a pure breeding short plant (tt).
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All Tall, All Heterozygous
T T
t Tt Tt
t Tt Tt
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• Perform a cross between two heterozygous tall plants.
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T T
T TT Tt
t Tt tt
Genotype Ratio =1:2:1
Phenotype ratio3:1
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• How about a heterozygous tall plant with a homozygous short plant?
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T t
t Tt tt
t Tt tt
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Law of Segregation
• States that each organism has two copies of every gene, but that only one of those