Mendelian Genetics

The Basics

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.

• 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.

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

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.

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).

• 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).

All Tall, All Heterozygous

T T

t Tt Tt

t Tt Tt

• Perform a cross between two heterozygous tall plants.

T T

T TT Tt

t Tt tt

Genotype Ratio =1:2:1

Phenotype ratio3:1

• How about a heterozygous tall plant with a homozygous short plant?

T t

t Tt tt

t Tt tt

Law of Segregation

• States that each organism has two copies of every gene, but that only one of those