Gregor Mendel

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Gregor Mendel

• The Father of Genetics

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Mendel

• Modern genetics had its beginnings in an abbey
  garden, where a monk named Gregor Mendel
  documented a particular mechanism of inheritance.
• He discovered the basic principles of heredity by
  breeding garden peas in carefully planned
  experiments.

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Why peas?

• Peas are normally self-pollinating.
• Peas are easily grown,
• Peas mature quickly
• Peas produce many seeds
• Peas show several pairs of obvious contrasting
  traits
• The use of plants also allowed strict control
  over the mating.

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Peas contrasting traits

• Tall plant vs. dwarf plant
• Yellow seeds vs. green seeds
• Smooth seeds vs. wrinkled seeds
• Inflated pod vs. constricted pod
• Purple flower vs. white flower
• Green pod vs. yellow pod
• Axial flower vs. terminal flower

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Mendel experiments

• Mendel spent several
• years self-pollinating
• the pea plants in order
• to establish purebred
• plants.

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Mendels experiments

• These pure-breeding plants were called the P
  generation.
• Mendel had strains of peas that were pure
  breeding for height the plants were either tall
  or dwarf
• He had strains that had either yellow pods or
  green pods, inflated pods or constricted pods.

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Mendels experiments

• When a pure breeding tall plant is crossed with a
  pure breeding dwarf plant, all the resulting
  plants are called hybrids.
• All the hybrids are called the F1 generation (
  first filial generation)
• All of the F1 generation were tall

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Mendels experiment

• The same pattern was observed in each of the 7
  contrasting characteristics.
• In the F1 generation, all of the plants showed
  one characteristic.
• Mendel called this characteristic the dominant
  trait
• The trait that was not expressed in F1 was called
  the recessive trait

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Mendels experiment

• Next, Mendel crossed the F1 generation plants.
• He wanted to know if the F1 plants were identical
  to the P generation
• So he crossed two of the F1 plants

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Mendels experiment

• The resulting plants (the F2 generation) yielded
  3 tall plants and 1 dwarf plant.
• A ratio of 31

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Mendels experiment

• This meant that the tall F1 plants ( the hybrids)
  were different than the tall P generation plants
  (the purebred plants) even though they were all
  tall.

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Mendels experiment

• Mendel repeated this experiment with all of the
  traits and the results were similar.
• The F2 generation displayed a phenotypic ratio of
  31

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Mendels explanation

• Units of inheritance, called factors (we now call
  them genes) were involved
• For any given characteristic there were several
  different forms (now called alleles)
• Each plants phenotype (what they look like) was
  determined by a pair of alleles that can be
  identical or different

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Mendels explanation

• In a hybrid plant one allele of a pair has the
  ability to express itself while the other one is
  not expressed.
• Recessive characteristics are expressed only when
  there is no dominant allele present

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Mendels explanation

• When gametes are produced, the members of each
  pair of alleles are separated into different
  reproductive cells.
• A gamete can contain only one allele of a
  particular characteristic.
• When fertilization occurs these alleles unite to
  give the zygote the necessary pair of alleles.

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Mendels first law

• The Law of Segregation
• Members of a pair of alleles for a given trait
  are segregated when gametes are formed

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Genotype

• The genotype represents the genes that exist in
  the organism
• Mendel used letters of the alphabet to represent
  genes.
• Capital letters represent the dominant allele
• Lower case letters represent the recessive allele

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Genotype

• A purebred tall plant would have a genotype of TT
• A purebred dwarf plant would have a genotype of
  tt
• Genotypes with identical alleles are called
  homozygous
• Genotypes with different alleles are called
  heterozygous

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• The sex cells of a TT plant would contain only
  the dominant allele T
• The sex cells of a tt plant would contain only
  the recessive allele t
• The F1 generation would all have the genotype Tt
• The phenotype of all the F1 generation would be
  tall

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Mendels impact

• Mendels theories of inheritance, first
  discovered in garden peas, are equally valid for
  figs, flies, fish, birds and human beings.
• Mendels impact endures, not only on genetics,
  but on all of science, as a case study of the
  power of hypothesis/deductive thinking.

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A Punnett Square

• Punnett squares illustrate the possible outcomes
  of a particular cross (the genotype of the
  offspring)
• Mendels first experiment could be illustrated
  with the following Punnett square
  TT X tt

tt gametes
t
TT gametes
T
Tt
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A 2 x 2 Punnett Square

• The F1 generation cross is a monohybrid cross Tt
  x Tt

Phenotypic ratio 31
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Single Trait Analysis

• In humans the ability to taste PTC, T is dominant
  to non-tasting t. Determine the expected
  genotypic and phenotypic ratios resulting from a
  cross between a heterozygous taster and a
  non-taster.

Tt (heterozygous taster) gametes
tt (non-taster) gametes
Genotype ratio 11 (Tttt) Phenotype 11
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Practice Problem

• In humans, the allele A, for pigment formation
• is dominant to the allele for a, the inability
  to
• form pigment.
• aa individuals are albino
• Determine the expected genotypic and
• phenotypic ratios expected from a cross
• between two individuals heterozygous for
• this trait.

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Solving Punnett Square ProblemsGRASP method

• Given
• pigment formation (A) is dominant to the
  inability to form pigment (a)
• aa individuals are albino
• The cross is Aa x Aa
• Required
• The expected genotypic and phenotypic ratio of
  the cross
• Analysis
• Solution
• Paraphrase

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Solving Punnett Square ProblemsGRASP method

• Analysis
• Solution
• Paraphrase
• The phenotypic ratio is 31 normal albino
• The genotypic ratio is 1 2 1 AAAaaa

A a


AA
Aa
A a
Aa
aa
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Recognizing Hybrids

• A geneticist crosses two parent plants that have
  the dominant trait of
• purple flowers.
• When the resulting seeds are planted the
  geneticist observes that 145 of
• the F1 plants have the recessive trait of yellow
  flowers and 430 of the F1
• plants have purple flowers.
• How can you explain these results? What are the
  genotypes of the parent
• plants and the F1 plants?
• Given F (purple flowers) is dominant to f
  (yellow flowers). Both of the P1
• plants possess at least one F gene F__ F__
• Required The genotypes of the parents and F1
  plants
• Analysis
• The key to this question is the appearance of the
  ff (yellow plants) in the F1.
• We know that yellow flowers can only appear if
  the plants are homozygous recessive for yellow
  flowers (ff)
• Produce a Punnett square of the offspring.

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• Let F be the dominant allele for flower colour
  purple and f be the recessive allele for colour
  yellow
• Phenotypic ratio is 430145

gametes
gametes
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• Solution
• The appearance of the recessive trait in the
• phenotype of the F1 plants can only occur if
• they are homozygous recessive (ff). This can
• only happen if both of the purple parent plants
• are heterozygous and each parent contributed
• the recessive allele to these yellow plants. In
• addition, recognize that the ratio of purple
• plants to yellow plants is approximately 31.
• This ratio indicates a monohybrid cross.

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Dihybrid crosses

• Dihybrid crosses show the genotypes of offspring
  using 2 different genes on two different
  chromosomes
• TtRr x TtRr hybrid tall plants with
  round seeds
• Gametes produced by independent assortment TR,
  tr, Tr, tR
• These gametes would be present in equal numbers
  (eg. ¼ of the total number)

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Dihybrid cross

• TtRr x TtRr

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Dihybrid cross

• TtRr x TtRr
• The expected phenotypic ratio
• 9/16 tall round plants, 3/16 tall wrinkled plants
• 3/16 dwarf round plants, 1/16 dwarf wrinkled
  plants

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Test cross

• A test cross is a cross with an individual whose
  genotype is being tested to a recessive
  individual
• We know that the recessive individual must be
  homozygous so it can only produce one type of
  gamete
• This way we can determine the genotype of the
  test individual.