Mendel

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Mendels Breakthrough

• Patterns, Particles, and Principles of Heredity

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Outline of Mendelian Genetics

• The historical puzzle of inheritance and how
  Mendels experimental approach helped solve it
• Mendels approach to genetic analysis including
  his experiments and related analytic tools
• A comprehensive example of Mendelian inheritance
  in humans

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Gregor Mendel (1822-1844)
Fig. 2.2
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Themes of Mendels work

• Variation is widespread in nature
• Observable variation is essential for following
  genes
• Variation is inherited according to genetic laws
  and not solely by chance
• Mendels laws apply to all sexually reproducing
  organisms.

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The historical puzzle of inheritance

• Artificial selection has been an important
  practice since before recorded history
• Domestication of animals
• Selective breeding of plants
• 19th century precise techniques for controlled
  matings in plants and animals to produce desired
  traits in many of offspring
• Breeders could not explain why traits would
  sometimes disappear and then reappear in
  subsequent generations.

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State of genetics in early 1800s

• What is inherited?
• How is it inherited?
• What is the role of chance in heredity?

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Mendels workplace
Fig. 2.5
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Historical theories of inheritance

• One parent contributes most features (e.g.,
  homunculus, N. Hartsoiker, 1694)
• Blending inheritance parental traits become
  mixed and forever changed in offspring

Fig.2.6
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Keys to Mendels experiments

• The garden pea was an ideal organism
• Vigorous growth
• Self fertilization
• Easy to cross fertilize
• Produced large number of offspring each
  generation
• Mendel analyzed traits with discrete alternative
  forms
• purple vs. white flowers
• yellow vs. green peas
• round vs. wrinkled seeds
• long vs. short stem length
• Mendel established pure breeding lines to conduct
  his experiments

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Monohybrid crosses reveal units of inheritance
and Law of Segregation
Fig.2.9
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Traits have dominant and recessive forms

• Disappearance of traits in F1 generation and
  reappearance in the F2 generation disproves the
  hypothesis that traits blend
• Trait must have two forms that can each breed
  true
• One form must be hidden when plants with each
  trait are interbred
• Trait that appears in F1 is dominant
• Trait that is hidden in F1 is recessive

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Alternative forms of traits are alleles

• Each trait carries two copies of a unit of
  inheritance, one inherited from the mother and
  the other from the father
• Alternative forms of traits are called alleles

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Law of Segregation

• Two alleles for each trait separate (segregate)
  during gamete formation, and then unite at
  random, one from each parent, at fertilization

Fig. 2.10
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The Punnet Square
Fig. 2.11
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Rules of Probability
Independent events - probability of two events
occurring together What is the probability that
both A and B will occur? Solution determine
probability of each and multiply them
together. Mutually exclusive events -
probability of one or another event occurring.
What is the probability of A or B
occurring? Solution determine the probability
of each and add them together.
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Probability and Mendels Results

• Cross Yy xYy pea plants.
• Chance of Y sperm uniting with a Y egg
• ½ chance of sperm with Y allele
• ½ chance of egg with Y allele
• Chance of Y and Y uniting ½ x ½ ¼
• Chance of Yy offpsring
• ½ chance of sperm with y allele and egg with Y
  allele
• ½ chance of sperm with Y allele and egg with y
  allele
• Chance of Yy (½ x ½) (½ x ½) 2/4, or 1/2

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Further crosses confirm predicted ratios
Fig. 2.12
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Genotypes and Phenotypes

• Phenotype observable characteristic of an
  organism
• Genotype pair of alleles present in and
  individual
• Homozygous two alleles of trait are the same
  (YY or yy)
• Heterozygous two alleles of trait are different
  (Yy)

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Genotypes versus phenotpyes
Yy ? Yy 121 YYYyyy 31 yellow green
Fig. 2.13
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Test cross reveals unkown genotpye
Fig. 2.14
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Dihybrid crosses reveal the law of independent
assortment

• A dihybrid is an individual that is heterozygous
  at two genes
• Mendel designed experiments to determine if two
  genes segregate independently of one another in
  dihybrids
• First constructed true breeding lines for both
  traits, crossed them to produce dihybrid
  offspring, and examined the F2 for parental or
  recombinant types (new combinations not present
  in the parents)

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Results of Mendels dihybrid crosses

• F2 generation contained both parental types and
  recombinant types
• Alleles of genes assort independently, and can
  thus appear in any combination in the offspring

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Dihybrid cross shows parental and recombinant
types
Fig. 2.15 top
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Dihybrid cross produces a predictable ratio of
phenotypes
Fig. 2.15 bottom
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The law of independent assortment

• During gamete formation different pairs of
  alleles segregate independently of each other

Fig. 2.16
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Summary of Mendel's work

• Inheritance is particulate - not blending
• There are two copies of each trait in a germ cell
• Gametes contain one copy of the trait
• Alleles (different forms of the trait) segregate
  randomly
• Alleles are dominant or recessive - thus the
  difference between genotype and phenotype
• Different traits assort independently

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Laws of probability for multiple genes
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Punnet Square method - 24 16 possible gamete
combinations for each parent Thus, a 16 ? 16
Punnet Square with 256 genotypes Thats one big
Punnet Square!
Loci Assort Independently - So we can look at
each locus independently to get the answer.
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Rediscovery of Mendel

• Mendels work was unappreciated and remained
  dormant for 34 years
• Even Darwins theories were viewed with
  skepticism in the late 1800s because he could
  not explain the mode of inheritance of variation
• In 1900, 16 years after Mendel died, four
  scientists rediscovered and acknowledged Mendels
  work, giving birth to the science of genetics

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1900 - Carl Correns, Hugo deVries, and Erich von
Tschermak rediscover and confirm Mendels laws
Fig. 2.19
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Mendelian inheritance in humans

• Most traits in humans are due to the interaction
  of multiple genes and do not show a simple
  Mendelian pattern of inheritance.
• A few traits represent single-genes. Examples
  include sickle-cell anemia, cystic fibrosis,
  Tay-Sachs disease, and Huntingtons disease (see
  Table 2.1 in text)
• Because we can not do breeding experiments on
  humans, we use model organisms.

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In humans we must use pedigrees to study
inheritance

• Pedigrees are an orderly diagram of a families
  relevant genetic features extending through
  multiple generations
• Pedigrees help us infer if a trait is from a
  single gene and if the trait is dominant or
  recessive

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Anatomy of a pedigree
Fig. 2.20
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A vertical pattern of inheritance indicates a
rare dominant trait
Fig. 2.20
Hunitingtons disease A rare dominant
trait Assign the genotypes by working backward
through the pedigree
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A horizontal pattern of inheritance indicates a
rare recessive trait
Fig.2.21
Cystic fibrosis a recessive condition Assign the
genotypes for each pedigree