Classical Genetics

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Classical Genetics

• The Legacy of Gregor Mendel
• Or
• The Monk with the Missing Peas

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The Big Question

• Why do children look like their parents?

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Who was Gregor Mendel?

• Father of Genetics
• Studied pea plants for eight years
• Published his results in 1865
• Grew over 10,000 pea plants, keeping track of
  progeny number and type.

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Father of Genetics

• 1st person to succeed in predicting how traits
  are passed from one generation to next

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Why is he so important?

• Studied one trait at a time
• Analyzed his data mathematically
• Looked at multiple traits
• Used multiple trials

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Why peas?

• Quick growing
• Lots of different traits

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Step 1 Start with pure-bred plants

• Pure-bred plants only produce one type of
  offspring
• Green pea plants only make green offspring
• Yellow pea plants only make yellow offspring

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Step 2 The First Generation

• Mendel chose true-breeding pea plants as his
  parental generation (when self-pollinated, always
  produced the same type of offspring)
• He crossed a true-breeding tall plant with a
  true-breeding short plant
• All of the offspring were tall!

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Step 3 The Second Generation

• Next, he crossed two tall offspring plants with
  each other
• ¾ of the offspring in the second generation were
  tall ¼ were short

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So how does this pea thing work?

• DNA from the Beginning

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The Second Generation, cont.

• Mendel did similar monohybrid crosses with the
  other traits as well.
• In every case, he found that one trait seemed to
  disappear in the F1 generation and reappear in ¼
  of the F2 plants
• This is where dominant and recessive come from

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Conclusion Law of Segregation

• Each parent has 2 genes that determine each trait
  
• Each parent can only give one of its two genes
  for each trait
• Therefore, these two genes must randomly separate
  to the sex cells so that sex cells contain only
  one gene of the pair.

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Step 4 Mendels Dihybrid Crosses

• Performed another set of crosses where he used
  peas that differed from each other in two traits
  rather than just one

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The first generation

• Took true-breeding pea plants that had round
  yellow seeds (RRYY) and crossed them with
  true-breeding pea plants that had wrinkled green
  seeds (rryy).
• The F1 plants all had round yellow seeds

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Step 5 The second generation

• F1 plants self-pollinated
• F2 plants
• 9 round yellow
• 3 round green
• 3 wrinkled yellow
• 1 wrinkled green

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Conclusion The Law of Independent Assortment

• Genes for different traits are inherited
  independently of each other
• When a pea plant with the genotype RrYy produces
  gametes, the alleles R and r will separate from
  each other (the law of segregation) as well as
  from the alleles Y and y and vice versa
• Alleles can then recombine in four different ways
  (see next slide)
• We now know that this is only true if genes are
  located on different chromosomes or are far apart
  on the same chromosome

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Testcross

• A cross of an individual of unknown genotype with
  an individual of a known genotype (usually
  homozygous recessive)
• Unknown R_ x rr
• If any offspring show the recessive phenotype,
  then the unknown parent must have been
  heterozygous

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Pedigrees

• A graphic representation of an individuals
  family tree, which permits patterns of
  inheritance to be recognized.

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When are pedigrees used?

• When testcrosses cannot be made
• When number of offspring is too small
• Or if results of testcross would take too long