Genetics and Mendel

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Genetics and Mendel
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Mendels Work

• Mendels pea experiments led him to conclude that
  inheritance is determined by factors (genes)
  passed from one generation to the next.
• These factors control all the various traits of
  an organism and may come in different forms
  (alleles).

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Principle of Dominance

• For a specific gene one allele for a trait is
  dominant over the others (recessive).
• If the dominant allele is present, this is the
  characteristic the organism displays.
• The only time the recessive allele is expressed
  is if there is NO dominant allele.

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Dominance

• The F1 generation from a cross of true-breeding
  plants are all Hybrids.
• The F2 generation shows the return of the
  recessive allele.

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Segregation

• Mendel showed that the recessive allele does not
  disappear. Crossing 2 Hybrid plants (F1) produced
  offspring that exhibited the recessive allele
  once again.
• For all the F2 offspring produced about ¼ of them
  exhibited the recessive allele.
• 31 ratio of DominantRecessive traits displayed.

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Segregation

• How did this occur?
• Mendel suggested that the 2 alleles for a
  specific trait that an organism contains
  segregate during sex cell formation (meiosis).
• Effectively, an organism will make two types of
  gametes 1 type has one allele for a gene and the
  2nd type holds the other allele.
• This way, of the 2 possible alleles a parent can
  contribute to offspring only one allele from each
  parent and becomes somewhat a game of chance.

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Punnett Squares

• We can use Punnett Squares to predict the outcome
  of a genetic cross for a particular trait.
• Homozygous Contains 2 of the same allele for a
  trait.
• Heterozygous Contains 2 different alleles for a
  trait (hybrids!)
• Genotype The genetic makeup of an organism for a
  trait.
• Phenotype The physical characteristic displayed
  for a specific trait.

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Independent Assortment

• Mendel then looked at the bigger picture. What
  happens to all of the genes of an organism during
  gamete formation.
• Does segregation of one gene influence another?
  In other words as one gene segregates does it
  impact the segregation of another?
• Mendel examined a cross of plants for two
  specific traits

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Independent Assortment

• He crossed a plant that was Homozygous for yellow
  round peas (YYRR) with a plant that was
  Homozygous wrinkled green peas (yyrr).
• The F1 generation was all yellow round as
  expected.
• Whats their genotype?__________

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Independent Assortment

• Mendel then let the plants self-pollinate (just
  as before) to produce the F2 generation.
• Of the resulting F2 generation every possible
  combination of traits were shown.
• 556 seeds produced 315 were round/yellow.
• 32 were wrinkled/green
• 209 were a combination of phenotypes round/green
  and wrinkled/yellow
• The results supports the idea that traits
  segregate separately.

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Independent Assortment

• Genes for different traits segregate
  independently during the formation of gametes.
• One genes segregation does not influence
  anothers.

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Exceptions to Mendel

• Mendels findings and rules hold true for many
  cases in genetics. But there are some exceptions.
  (Figures, right?)
• Some genes do not have a dominant or recessive
  form, and some have more than just 2 alleles.

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Incomplete Dominance

• One allele is not dominant over another.
• The result of these crosses produce heterozygous
  offspring showing traits between both homozygous
  parents.
• Example snapdragon

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Codominance

• Similar to incomplete dominance. In codominant
  cases both alleles contribute to the organisms
  phenotype.
• What you see is both phenotypes being expressed!
• Example Roan cattle color of chicken feathers

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Multiple Alleles

• Many genes have more than 2 possible forms
  (alleles) in a population.
• Makes for greater diversity for a trait.
• Example Blood type in humans

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Polygenetic Traits

• Not every trait is controlled by one gene. Often,
  multiple genes influence how certain traits are
  displayed.
• Traits controlled by two or more genes are said
  to be polygenetic.
• Example Skin color in humans, fruit fly eye
  color.