How are traits inherited?

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How are traits inherited?
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Darwin's dilemma

• Darwin found a lot of evidence supporting his
  theory of evolution by natural selection, but one
  thing troubled him.
• He knew that species occasionally would appear
  with advantageous traits, but feared those traits
  would get blended away.

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• Consider a giraffe that is born with a longer
  neck.
• It would eat better than other giraffes, and
  would have a better than average chance to mate.
• But who would it mate with?
• He thought the kids would be half-way between
  both parents.
• Who would the kids mate with?
• Darwin died wondering about this paradox.

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Blending vs. Particulate Inheritance

• Ironically, the answer was available in a journal
  in Darwin's library, but he never read that
  article.
• Gregor Mendel, an Austrian monk, published his
  data on pea plants providing evidence that
  blending inheritance was incorrect. 

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Mendels particulates factors were genes.

• Mendel determined that hereditary factors
  occurred in discrete pairs, with one of each
  coming from each parent.
• 40 years later, we discovered that these
  factors (now called genes) were carried on
  chromosomes.

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Once the connection was made between genes and
inheritance, Darwin's theory of evolution by
natural selection was greatly strengthened. 

• Incidentally, the combination of Darwinian
  evolution and population genetics is called the
  New Synthesis.

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Mendelian Genetics

• Mendel knew that his 'factors' were discrete and
  non-blending. 
• He also knew much more about the behavior of
  these units of inheritance.
• So lets revisit his peas!

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Law of Segregation

• Mendel's First Law (Law of Segregation) Mendel
  determined that each individual has two copies of
  each gene (e.g., Pp).
• These copies are called alleles.  If both alleles
  are the same, then the individual is homozygous
  (e.g., PP or pp). 
• If the two alleles are different, then the
  individual is heterozygous (e.g., Pp).

• When an individual creates gametes (sex cells
  egg or sperm in humans, egg or pollen grain in
  plants), only one of each allele is packaged in
  the gamete. 
• Mendel determined that which allele appears in
  the gamete is random, with each allele having a
  50 chance.  This rule is the Law of Segregation.

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Flower color

• Pea flowers are either purple or white.
• Peas fertilize themselves, so
• white ? white and purple ? purple.
• called true-breeding
• But

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• if you cross a true-breeding purple with a
  true-breeding white
• all of the offspring have purple flowers.
• Hence Mendel said that purple was dominant to
  white.
• PP purple
• pp white
• Pp purple!

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Terms to understand

• gene stretch of DNA that codes for a particular
  trait. (e.g., flower color)
• allele a particular variant of a gene (e.g.,
  purple)
• genotype what alleles an individual has for a
  particular trait or set of traits (e.g., Pp)
• phenotype the expression of the genes what the
  individual looks like (e.g., purple)

• dominant trait an allele that is expressed no
  matter what the other allele is (e.g., purple
  flower color being dominant to white flower color
  in pea plants)
• recessive trait an allele that is only expressed
  if it is the only allele present (i.e., both
  alleles are the same) (e.g., white flower being
  recessive to purple flower color)

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Terms to understand

• homozygous has 2 copies of the same allele for a
  given trait (e.g., PP or pp)
• heterozygous has 1 copy of each of two alleles
  for a given trait (e.g., Pp)
• F1 generation the kids of the parents
• F2 generation the grandkids of the parents (kids
  of F1)

• gamete sex cell (egg or sperm) only has ONE
  allele for each gene since it only has one
  homologous chromosome (either the one you
  received from Mom or the one you received from
  Dad)
• True-breeding homozygous for the trait.

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Forming gametes

• How many different gametes can PP make?
• 1
• P
• How many different gametes can Pp make?
• 2
• P or p

• When forming gametes, you always need one allele
  for each gene.
• How many different gametes can PPTt make?
• 2
• PT or Pt

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Determining the number of different gametes
possible

• AaBBCc?
• 2 x 1 x 2 4
• AaBbCC?
• 2 x 2 x 1 4
• AaBbCcDd?
• 2 x 2 x 2 x 2 16
• AAbbCCddEE?
• 1 x 1 x 1 x 1 x 1 1
• What is it?
• AbCdE

• Which of the following gametes can this parent
  (AABbCCDdeeFf) make? a. AAbCEf b. ABCDEF c.
  abcdef d. ABCdef
• d is the answer.
• What is the chance of that parent producing that
  gamete?
• 1/8 Why?

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Determining the number of different alleles

• AaBBCc?
• 2 1 2 5 alleles
• AaBbCC?
• 2 2 1 5 alleles
• AaBbCcDd?
• 2 2 2 2 8
• AAbbCCddEE?
• 1 1 1 1 1 5

• How many different genes are shown at right?
• 3, 3, 4, and 5 (top to bottom)