Mendelian Genetics

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Mendelian Genetics

• Pea Plants and Punnett Squares

Chapter 6, sections 3-5
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Gregor Mendel Father of Genetics

• Austrian monk in mid-1800s
• Interested in nature
• Studied inheritance of traits in pea plants
• Conducted experiments and recorded data
• Developed some basic laws of inheritance

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Pea Plants because

• fast-growing
• mature quickly
• easy to care for
• have many easy-to-distinguish traits /
  characteristics examples flower color
  either purple or white, plant height either
  tall or short, seed color either green or yellow

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Mendels Experiments
Video clip 430

• Step One Mendel allowed plants to
  self-pollinate for several generations to make
  sure that they were pure, or true-breeding, for
  flower color.
• Today, we would describe each of these pure
  plants as homozygous.

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Self-Pollination
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Experiments, contd

• Step Two Mendel took two pure flowers one
  purple and one white and cross-pollinated them.
  He called these two flowers the P generation (P
  for parental).
• Results all offspring were purple he called
  these offspring the F1 generation (F1 for first
  filial)

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Cross-Pollination
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Experiments, contd

• Step Three Mendel allowed the F1 flowers
  which were all purple - to self-pollinate. The
  new offspring were called the F2 generation
  (second filial).
• Results Roughly 3 out of every 4 flowers was
  purple and 1 out of every 4 was white.

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Interpreting the Results

• Today, we know that
• purple flower color is dominant to white (in pea
  plants)
• the white flower color did not completely
  disappear it was simply hidden (masked) in the
  F1 generation

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Results, contd

• Remember
• Dominant the allele that always shows up in the
  phenotype as long as at least one copy is present
• Recessive the allele that will only show up in
  the phenotype if there are no dominant alleles

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Results, contd

• In the F1 generation, all of the plants were
  heterozygous (genotype) for flower color they
  each had one purple allele and one white allele,
  but they all had a purple phenotype (appearance)

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Results, contd

• In the F2 generation
• one of the purple flowers was homozygous
  dominant
• the other two purple flowers were heterozygous
• the white flower was homozygous recessive

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Punnett Squares

• Now, we can use Punnett squares to show what
  happened in Mendels initial experiments

The Punnett Square at left shows the cross
between two pure plants one parent is homozygous
dominant and the other parent is homozygous
recessive. This is like Mendels P generation.
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Setting up Punnett Squares
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Punnett Square F1 cross

• For Mendels F1 cross, he allowed plants to
  self-pollinate.
• The results are shown at right ?
• 75 (3/4) were tall and 25 (1/4) were short.

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Mendels Conclusions

1. Inheritance is based on genes, which are passed
   on from parents to offspring.
2. Some versions of a gene may be dominant while
   others may be recessive.
3. In most sexually reproducing organisms, each
   individual has two alleles for each gene one
   from each parent.

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Conclusions, contd

1. When gametes (eggs and sperm) form, the two
   alleles for each gene separate (segregate) from
   each other (Law of Segregation).
2. When gametes form, different genes separate
   (segregate) independently of each other (Law of
   Independent Assortment).

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Law of Segregation
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Law of Independent Assortment
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Factors in Mendels Success

1. Mendel concentrated on one trait at a time.
2. Only after he established the behavior of one
   trait (monohybrid cross) did he consider two
   traits together (dihybrid cross).
3. He conducted a large number of crosses
   (2000-3000) to eliminate chance and to obtain a
   valid and accurate explanation

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Success, contd

• He actually counted the number of offspring of
  each category and maintained accurate records for
  each generation in each experiment.
• Luck -- though Mendel did not know this -- in the
  selection of pea plants as well as in the
  selection of the particular traits he studied.
• In each pair, one form of the trait is completely
  dominant over the other.
• It also helped that the genes for the traits
  Mendel studied are present on seven different
  chromosomes in the pea plant.