Gregor Mendel 18221884

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Mendelian Genetics
Gregor Mendel 1822-1884
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Why Sex?
John Maynard Smith
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Asexual Reproduction

• Bacteria can reproduce as often as every 12
  minutes and will go through 120 generations in
  one day
• Thus capable of producing 6 x 1035 offspring per
  day
• Bacteria often produce 1 mutation per 1000
  replications of DNA
• So for fast-growing species, mutation is a good
  way to respond to a changing environment

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Independent Assortment
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Crossing Over
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• The random nature of fertilization adds to the
  genetic variation arising from meiosis.
• Any sperm can fuse with any egg.
• A zygote produced by a mating of a woman and man
  has a unique genetic identity.
• An ovum is one of approximately 8,388,608
  possible chromosome combinations (223).
• The successful sperm represents one of 8,388,608
  different possibilities (223).
• The resulting zygote is composed of 1 in 70
  trillion (223 x 223) possible combinations of
  chromosomes.
• Crossing over adds even more variation to this.

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Mendelian Genetics
Gregor Mendel 1822-1884
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Two possible types of inheritance

• One possible explanation of heredity is a
  blending hypothesis
• The idea that genetic material contributed by two
  parents mixes in a manner analogous to the way
  blue and yellow paints blend to make green
• An alternative to the blending model is the
  particulate hypothesis of inheritance the gene
  idea
• Parents pass on discrete heritable units, later
  known as genes

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Mendels time Today
Mendels garden at Brunn (Brno) Monastery
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Some genetic vocabulary

• Character a heritable feature, such as flower
  color
• Trait a variant of a character, such as purple
  or white flowers

Garden Pea
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Flower Structure
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In Mendels Experiments

• Mendel chose to track
• Only those characters that varied in an
  either-or manner
• Mendel also made sure that
• He started his experiments with varieties that
  were true-breeding
• In a typical breeding experiment
• Mendel mated two contrasting, true-breeding
  varieties, a process called hybridization

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Breeding Terminology

• The true-breeding parents
• Are called the P (parental) generation
• The hybrid offspring of the P generation
• Are called the F1 (filial) generation
• When F1 individuals self-pollinate
• The F2 generation is produced

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Mendel developed a hypothesis to explain his
results that consisted of four ideas

• Alternative versions of genes (different alleles)
  account for variations in inherited characters
• For each character, an organism inherits two
  alleles, one from each parent
• If two alleles differ, then one, the dominant
  allele, is fully expressed in the organisms
  appearance. The other, recessive allele has no
  effect on a hybrid organisms appearance
• The two alleles for each character segregate
  (separate) during gamete formation

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The Law of Segregation
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The Testcross
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The Law of Independent Assortment
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Rule of Multiplication