Mendelian Genetics

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Gregor Johann Mendel

• 1822- 1884
• Austrian monk
• Experimented with pea plants
• He thought that heritable factors (genes)
  retained their individuality generation after
  generation

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Time Line

• 1831 Charles Darwin joins crew of Beagle
• 1839 Schleiden and Schwann propose the Cell
  Theory
• 1856 Mendel begins hybridization studies with
  garden peas
• 1857 Louis Pasteur (France) introduces the Germ
  Theory of Disease.
• 1859 Darwin publishes Origin of Species
• 1865 Mendel presents his results in
  transmission of phenotypic traits between
  the generations to the Brünn Society of Natural
  Sciences.
• 1900 Hugo de Vries in Holland, William Bateson
  in Great Britain, Franz Correns in Germany, and
  Erich Tschermak in Austria acknowledged Mendel's
  legacy, and hailed him as the true father of
  classical genetics.

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Terms to Know and Use

• Gene A DNA blueprint controlling synthesis of a
  protein
• Trait - variant for a gene i.e. a purple flower,
  determined by alleles
• Dominant trait - expressed over recessive trait
  when both are present
• Recessive trait - not expressed when the dominant
  trait is present
• Co-Dominant expressed as blended traits

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• Allele - a variation of a gene responsible for
  different traits, often represented as A or a
• Locus - location of a gene,or allele, on a
  chromosome
• Chromosome - strand of DNA containing the genes
• Haploid - one copy of a chromosome
• Diploid - two copies of a chromosome
• Gamete - a spermatozoa or oocyte (egg) cell, they
  are haploid

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• Zygote - cell resulting from the fusion of two
  gametes, they are diploid
• Genotype - the type of alleles on a chromosome
  genetic makeup
• Phenotype - The way a genotype is expressed
  i.e. the color of a flower
• True breeding line - organisms that always pass
  the same genotype to their offspring
• Hybrid - offspring resulting from crossbreeding
  two true breeding lines F1

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Pea Characteristics
Trait on the left is dominant. Trait on the right
is recessive.
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Mendels Hypotheses

• There are alternate forms of genesalleles
• For each trait, organisms have 2 genes, one from
  mom one from dad
• Pollen and egg each carry 1 allele/trait because
  alleles segregate
• When only one allele is expressed other has no
  noticeable effect, it is dominant

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Mendels Experiments

• Plants must possess constant differentiating
  characteristics.
• The hybrids of such plants must, during the
  flowering period, be protected from the influence
  of all foreign pollen, or be easily capable of
  such protection.
• The hybrids and their offspring should suffer no
  marked disturbance in their fertility in the
  successive generations.

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Mendel's Laws of Inheritance

• Law of Segregation during gamete formation
  allele pairs separate or segregate, into
  different gametes (Demonstrated with a test
  cross).
• Law of Independent Assortment suggested that
  each allele pair segregates independently of
  other gene pairs during gamete formation
  (Demonstrated with a dihybrid cross).

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Mother contributes
True Breeding
A
A
or
A
AA
AA
or
Father contributes
AA
AA
A
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Mother contributes
Cross Breeding
a
a
or
A
Aa
Aa
or
Father contributes
Aa
Aa
A
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Monohybrid Cross
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Test Cross
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Dihybrid Cross