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
• 1847 Semmelweiss proposes that infection is
  spread by contaminated hands of physicians.
• 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 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 heritable feature i.e. flower color
  coded for on a chromosome
• 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

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• Allele - a variation of a gene responsible for
  different traits, often expressed as A or a
• Locus - location of a gene,or allele, on a
  chromosome
• Chromosome - strand of DNA that codes for 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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Garden Pea Experiments 1856-64

• Mendel disagreed with the Blending Theory of
  inheritance.
• Started with34 kinds peas Pisium sativum
• After 2 years he had 22 purebreds
• http//www.dnaftb.org/dnaftb/1/concept/index.html

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Pea Characteristics
Trait on the left is dominant. Trait on the right
is recessive.
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Flower Parts
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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
• Sperm and egg each carry 1 allele/trait because
  alleles segregate
• When only one allele is expressed other has no
  noticeable effect, it is dominant
• http//www.sumanasinc.com/webcontent/anisamples/no
  nmajorsbiology/independentassortment.html

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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).
• http//www.sumanasinc.com/webcontent/anisamples/ma
  jorsbiology/independentassortment.html

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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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Mendel's Laws of Inheritance

• Law of Segregation during gamete formation
  allele pairs separate or segregate, into
  different gametes (Demonstrated with a test
  cross).
• http//www.sumanasinc.com/webcontent/anisamples/no
  nmajorsbiology/independentassortment.html
• Law of Independent Assortment suggested that
  each allele pair segregates independently of
  other gene pairs during gamete formation
  (Demonstrated with a dihybrid cross).
• http//www.sumanasinc.com/webcontent/anisamples/ma
  jorsbiology/independentassortment.html

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Dihybrid Cross
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wrinkled
wrinkled
Smooth
Smooth
Smooth
Tall
Tall
short
Tall
short
Two chromosomes of one parent are represented on
the left. Possible alleles passed on to the
offspring are on the right. (Consider smooth or
wrinkled peas AND tall or short plants)
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Mother contributes
Dihybrid Cross
SB
sb
Sb
sB
SB
SsBb
SSBB
SSBb
SsBB
Sb
SSbB
SsbB
Ssbb
SSbb
Father contributes
sB
sSBB
sSBb
ssBB
ssBb
sb
sSbB
sSbb
ssbB
ssbb
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Chromosomes DNA
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Meiosis I
One diploid sex cell divides
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Meiosis II
Result One diploid cell four haploid cells
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Crossing Over in Meiosis I
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DO ALL GENES ASSORT INDEPENDENTLY?

• Genes on the same chromosome tend to be inherited
  together linked
• Sex-linked genes color blindness, MD,
  hemophilia. X Y
• X
• X

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Credits

• Mendel Image pw1.netcom.com/aguldo/
  agga/bt/txt/bt1899.htm
• Pea trellis http//www.floridata.com/ref/p/images
  /pisu_sa1.jpg
• Experiments in Plant Hybridization (1865)by
  Gregor Mendel Read at the meetings of February
  8th, and March 8th, 1865 to the the Natural
  History Society of Brünn (paper can be read at
  http//www.mendelweb.org/home.html
• MendelWeb, edited by Roger B. Blumberg.
  (http//www.netspace.org/MendelWeb/, Edition 97.1
  1997)
• The Biology Project http//www.biology.arizona.e
  du/default.html
• Meiosis Images www.micro.utexas.edu/.../bio304/
  genetics/meiosis.4.gif
• Crossing Over image http//medlib.med.utah.edu/b
  lock2/biochem/Formosa/Figures/Lecture5/5-1520Meio
  sis.JPG
• Monohybrid, dihybrid, test cross pea flower
  ntri.tamuk.edu/homepage-ntri/lectures/
  biology/test-cross.gi

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Who Cares About Genetics?
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