Mendelian Genetics

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Mendels Legacy

• Genetics is everywhere these days and it will
  continue to become even more important in decades
  to come.
• So wouldnt it be nice if people understood it
  better?

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Terms to Know and Use

• Genetics the study of heredity

• Heredity the passing of traits from the
• parents to their offspring

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• Trait - variations of a gene (i.e. black or
  brown hair) determined by alleles
• Allele different forms a gene, often expressed
  as Y or y, X or x, Z or z

• Gene - a heritable feature on a chromosome
  
• (i.e. hair color)

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• Chromosome - strand of DNA that codes for genes
• Locus - location of a gene, or allele, on a
  chromosome

• Monohybrid cross cross involving parents
• differing in only 1 trait
• True-breeding - organisms that always pass
• the same genes to their offspring

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• Dominant trait - expressed over a recessive trait
  when both are present
• Recessive trait - not expressed when the dominant
  trait is present

• Homozygous when both alleles of a gene are
• the same (i.e. pure, TT)
• Heterozygous when the two alleles are not
• the same (i.e. hybrid, Tt)

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• Genotype - the type of alleles on a chromosome
  (gene makeup)

• Phenotype - The way a genotype is expressed
  (physical appearance)

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Gregor Johann MendelFather of Genetics

• 1822- 1884
• Austrian monk
• Experimented with pea plants
• Trained in math and science

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Garden Pea Experiments 1856-64

• Why pea plants? (Pisium sativum)
• Because they
• Were easy to grow
• Produced a large number of offspring
• Matured quickly
• Had both male and female parts

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The Problem

• T.A. Knight, a scientist, saw that if he crossed
• true bred true bred
• purple pea plant (P) x white pea plant (P)
• ALL the offspring would be purple (F1).

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The Problem (contd)

• If he then crossed the purple (F1) offspring
• hybrid hybrid
• purple pea plant (F1) x white pea plant (F1)
• Most offspring are purple (F2) few white (F2).

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Mendels Answer
Mendel used math with science to explain
heredity. He counted 705 purple 224
white total 929 pea plants Thus he saw a 31
ratio.
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Mendels Answer (contd)

• To explain this ratio he came up with Rules of
  Heredity
• Parents transmit genes to offspring
• 2) Each individual has 2 genes (1 from each
  parent)
• 3) Some genes are dominant and others recessive

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Mother contributes
P Generation
P
P
or
p
Pp
Pp
or
Father contributes
Pp
Pp
p
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Mother contributes
F1 Generation
P
p
or
P
Pp
PP
or
Father contributes
pp
Pp
p
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Lucky or Right On?

• Mendel repeated his experiments while observing
  other traits such as
• height of plant (tall vs. short)
• pod appearance (inflated vs. constricted)
• pod color (green vs. yellow)
• seed texture (round vs. wrinkled)
• seed color (yellow vs. green)

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Pea Characteristics
Trait on the left is dominant. Trait on the right
is recessive.
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Mendel's Laws of Inheritance

• Law of Segregation - states that the two alleles
  separate when gametes (sperm/egg) form. Thus, a
  gamete receives only one allele from each parent.
  (test cross)
• Law of Independent Assortment - states that
  different alleles (traits) separate
  independently. Thus, color, height, pod shape,
  etc. are not connected together. (dihybrid cross)

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Determining Unknown Genotypes

• How do you know if a dominant phenotype is
  homozygous or heterozygous?
• Scientists can perform a test cross, where they
  cross the unknown with a recessive (known)
  phenotype.

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Mother contributes
OPTION 1 Test Cross
a
a
or
A
Aa
Aa
or
Father contributes
Aa
Aa
A
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Mother contributes
OPTION 2 Test Cross
a
a
or
A
Aa
Aa
or
Father contributes
aa
aa
a
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Mendel's Laws of Inheritance

• Law of Segregation - states that the two alleles
  separate when gametes (sperm/egg) form. Thus, a
  gamete receives only one allele from each parent.
  (test cross)
• Law of Independent Assortment - states that
  different alleles (traits) separate
  independently. Thus, color, height, pod shape,
  etc. are not connected together. (dihybrid cross)

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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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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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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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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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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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Any Questions?

• The problem with the gene pool is that there is
  no lifeguard. Unknown
• Life can only be understood backwards but it
  must be lived forwards.
• Kierkegaard