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Genetics
By Samuel and Daniel Mueller
What is the Study of Genetics?
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The study of heredity and the variation of
inherited characteristics.
First beginning in the mid-1800’s by
Gregor Mendel, whom documented the
patterns of inheritance in peas.
Mendelian Genetics
The simplest of genetics
Dominant=functioning gene.
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Mendel bred true breeding pea
plants, meaning one plant had a
homozygous dominant genotype
and the other a homozygous
recessive genotype.
He found that the offspring of the
plants inherited their traits in a
repeated pattern.
With a dominant purple plant
breeding with a recessive white
plant, the first generation was
always pink 100% and the 2nd
generation was a ratio of 3:1 of
purple to white offspring.
Recessive=malfunctioning gene
dominant genes almost always
mask.
What does this mean?
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Inherited traits can
come in multiple
versions
Different versions of
the same gene in the
same location on
homologous
chromosomes.
If an organism has a
chromosome with one
allele for a dominant purple
color and another
chromosome with a
recessive white color, The
expressed gene will be the
dominant gene (in
Mendelian genetics).
Genotype VS Phenotype
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Genotype
The organism’s actual
genetics, often expressed in
letters symbolizing alleles.
PP=homozygous dominant
purple flower
Pp=heterozygous dominant
purple flower
Pp=homozygous recessive
white flower
Phenotype
The organism’s
“expressed” gene, the
physical gene.
The flower being
either Purple or White
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How can you tell if an expressed gene is the
organism’s phenotype of genotype
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Breeding with a homozygous recessive
organism!
Since organisms can have both homozygous and
heterozygous genotypes and still the same
genotype.
If the offspring has any recessive traits, the
original organism is heterozygous, if not, then it
is homozygous dominant.
Punnett Squares
The Core of Mendelian Genetics
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A diagram to display
the probability of an
organism’s
inheritance.
Each of the parent’s
alleles are placed next
to each column/row
of a square to
determine the
possible genotypes.
Monohybrid Cross
Mendel’s 1st Law of Heredity
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The Law of Segregation
During Metaphase I of
meiosis, homologous
chromosomes separate and
each allele is given
separately to each gamete.
One allele for each sperm
and egg cell.
Mendel’s 2nd law of Heredity
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The Law of Independent Assortment
Again during Metaphase I, different loci,
or genes, separate into gametes
independently and in equal amounts.
Dihybrid crosses
Incomplete Dominance
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This is when alleles are
not completely
dominant.
Heterozygous
organisms will show a
blend of phenotypes.
RR=Red
R’R=Pink
R’R’=white
Co-dominance
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Two alleles affect the
phenotype equally
and separately. Ex.
Human blood types!
This flower’s
phenotype is a
mixture of red and
white but not
blended.
Genotype: RW
Pleiotropy
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One gene has multiple effects on an
organism.
EX. Acromegaly and Dwarfism (gigantic and
dwarfed respectively).
<- Dwarf
Sex Linked Traits
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Genes which lie on either one
of an organism’s sex
chromosomes (other 22 are
autosomal chrmosomes)
Discovered by T.H. Morgan.
Worked with drosophila (fruit
flies)
An example is hemophilia in
humans. When an X
chromosome with this recessive
gene is inherited by having a
female with two of these
chromosomes or a male with
one, then the gene is
expressed
Linked Genes
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Linked genes are merely a combination of
genes that are inherited together as they
lie on the same chromosome.
For example, a fruit fly's eye color and
wing length lie on the same chromosome
and are thus inherited together.
(Not to be confused with sex-linked genes which are
genes that lie on one of the sex chromosomes)
X-inactivation
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When an organism, male or female, suffers one
of its X chromosomes to become inactivated
(obviously), which usually results in the early
deaths of males since they only have a single X
chromosome.
Not every cell in an organism’s body has to have
an inactivated X chromosome which is how tricolor cats form. In the cells with inactivated X
chromosomes, that patch of fur may be black
while another with both of its chromosomes
activated would be orange.
Epistasis
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The diagram is similar to
Dihybrid crosses, however one
gene has a profound effect on
the other gene.
One gene masks the other, in
this case, with its pigment.
If the mouse has the dominant
CC or Cc genotype, they are
allowed pigment in their fur as
their BB, Bb, or bb genotypes
encode. (black or brown if
CC/Cc and no pigment
whatsoever if recessive for c
gene.)
Acronyms!
M
 M
 M
 M
 C
 Why
 ChoColate
 Hershey
 So
 Good
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•T.H. Morgan
•Meselson and Stahl
•Avery, McCarty and MacLeod
•Watson and Crick
•Erwin Chargaff
•Hershey and Chase
•Frederick Griffith
Yolo because why not
Moving on.
T.H. Morgan
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Worked with Drosophila (fruit flies).
Associated phenotype with chromosome.
(genes located on chromosomes).
White eyed males had a certain X
chrmosome (sex-linked traits as well).
End conclusion was that he found that
either DNA or proteins are the genetic
material of a cell.
Frederick Griffith
“Transforming Principle”
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Mixed Heat-killed, deadly pathogens with
harmless, live bacteria
The DNA from the deadly pathogens
entered the harmless bacteria and
changed their phenotype to make them
deadly as well.
“something” had been transmitted to the
harmless bacteria but was it Protein or
DNA.
How Griffith found this out.
Rough=harmless bacteria
Smooth=deadly bacteria
Avery, McCarty, and MacLeod
DNA is the “transforming principle”
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Purified both the protein and DNA from
the deadly pathogens and individually
injected them into the harmless pathogens
separately.
Both became deadly, however, the
offspring of the one injected with DNA had
deadly offspring.
Showed DNA is what is passed down.
Hershey and Chase
“Blender” experiment
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Uses radioactive isotopes, 35S and 32P
(number=neutrons) to trace where
proteins and DNA go.
When cells multiplied, the sulfur, initially in
the proteins, were found in the medium
(not in the cell), while the phosphorus was
found in the cell as DNA.
Solidified DNA as the genetic material.
Erwin Chargaff
Chargaff’s Rules
Showed that all 4 bases in DNA did not
appear in equal amounts and varied
greatly from species to species.
He did find that the amount of adenine
usually always equals the amount of
thymine and similarly for cytosine to
guanine.
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Watson and Crick
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Found double helix structure of DNA
How is DNA copied
(Meselson and Stahl)
How did they find this out?
14N and 15N (radioactive isotopes)
One strand of DNA only had 14N and the
other only had 15 N changing their
weights
Meselson and Stahl Found Semiconservative replication was the correct
theory
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How does DNA replicate?
http://www.youtube.com/watch?list=EC3EE
D4C1D684D3ADF&v=8kK2zwjRV0M&featur
e=player_detailpage#t=547s
Transcription
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Making mRNA (done by RNA polymerase)
RNA is only single stranded so it is always
copied off the 3’ to 5’ DNA strand so that
it can easily grow in the 5’ to 3’ direction.
Binding site where transcription begins is
called the TATA box
Introns vs Exons
Introns
“Junk” DNA that is
not expressed when
transcribed into RNA
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Exons
The “expressed” DNA
that is used for
translation.
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mRNA splicing
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Molecules called Snurps combine to create
spicesomes which bind to the beginning
and end of an intron segment and extracts
it from the pre-mRNA into regular mRNA
for translation (only exons left).
Translation
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Every 3 letters of mRNA are called triplet
codons and are used to code for one of 20
amino acids.
tRNA can translate mRNA by finding its
anticodon which assures the gene is being
read correctly and attatches the following
amino acid to the primary structure of the
protein being created.
EPA sites
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mRNA enters a ribosome like a dollar bill into a
vending machine, first entering through the A
site (aminoacyl-tRNA site). This site carries the
next amino acid to be added to the polypeptide
chain
Next is the P site (peptidyl-tRNA site) which
holds tRNA which carries the growing
polypeptide chain
Finally is the E site (exit site) where the empty
tRNA leaves the ribosome to be reused later.
Protein Synthesis in Prokaryotes
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DNA in prokaryotes is located in the
cytoplasm already so no 5’ cap or poly-A
tail is needed.
No introns are found, so no splicing occurs
Transcription and translation happen
simultaneously, ribosomes read mRNA as
it is being transcribed.