Educational Technology Overview

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Gregor Mendel1822 - 1884
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Background
Gregor Mendel was an Augustinian priest who lived
in a monastery in Austria and discovered the
basic laws of inheritance and genetics. He was
educated at the University of Vienna, and entered
the Augustinian Abbey of St. Thomas in Brno where
he was the physics instructor. In addition, the
abbot assigned him to work in the garden, where
he worked from 1856 - 1863.
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Humility
Mendel, a university-educated physics instructor,
could have taken this gardening assignment as a
waste of time for an educated person like
himself. Had he done so, he would have been just
an obscure monk who died a long time ago in a
far-away country. Fortunately, he saw the
opportunity in this relatively menial task, and
he became one of the founders of modern biology.
Now, 150 years later we are still studying his
work.
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Peas
Mendel began his work on pea plants. He had a
number of different varieties of peas, with many
different characteristics. Among these, he
identified 7 different characteristics, each of
which had two possible traits. He found that by
self-pollinating plants with particular traits,
he obtained hereditary lines which always had
offspring with that particular trait.
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Self-Pollination
Mendel could force plants to self-pollinate by
shielding each flower from other flowers. Then
the pollen (male cells) made in the stamen of a
flower could only reach the pistil (female part)
of the flower which contains the eggs.
Why are gardens dangerous?
Because all the flowers have pistils!
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The Offspring of Self-Pollination
He found that by self-pollinating plants with
particular traits for a number of generations, he
obtained hereditary lines which always had
offspring with that particular trait.
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True-Breeding Peas
Mendel called these True-Breeding lines of
plants, because their offspring always had the
same trait of one particular characteristic. He
continued his work with 7 particular
easily-distinguished characteristics, each of
which had two traits. He assigned a letter to
each of these characteristics.
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Mendels True-BreedingPea Characteristics
Characteristic
Trait 1
Trait 2
LTR
Tall
Short
T
Stem Height
Flower Color
Purple
White
P
Seed Shape
Round
Wrinkled
R
Seed Color
Yellow
Green
Y
Pod Shape
Smooth
Constricted
S
Pod Color
Green
Yellow
G
Flower Position
Axial
Terminal
A
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Cross-Pollinating Peas
Once Mendel had established these True-Breeding
lines of plants, he tried cross-pollinating them.
That is he took the pollen (male cells) from a
plant with one trait of one of the 7
characteristics (e.g. Stem Height - Tall) and
applied it to the female part of a plant with the
other trait of that characteristic (e.g. Stem
Height - Short).
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Cross-Pollinating Peas (2)
He called the original True-Breeding generation
the P or parent generation, and the offspring
resulting from the cross-pollination the F1 or
First Filial Generation. Remember, although we
will look at small numbers of plants, the results
are RANDOM, so you only get accurate ratios when
you do this to hundreds of plants.
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Cross-Pollinating Peas (3)
Surprisingly, he found that the F1 generation of
pea plants showed only one of the two traits.
The other trait had seemingly disappeared.
X
P
F1
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Cross-Pollinating Peas (4)
This was puzzling, so he again cross-pollinated
plants from the F1 generation to produce an F2 or
second filial generation.
!!!
X
F1
F2
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Cross-Pollinating Peas (5)
In the F2 or second filial generation, the
missing trait re-appeared, but only in
one-quarter of the plants in the F2
generation. Realizing that each plant inherited
from both the male and female side, and that it
must carry characteristics from both, he
visualized the inheritance of any characteristic
as being like a double-coin flip.
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Double Coin-Flip
If you flip two coins there are 4 equally
probable outcomes HH, HT, TH, TT, as illustrated
below
Coin 1
H
T
TH
HH
H
Coin 2
TT
HT
T
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Double Coin-Flip (2)
Each of the 4 possibilities has equal
probability, but HT and TH are not
distinguishable unless you see the actual coins.
Coin 1
H
T
TH
HH
H
Coin 2
TT
HT
T
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Double Coin-Flip (3)
Interestingly, the chance of flipping either HH
or TT is exactly 1/4. The same as the
probability of a short plant in the F2 Generation.
Coin 1
H
T
TH
HH
H
Coin 2
TT
HT
T
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Mendels Hypotheses
Mendel thought about this and developed 4
hypotheses that appeared to explain the data.
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Hypothesis 1
For each characteristic, each plant inherits two
copies of each gene, one from each parent.
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Hypothesis 3
When a plant inherits both alleles, one may be
expressed, one hidden. In these cases, the
expressed allele is called dominant and the
hidden allele is called recessive. Each of
Mendels 7 characteristics (e.g. Tall, Short)
turned out to have a dominant and recessive form.
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Hypothesis 4
When gametes (pollen or eggs) are formed, the two
alleles which an individual carries for each
characteristic separate, and each gamete randomly
gets only one of them. When fertilization occurs,
forming a zygote or fertilized egg, the two
alleles from the male (pollen) and female (egg)
gametes are combined, and these become the two
alleles carried by the new individual (or seed,
at this point).
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Genotypes and Phenotypes
A genotype is the combination of alleles that an
individual carries in its genes. For example a
plant may have a genotype of Tall, Short.
A phenotype is the way a plant looks externally.
In the example above, the phenotype of the plant
would be Tall, because it carries both Tall and
Short alleles, and Tall is dominant while Short
is recessive.
Mendel represented the dominant trait with an
upper-case letter and the recessive trait with a
lower-case letter as in the following chart.
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Pea Characteristics
Gene
Dominant Trait
RecessiveTrait
Tall (T)
Short (t)
Stem Height
Flower Color
Purple (P)
White (p)
Seed Shape
Round (R)
Wrinkled (r)
Seed Color
Yellow (Y)
Green (y)
Pod Shape
Smooth (S)
Constricted (s)
Pod Color
Green (G)
Yellow (g)
Flower Position
Axial (A)
Terminal (a)
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Tall and Short Phenotypes
We know from the previous chart that for the Stem
Height characteristic the Tall (T) allele is
dominant and the Short (t) allele is
recessive. Lets look at the P, F1 and F2
generations in light of this knowledge, our
coin-flip experiment, and Mendels 4 hypotheses.
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Cross-Pollination Genotypes
P Generation
F1 Generation
F2 Generation
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Confirmation!
So we see that Mendels four hypotheses and his
chart of dominant and recessive traits explains
the data from the P, F1, and F2 generations very
nicely.
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Punnet Squares
A Punnet Square is just our coin-flipping diagram
relabeled with alleles (traits) instead of heads
and tails. This one illustrates going from the
F1 to the F2 generation.
Male Ancestor
T
t
Tt
TT
T
Female Ancestor
tt
tT
t
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Homozygous Heterozygous
The pink and green genotypes are homozygous (both
alleles the same) while the yellow genotypes are
heterozygous (different alleles).
Male Parent
T
t
Tt
TT
T
Female Parent
tt
tT
t
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Incomplete Dominance
In tulips there are red, pink, and white flowers.
The red (R) allele has incomplete dominance, so
tulips with heterozygous color are pink.
Male Parent
R
r
Rr
RR
R
Female Parent
rr
rR
r
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Complex Punnet Squares
A Punnet Square with 2 or more characteristics.
RY
Ry
rY
ry
RrYy
RrYY
RRYy
RY
RRYY
Rryy
RrYy
RRyy
Ry
RRYy
rY
RrYY
RrYy
rrYY
rrYy
ry
rryy
rrYy
Rryy
RrYy
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Vocabulary
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Title Objectives Vocabulary Lesson Homework Answer
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Title Objectives Vocabulary Lesson Homework Answer
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