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Gregor Mendel and Heredity
R. H. Beeman
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2008-03-01
Gregor Mendel
1822 - 1884
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Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
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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 faraway 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.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
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R. H. Beeman
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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!
Gregor Mendel and Heredity
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R. H. Beeman
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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.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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 easilydistinguished characteristics, each of which
had two traits. He assigned a letter to each of
these characteristics.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
Mendel’s True-Breeding
Pea Characteristics
Characteristic
LTR Trait 1
Trait 2
Stem Height
T
Tall
Short
Flower Color
P
Purple
White
Seed Shape
R
Round
Wrinkled
Seed Color
Y
Yellow
Green
Pod Shape
S
Smooth
Constricted
Pod Color
G
Green
Yellow
Flower Position
A
Axial
Terminal
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
Cross-Pollinating Peas
Once Mendel had established these TrueBreeding lines of plants, he tried crosspollinating 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).
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
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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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2008-03-01
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
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
Cross-Pollinating Peas (5)
In the F2 or second filial generation, the
missing trait re-appeared, but only in onequarter 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.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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
H
HH
TH
T
HT
TT
Coin 2
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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
H
HH
TH
T
HT
TT
Coin 2
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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
H
HH
TH
T
HT
TT
Coin 2
Gregor Mendel and Heredity
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R. H. Beeman
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Mendel’s Hypotheses
Mendel thought about this and developed 4
hypotheses that appeared to explain the data.
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R. H. Beeman
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Hypothesis 1
For each characteristic, each plant inherits two
copies of each gene, one from each parent.
Hypothesis 2
There are alternative versions of genes. For
example, Tall and Short.
In modern biology the two versions of each gene
are called alleles.
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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 Mendel’s 7 characteristics (e.g. Tall,
Short) turned out to have a dominant and
recessive form.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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).
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
Pea Characteristics
Gene
Dominant Trait
RecessiveTrait
Stem Height
Tall (T)
Short (t)
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)
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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
Mendel’s 4 hypotheses.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
Cross-Pollination Genotypes
P Generation
F1 Generation
F2 Generation
T
T
T
t
T
T
t
t
T
t
T
t
T
t
t
T
T
t
t
t
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R. H. Beeman
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2008-03-01
Confirmation!
So we see that Mendel’s four hypotheses and his
chart of dominant and recessive traits explains
the data from the P, F1, and F2 generations very
nicely.
Gregor Mendel and Heredity
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R. H. Beeman
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2008-03-01
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
t
TT
Tt
tT
tt
Female Ancestor
t
Gregor Mendel and Heredity
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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
t
TT
Tt
tT
tt
Female Parent
t
Gregor Mendel and Heredity
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R. H. Beeman
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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
r
RR
Rr
rR
rr
Female Parent
r
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2008-03-01
Complex Punnet Squares
A Punnet Square with 2 or more characteristics.
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Gregor Mendel and Heredity
Vocabulary
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R. H. Beeman
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2008-03-01
Heredity:
The passing of genetic traits from parent to
offspring
Genetics:
The science of heredity and of the
mechanisms by which traits are passed from
parents to offspring.
Monohybrid Cross:
A cross between individuals that involves one
pair of contrasting traits.
Gregor Mendel and Heredity
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True-Breeding:
Organisms or genotypes that are
homozygous for a specific trait and thus
always produce offspring that have the same
phenotype for that trait
P Generation:
Parental generation, the first two individuals
that mate in a genetic cross.
F1 Generation:
The first generation of offspring obtained from
an experimental cross of two organisms
Gregor Mendel and Heredity
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R. H. Beeman
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F2 Generation:
The second generation of offspring, obtained
from an experimaental cross of two
organisms; the offspring of the F1 generation
Alleles:
Parental generation, the first two individuals
that mate in a genetic cross.
Dominant:
The trait or allele that is expressed when two
different alleles for the same characteristic are
inherited.
Gregor Mendel and Heredity
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Recessive:
The trait or allele that is expressed only when
two recessive alleles for the same
characteristic are inherited.
Homozygous:
An individual that identical alleles for a trait on
both homologous chromosomes.
Heterozygous:
An individual that has two different alleles for
a trait.
Gregor Mendel and Heredity
Vocabulary
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R. H. Beeman
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2008-03-01
Genotype:
The entire genetic makeup of an organism;
also the combination of genes for one or more
specific traits.
Phenotype:
An organism’s appearance or other
detectable characteristic that results from the
organism’s genotype and the environment.
Gregor Mendel and Heredity
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2008-03-01
Law of Segregation:
Mendel’s law that states that the pairs of
homologous chromosomes separate in
meiosis so that only one chromosome from
each pair is present in each gamete.
Law of Independent Assortment:
The law that states that genes separate
independently of one another in meiosis.
Punnet Square:
A graphic used to predict the results of a
genetic cross.
Gregor Mendel and Heredity
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Test Cross:
The crossing of an individual of unknown
genotype with a homozygous recessive
individual to determine the unknown
genotype.
Probability:
The liklihood that a possible future event will
occur in any given instance of the event; the
mathematical ratio of the number of times one
outcome of any event is likely to occur to the
number of possible outcomes of the event.
Gregor Mendel and Heredity
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Pedigree:
A diagram that shows the occurrence of a
genetic trait in several generations of a family.
Sex-linked gene:
A gene found on one of the sex
chromosomes, such as the X chromosome or
Y chromosome in humans.
Polygenic Inheritance:
A characteristic of an organism that is
determined by many genes.
Gregor Mendel and Heredity
Vocabulary
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R. H. Beeman
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2008-03-01
Incomplete Dominance:
A condition in which a trait in an individual is
intermediate between the phenotype of the
individual’s two parents because the dominant
allele is unable to express itself fully.
Multiple Alleles:
More than two alleles (versions of the gene)
for a genetic trait.
Codominance:
A condition in which both alleles for a gene
are fully expressed.
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Gregor Mendel and Heredity
R. H. Beeman
2008-03-01
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Ω
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Gregor Mendel and Heredity
2008-03-01