Transcript “Come to the garden and see my children,” said the monk to the

“Come to the garden and see my
children,” said the monk to the
bishop.
The Gregor Mendel Story
Gregor Mendel
Born in the 1822 in
Moravia (now in the
Czech Republic).
Son of a tenant farmer;
joined the monastery to
get an education.
Deeply interested in
science, particularly in
heredity.
Gregor Mendel
At the monastery in
Brno, Moravia,
Mendel received the
support of Abbot
Napp.
From 1851 - 1855,
studied at the
University of
Vienna, but did not
finish any degree.
What was known...
At the time, it was thought that heredity was
more or less random and unpredictable.
Most scientists believed heredity was
controlled by a liquid factor that blended in the
offspring. For example, a child with one lightskinned and one dark-skinned parent often had
medium-toned skin.
Mendel wondered if there were predictable
patterns to heredity.
Gregor Mendel
With the encouragement of
Abbot Napp, Mendel
conducted his own studies of
heredity in peas.
Collected data from 1856
through 1865.
Mendel chose to study
single, simple traits
individually instead of all
traits in all plants at once.
Gregor Mendel
Mendel presented his work to
the Association of Natural
Research of Brno in 1865.
However, his quantitative
approach to research went
over everyone’s heads!
Further, the “blending”
hypothesis was so widely
accepted, no one saw any
reason to doubt it. Mendel’s
work was seen as a mere
curiosity.
Gregor Mendel
In 1868, Gregor was made
Abbot of the Brno
monastery.
His religious work left
little time for research,
and he eventually gave up
research entirely, though
still convinced he was
right.
Gregor Mendel
In 1900, Mendel’s work
was rediscovered by Hugo
de Vries in the Netherlands
and Erich von TschermakSeysnegg in Austria.
Though sometimes
criticized in its detail, the
main body of Mendel’s
work still stands.
Mendel’s Law of Dominance
Traits are controlled by pairs of “genes.”
Each “gene” can be one of two “alleles”:
dominant or recessive.
If at least one dominant allele is inherited,
the individual will show the dominant trait.
Recessive traits are seen only when two
recessive alleles are inherited.
Now we know this only holds true for single
gene traits.
Law of Segregation
In the cell are pairs of “genes,” one pair for
each trait.
During gamete formation, these pairs are
separated from one another. Each member of
the pair ends up in a separate gamete.
This law still fits what we know today about
gamete formation, except that most traits are
controlled by multiple genes.
Law of Independent
Assortment
At the end of gamete formation, the “genes” for each
trait have been sorted into separate gametes.
Each pair of traits sorts independently of other pairs of
genes; that is, a dominant allele for one trait doesn’t
necessarily follow the dominant allele for another trait.
We now know that while we have millions of genes,
we have only 23 pairs of chromosomes. Obviously
most genes must be linked with others on the same
chromosome, though crossing over during meiosis
does mix genes further.
Monohybrid Cross
A monohybrid cross
illustrates the Law of
Dominance and the
Law of Segregation.
Pure-breeding purple
and pure-breeding
white plants produce
all purple plants in
the F1 generation.
Monohybrid cross
When F1 plants are
bred together, 3/4 of
the offspring are
purple, 1/4 are white.
The recessive trait did
not disappear, but it
only appears if an
individual inherits
two recessive alleles.
The parents were all
carriers of the
recessive allele.
Monohybrid Cross
(Law of Segregation is at work
here.)
How it works: Each plant has two chromosomes with the flower color
gene (one from each parent). All of the F1 plants were homozygous,
having either two dominant alleles or two recessive alleles. Each
individual can put ONE of each chromosome in their gametes. In this
case, the P generation plant with the dominant traits only had the
dominant allele.
Monohybrid Cross
(Law of Dominance
is at work here.)
A purple P generation parent was always crossed with a white P
generation parent, so that each offspring received one dominant (P)
allele and one recessive (p) allele. All offspring were heterozygous.
Monohybrid Cross
All of F1 plants had purple flowers because of the dominant allele they
received from the purple P generation parent. However, the recessive
allele did not disappear. Each plant F1 could donate either the dominant
allele OR the recessive allele to the next generation. Once again, this
shows the Law of Segregation.
Monohybrid Cross
The appearance of the F2 offspring
depends on which combination of
alleles it received from the F1
parents. Each F1 parent had a
50:50 chance of donating either the
dominant (P) or the recessive (p)
allele. There are three ways of
getting a purple F2 offspring and
one way of getting a white
offspring. Notice how the Law of
Dominance works here.
Punnett Square
A Punnett square is one way to determine
the possible outcomes and the probability of
each outcome in a monohybrid cross. It can
also be figured mathematically. Notice that
what goes across the top and down the side
of the square are the possible gametes that
each parent can produce.
Exceptions to the rules...
The traits that Mendel studied in peas
showed complete dominance.
Not all single-gene traits show complete
dominance, however. Some traits appear to
“blend,” which is why the blending
hypothesis was so strong for such a long
time.
Incomplete Dominance
Notice the appearance of
“blending” in the heterozygous
snapdragons. Even in complete
dominance, both alleles in a
heterozygote are expressed,
but only the dominant one is
apparent in the phenotype. In
incomplete dominance, both
are expressed, and both show
in the phenotype in such a way
that they appear to blend
together.
Incomplete Dominance
An example of incomplete dominance in animals.
Co-dominance
Genetically, co-dominance works just like incomplete
dominance: both alleles are expressed in the
phenotype.The only difference between the two is that
the expressions of the two alleles do not blend, but are
both seen in the phenotype.
A classic example is the roan horse, offspring of a
white and a chestnut horse, which has both white and
red hairs, making it look pink.
Co-dominance
Red Roan
Blue Roan
Summary
Mendel’s work showed that heredity is controlled not
by “blending” of liquid factors, but by some kind of
particle that we now call “genes.”
His discovery showed that heredity follows the laws of
probability, and the probability of certain outcomes of
crosses can be predicted.