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Gregor Mendel
“Give peas a chance!”
How a monk learned about inheritance using
a tasty vegetable
Gregor Mendel Born in 1822.
Gregor Mendel was a
monk who taught high
school and worked in the
monastery gardens.
In fact, he loved plants so
much that he spent most
of his adult life studying
how certain traits were
passed from “parent”
plant to “baby” plant.
Gregor Mendel
Considered the “Father
of Genetics” due to his
work studying
inheritance.
Genetics: the
scientific study of
heredity.
Mendel and his peas Gregor Mendel is most famous for his scientific
study of pea plants.
He chose pea plants because they reproduce
quickly and produce large amounts of offspring.
Pea plants have seven observable characteristics.
Each characteristic has two possible traits.
Characteristic Possible Traits
1. Plant height -----------------------------Long or short stems
2. Flower position along stem--------------Axial or terminal
3. Pod color-----------------------------------Green or yellow
4. Pod appearance------------------------Inflated or constricted
5. Seed texture-----------------------------Smooth or wrinkled
6. Seed color----------------------------------Yellow or green
7. Flower Color--------------------------------Purple or white
Purebred
By controlling plant pollination
(fertilization), Mendel was able to create
pea plants that were purebred for each
trait.
This is also known as true breeding.
When they self-fertilize, true breeding
parent plants always produce offspring
with the same trait.
Purebred Examples of True Breeding:
A purebred Labrador bred with a purebred Labrador
will always produce fully Labrador offspring. No
Chihuahuas???
A self-pollinating plant purebred for purple flowers
will always produce offspring with purple flowers.
A self-pollinating plant
purebred for white flowers
will always produce offspring
with white flowers.
What happens if you cross two
purebred plants?
purple flowers x white flowers
or
wrinkled seeds x smooth seeds
or
tall plants x short plants
or
Labrador x Poodle
Let’s take a look at flower color….
Mendel called the purebred parents
the P generation.
For each cross (plant “parents”),
Mendel cross-pollinated plants who
were true breeding with opposite traits.
Example: the P generation consisted of
a purebred purple flower plant and a
purebred white flower plant
Flower color cross
Mendel called the offspring plants the F1
generation (1 for 1st , F for filial- son or daughter)
Plants in the F1 generation are called hybrids
because their parents have different traits.
What do you think Mendel expected to see when
he crossed a purebred purple flower plant with a
purebred white flower plant?
All of the hybrid, or F1 Generation,
offspring had purple flowers!
Crossing two purebred plants
In every case of crossing two purebred plants,
one trait “won out” in the F1 generation.
Examples:
Purple flower color “won out” over white flower
colors
Smooth seed texture “won out” over wrinkled seed
texture.
Why do you think this is???
Dominant and Recessive
Traits can be dominant or recessive!
Mendel called the trait that appeared in the F1
generation (purple flowers in this case) the
dominant trait.
Mendel called the trait that did not appear in
the F1 generation (white flowers) recessive
trait.
What do you think happened when
Mendel let plants from the F1
generation self-pollinate?
Did he end up with purebred purple
flowers?
No!!!
Dominant and Recessive Traits
About 25% of the flowers in the F2 generation
were white!
Even though the F1 generation looked like
purebred purple plants, they carried the trait
for white flowers somewhere inside.
Where was this white flower trait “hidden”??
The traits are hidden in the genes!
A gene is a segment of DNA that codes for a particular protein. The protein results in the trait.
So….. Each characteristic = One gene
For example:
There’s a gene for plant height
There’s a gene for seed color
There’s a gene for flower color
etc...
Alleles
Each alternative (different) form of a gene is
called an allele.
Examples:
The plant height gene has two alleles, tall and
short.
The flower color gene has two alleles, purple and
white
The seed texture gene has two alleles, wrinkled
and smooth.
In other words… Inheritance is determined
by factors (genes) that are
passed from one generation
to the next.
These genes can come in
different forms called
alleles.
Law of Dominance: Some
alleles are dominant and
others are recessive
For every gene, how many alleles do you get from each parent?
This is a result of what process??
From his research, Mendel came to
two conclusions:
1. The Law of Segregation: Two factors (alleles)
control each specific characteristic (gene). These
factors (alleles) are separated during the formation
of gametes (meiosis).
2. The Law of Independent
Assortment: Factors (alleles)
for different characteristics (genes)
are distributed to gametes
independently. This means that the
allele for seed texture isn’t dependent on the allele
for plant height, etc.
As humans, our alleles are what
create the diversity among us!
Terminology
Alleles are represented with letters
Dominant – The allele/trait that is expressed (T)
Recessive – The allele/trait that is hidden (t)
Genotype – The allele makeup of a gene (TT,
Tt, tt)
Phenotype – The physical appearance of a trait
(Tall plant, purple flowers, etc)
Genotypes and Phenotypes
There are three possible genotypes:
Homozygous Dominant (BB, TT, PP)
Homozygous Recessive (bb, tt, pp)
Heterozygous (Bb, Tt, Pp)
Homozygous: Has two of the same allele.
Heterozygous: Has one of each allele.
Mendelian Genetics
Punnett Squares
Mendel’s work has allowed
us to be able to predict
possible offspring
outcomes between two
known genotypes.
This prediction is illustrated
through a punnett square.
Monohybrid Crosses
Two heterozygous
tall plants are
allowed to cross-
pollinate.
Use a punnett square
to predict the
genotype and
phenotype
probabilities.
Monohybrid Crosses
Genotype Probabilities:
TT: 25%
Tt: 50%
Tt: 25%
Phenotype Probabilities:
Tall: 75%
Short: 25%