Transcript Mendel`s Experiments

Gregor Mendel was a priest from the mid
19th century who conducted
experiments in his garden.
 Mendel is considered the
“Father of Genetics!”

While working in his garden, Mendel
wondered why different pea plants grew
tall, while others were short.
 Some had green seeds, others yellow.
 He called all these characteristics traits.

Mendel experimented with thousands of
pea plants to understand the process of
heredity – the passing down of traits
through generations.
 Genetics is the scientific study of
heredity.

If you look at a flower, you can see that
the petals surround two structures.
 The first is the Pistil. It produces eggs, the
female sex cell of plants.
 The second is the stamen. Stamens
produce pollen – the male sex cell. It
contains sperm.

When the egg and pollen (sperm) cells
join, this is called fertilization.
 Pea plants are usually self-pollinating –
meaning they can pollinate themselves,
even when they are alone.
 Mendel developed a method to crosspollinate – meaning he moved the
pollen from one plant to the pistol of
another. This is called “cross pollination”
or “crossed” for short.

Mendel decided to cross plants that
looked completely different to see what
would happen.
 He started with purebred plants. Plants
that he saw that generation after
generation, produced plants that looked
identical to the parent plant.
 He called the Parent generation the “P
generation” for short.

In one experiment, Mendel crossed purebred tall plants with purebred short
plants.
 Scientists today call these parent plants
the parental generation (P generation).
 All of offspring were Tall!

The offspring generation are called “F
generations.”
 The “F” stands for filial – the Latin word for
“daughter” and “son.”
 The first generation of offspring are noted
by a little number 1 after the letter “F”
(F1).
 The second generation (from mating the
F1’s together) form the F2 generation
(and so forth).

When the plants in the F1
generation were full grown,
Mendel allowed them to selfpollinate.
 Surprisingly, the plants in the F2
generation were a mix of tall
and short plants!
 The shortness trait had
reappeared!

Mendel counted the tall and short
plants.
 About 3/4ths of the plants were tall, while
only 1/4th of the plants were short.

Mendel also crossed pea plants with
other contrasting traits.
 In all of Mendel’s crosses, only one form
of the trait appeared in the F1
generation.
 However, in the F2 generation, the “lost”
form of the trait always reappeared in
about 1/4th of the plants!

Mendel reached several conclusions on
the basis of his experimental results.
 He reasoned that individual factors, or
sets of genetic “information,” must
control the inheritance of traits from
peas!
 The factors must come in PAIRS!
 One from the male parent and one from
the female parent!

The last conclusion Mendel made was
that one factor in the pair can Mask or
Hide the other!
 In Mendel's experiments, Tallness in pea
plants masked the Short pea
characteristic.


Today, we use the word “Gene” instead
of “factor.”
 Alleles are the different forms of the
same Gene.
 Plant Height = Gene

› Tall plants = allele
› Short plants = allele
Each pea plant inherits two alleles from
its parents – one from the egg and one
from the sperm.
 A pea plant may inherit two alleles for
tall stems, two alleles for short stems, or
one of each.

An organism’s traits are controlled by the
alleles it inherits from its parents.
 Some alleles are Dominant, while other
alleles are recessive.
 A Dominant allele is one whose traits
ALWAYS shows up in the organism when
it is present!
 A recessive allele, on the other hand, is
hidden by a dominant trait. It only shows
up when it is ALONE!

In Mendel’s cross for stem height, the
purebred tall plants in the P generation
had two alleles for tall stems.
 The purebred short plants had two alleles
for short stems.
 The F1 plants each inherited an allele for
tall stems from the tall parent and an
allele for short stems from the short
parent.


When you have two genes that are
different (not purebred) then you are
called a Hybrid!
› Name another thing you think about when
you hear the word “Hybrid.” What does it
mean in your example?
Geneticists use letters to represent alleles.
 A Dominant allele is ALWAYS represented
with a capital letter!

› Tall plants = T

A recessive allele is ALWAYS represented by
a lowercase version of THE SAME LETTER OF
THE DOMINANT ALLELE!
› Short plants = t

This way, you always know that the “T’s” go
together!
When a plant inherits two Dominant
alleles for tall stems, its alleles are written
as TT.
 When a plant inherits two recessive
alleles for short stems, its alleles are
written as tt.
 When a plant inherits one allele for tall
stems and one allele for short stems, its
alleles are written as Tt.

Unfortunately, the importance of
Mendel’s discovery was not recognized
during his lifetime.
 Then, in 1900, three different scientists
rediscovered Mendel’s work.
 These scientists quickly recognized the
importance of Mendel’s ideas.
 Because of his work, Mendel is often
called the Father of Genetics.
