MENDEL used PEA
PLANTS to find the rules of
GENETICS
Why?
Do you know the shape of pea flowers?
Pollen matures in
anthers.
When pollen comes
into contact with the
stigma, the ovules
in the ovary are
fertilized
and form the seeds,
then ovary becomes
the pod.
Ovules in
Now, in these pea flowers, do
you see the anthers with
pollen and do you see the
stigma???
No, of course, because
petals close pollen and
anthers into the flower.
That is why Mendel decided to use peas to understand genetic rules: it is sure
that pollen, with the father information, drops only on the ovules with the
mother information of the same flower (You know that often pollen is
transported by wind or by insects but this in not possible in the pea flowers
because the pollen is hidden into the flower).
The shape of a pea flower is very important : petals cover the
stigma from pollen of other flowers, thus promoting self-pollination
Mendel’s genetics
The seven characters
Moreover pea plants are easy to grow and are not expensive.
So Mendel thought they were an optimum material to study how
characters can be transmitted from parents to next generations.
Before starting the study of the characters’ transmission,
Mendel started to study a lot of different pea plants:
he found 22 different pea plants
and for two years he studied these plants, how they grew, how
the children were, to find traits (characters) that were very
easy to recognize.
SEVEN were the characters that he found
These are the seven characters that
Mendel studied:
Then Mendel decided to be sure that
So for many generations he planted
• At the end he called these plants
PURE LINES
An example: the pure line of a red
flower
• Individuals that belong to pure line have the same
characters from one generation to another
START!!!!
Mendel could start: he had all that he needed for his
experiments because he had:
- 7 different
characteristics to
analyze
- pure lines for each
characteristic
CROSS-POLLINATION ….
Mendel chose to artificially breed pure lines of plants such as red flowers and white
flowers.
So Mendel kept the pollen of a
pure line red flower and put it on
a stigma of pure line white
flower: we call this cross
pollination
Parental generation＝
P
QUIZ: what did he obtain? Red flowers or white flowers
or some red and some white flowers???
Parental
generation＝
P
He obtained
only
red flowers!!!!
1st generation
of children＝
F1
Experiments with pure lines of
green and yellow peas,
axial or terminal flowers
These are the results:
Parental generation＝ P
1st generation of children＝
X
F1
Parental generation＝ P
1st generation of children＝
X
F1
Why does a character always disappear?
Where has the information for white (flower), green (seed), or terminal
(position of flower) gone ?
…AND SELF POLLINATION
Hoping to understand more about this problem, Mendel decided
to analyze another generation: the 2nd filial generation (F2).
To obtain the F2 Mendel carried out a self pollination: he didn’t
touch flowers of F1 so the pollen went on the stigma of the same
flower.
Mendel let F1
generation
plants selfpollinate and
obtained the
following
generation (F2).
F1
The white (flower) character reappeared!
The missing characters in the F1 generation (white) reappeared in
the following generation (F2)!!!.
pollen
P
CROSSPOLLINATION
F1
SELFPOLLINATION
F2
Mendel thought that the information white was present also in
the F1 but was hidden.
So in the F1 is present the white information but obviously is
present also the red information!!!
P
F1
F2
red information+
white information=
red colour
Mendel called
DOMINANT the
character that
appeared in the F1
generation (red
flower) and
RECESSIVE the
character (white)
that was present
but not expressed.
QUIZ: how much information 1s present in the F1 about the colour of flowers?
QUIZ: how much information is present in each flower about its colour ?
Also the pure lines have two factors but of the same type.
red inf.+
red inf.=
red colour
P
So it is
white inf.+
white inf.=
white colour
reasonable to
say that the
colour of pea
pure lines
flowers is
always
F1
red information+
white information=
red colour
controlled by
two factors:
red (dominant)
F2
and
white (recessive).
Only one item of information for the colour of the flower (red or white) is
present in the pollen and in the ovule (one in the pollen, one in the ovule).
Since the new flower has two parents, it receives 2 items of information (one
from the pollen, one from the ovule).
P
F1
The meiosis separates the two items of
information so
in the gamets (pollen and ovule) the
information is again one.
red information+
white information=
red colour
…and the F2 generation? Also the flowers of the F2 has two factors (=alleles) received
by its parents (the F1 flowers,obviously!)
The colour of pea flower is controlled by two alleles: red ( dominant) and white
(recessive)
P
The pure lines have only
one kind of factor .
F1
F2
The F1 is only red because
all flowers have one red
(dominant) information.
In F2 the white
colour reappears
because some
flowers have the two
items of information
for the white colour.
Mendel carried out similar experiments with all characters (do you remember the 7
characters that he found?) and always one character disappeared in the F1 but
reappeared in F2.
P
X
Cross-pollination of
pure lines
F1
Self-pollination
F2
Every 3 dominant
characters there is 1
recessive character
(3:1)
CONCLUSION
Every character is given by two alleles (factors, units): mother transmits 1 allele to every
child and so father does .
Trait
So each child receives 2
alleles. Conventionally the
allele is designed by a
letter.
If one allele (factor, unit) is
dominant, the character
will be dominant, if no
allele is dominant,
character will appears
recessive.
The allele is designed with
a capital letter if
dominant, small letter if
recessive.
Flower
colour
Characte Allele
r
s
dominan
t Re
d
Character
e
recesseiv
e
Allele
s
RR
or
Rr
rr
Flower
position
AA
or
Aa
aa
Seed
colour
YY
or
Yy
yy
Seed
shape
RR
or
Rr
rr
Pod
shape
II or
Ii
ii
•
•
•
•
•
•
•
•
•
Let us summarize…and
formalize
Mendel cross-pollinated two pure plants, one with red flowers and one with white flowers and called they
PARENT GENERATION. From this cross he obtained a FIRST GENERATION (F1), in which all the
plants showed only red flowers, while the character of the white flower seemed to have disappeared.
Mendel called the character that appeared in the F1 generation DOMINANT (red flower).
He then let F1 generation plants self-pollinate and saw that the missing character in the F1 generation
(white) reappeared in the following generation (F2). This character was called RECESSIVE. In the F2
generation the dominant and recessive characters were in a ratio of 3:1.
a) The inheritance of each character is determined by units or factors that are passed unchanged to the
descendents;
b) Each individual inherits one unit from each parent for each character;
c) The character may not appear in an individual but can still be passed to the next generation.
Alleles are alternative forms of the same gene which has a defined character.
There are individuals:
HOMOZYGOUS, having two identical alleles. In this case they can be dominants if they have both
dominant alleles or recessives if both alleles are recessive
HETEROZYGOUS, who have inherited different alleles from each parent.