THE LAWS OF MENDEL

THE LAWS OF
MENDEL
SUBMITTED TO: SUBMITTED BY :
HIMANSHU BHATT

GREGOR MENDEL (1822-1884)
 Austrian monk
 Born in 1822 near Brunn in Austria, in a
poor family.
 Joined the St. Augustinian monastery .
 Was sent to University of Vienna.
 Presented his findings before National
History Society of Brunn in 1965.
September 13, 2014 THE LAWS OF MENDEL

 But his findings were discarded due to the prevailing theories of
Evolution.
 Later on, his theories were again discovered simultaneously by:
 Hugo de Vries in Holland
 Carl Correns in Germany
 Eric von Tschermak in Austria ; in 1900.
“FATHER OF GENETICS”

WHY PEA PLANT…??
Flower structure of pea ensured self-pollination
and also, well defined male
and female parts.
Single season crop.

Emasculation and pollination of pea
flowers is quite easy.

reproduce sexually.
In the varieties available, several
characters had two contrasting
forms, which were easily
distinguishable from each other.

HYPOTHETICO-DEDUCTIVE METHOD APPLIED BY
MENDEL (APPARENTLY UNKNOWINGLY)
Theory

SEVEN PAIRS OF SIMPLE DIFFERENCES

TERMINOLOGY
• Character - morphological, anatomical or behavioral feature of an organism.
• Allele - different alternative forms of gene ; e.g., a gene for height has two
alleles: one for tall and one for dwarf.
• Homozygous – An individual with two identical alleles.
• Heterozygous - An individual with two different alleles.
• Phenotype: observable characteristic of the some organism like color, size, form
and structure
Eg: tall, short
• Genotype: genetic composition of an organism and cannot be seen
Eg: TT, tt, Tt

F1 generation : the hybrid offspring of the parental generation.
F2 generation: the self fertilized progenies of F1 generation plants.
Monohybrid cross: a cross between parents differing in only one trait or
characters.
Dihybrid cross: a cross between parents differing in two traits or characters.
Test cross: a cross of F1 hybrid to recessive parent type.
Back cross: the cross of F1 hybrid to one of parents.

LAW OF DOMINANCE (GIVEN BY ERIC VON TSCHERMAK)
 Every gene has two alleles that can code for a trait.
 One allele is dominant, meaning it will always show.
 One allele is recessive, meaning it will be masked by the presence of
the dominant allele.
Tall plant (TT) Dwarf plant (tt) F1 Tall plant (Tt)

F1 Tall plant (Tt) F1 Tall plant (Tt) Tall plant (TT) Tall plant (Tt) Dwarf plant (tt)
. . . . . . .
However, this law was not fully accepted as due to :
• co-dominance ;
• incomplete dominance ;
• over dominance.

LAW OF SEGREGATION
•During the formation of gametes, the paired alleles separate (segregate) randomly
so that each gamete receives one allele or the other.
•The two alleles of a gene present in the F1 do not contaminate each other;
they separate and pass into different gametes in their original form producing two
different types of gametes in equal proportion.

•The pair of alleles of each parent separate and only one allele passes
from each parent on to an offspring
•Which allele in a parent's pair of alleles is inherited is a matter of
chance
•Segregation of alleles occurs during the process of gamete formation
(meiosis)
•Randomly unite at fertilization

 Mendel experiment , he crossed tall and dwarf
varieties of garden peas.
 F1 produced were tall.
 When F1 were self-fertilized, both tall and dwarf
progenies were produced in F2 generation.
75% were tall and 25% were dwarf
 An F2 of 1064, consisted of : 787 tall plants and 277
dwarfs: an almost 3:1.

 When F1 plants from Mendel’s experiments were
crossed back to the dwarf variety, half the progeny
were tall and half dwarf.
 The phenotypic ratio observed was 1:1.
 This further demonstrated more conclusively the
Principle of Segregation.

When a similar experiment was done with each pair of characters,
the results were the same:

LAW OF INDEPENDENT ASSORTMENT
 This law states that member of different pairs of alleles
assort independently during the formation of gametes.
 This means that traits are transmitted to offspring
independently of one another.

 Mendel crossed plants that differ in two pairs of alleles.
e.g., round green seeds and wrinkled yellow seeds.
 F1 hybrids were round and yellow seeds, as expected.
 When F1 were self-fertilized , four F2 phenotypes were
observed.
 From a total of 556 seeds, the following distribution :-
315 round green ; 108 round yellow ; 101 wrinkled green; and 32
wrinkled yellow.
 This results closely fit a ratio of 9 : 3 : 3 : 1 .

When Mendel crossed the F1
generation obtained with the
recessive parent, four different
phenotype were obtained.
It verified the ratio in which the
different gametes of the F1 hybrid
were produced..

WHY WAS MENDEL SO SUCCESSFUL..??
Studied the inheritance of only one pair of contrasting
characters at a time.
Selected pea varieties that had clearly different forms of
one or more characters, e.g., round and wrinkled seeds,
yellow and green cotyledons, etc.
Carried out his experiments with great care and
elaborateness.

 Accurate and incisive analysis of the
reasons.
 His knowledge of mathematics was a
definite asset for the interpretation.
 Classified all the plants of a
population on the basis of
contrasting characters under study.

NEGLECTION OF MENDEL’S FINDING :
 Using mathematical principles was something new and unacceptable to
the biologists.
 Studied contrasting pairs of characters exhibiting discontinuous
variation unlike his contemporaries like Darwin, Galton and others.
 The phenomena of fertilization and the behavior of chromosomes during
cell divisions were not known at that time.
 No further publishing of his work.

 Studies of Mendel appeared to establish the constancy of
characters of a species generation after generation, which was
contrary to the theory of evolution at that time.
 Mendel failed to demonstrated the validity of his conclusions in
other species.
 Mendel corresponded extensively with his contemporary, the
noted botanist, Nageli.

BIOLOGICAL SIGNIFICANCE OF MENDEL’S
LAWS
It showed the pattern of inheritance of the character from parent to the
offsprings.
The Law of Independent Assortment has laid the foundation of inheritance
which claimed that differernt genes assorted themselves independently of
one another during gamete formation and had established the relationship
between dominant and recessive gene type.

 It also provided the reason for variation in offspring
transferred from the parents to the offspring in the
form of gene (factor) present in chromosome of
gamete.
 The Law of Segregation ; according to this law, when an
individual of a species mate, the copy of genes are
separated and assorted so that each of gamete receive
only one copy. By the discovery of meiosis, this notion
was confirmed.

“Genetics has moved far head than the protoidea that had been laid down by Mendel , but still the
science owes its existence to the still the science owes its existence to the discovery & ideas that had
originally been laid down Mendel.”
September 13 2014 31 THE LAWS OF MENDEL

THE LAWS OF MENDEL

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