the laws of mendel

THE LAWS OF THE LAWS OF MENDELMENDEL

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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.

THE LAWS OF MENDELSeptember 13, 2014

September 13, 2014 THE LAWS OF MENDEL

“FATHER OF GENETICS”

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.

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)

. . . . . . . However, this law was not fully accepted as due to :• co-dominance ;• incomplete dominance ;• over dominance.


F1 Tall plant (Tt)F1 Tall plant (Tt) Tall plant (Tt)Tall plant (TT) Dwarf plant (tt)

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 . THE LAWS OF MENDELSeptember 13, 2014

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 LAWSIt 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.


September 13 2014

THE LAWS OF MENDEL31

“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.”

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