It includes those traits or characters which are transmitted from generation to generation and are therefore fixed for a particular individual.

In other words, the transmission of characters from parents to their offspring is called heredity.


The differences in the characters of the individuals of a species are called variations.

Variations present in sexual reproduction and its absent in asexual reproduction.

Type of variation

A. Somaticc variations-somatic variations affect the somatic cells of an is neither inherited from parents nor transmitted to the next generation.

 It is also called acquired variation or blastogenic variations.

1. Germinal variations-This variation affects the the germ cell of an organism.

It is received by the individual from the parents and is transmitted to the the next generation.

Significance of variation

1. It forms raw material for evolution.

2. It is the basis of heredity.

3. It enables the organisms to face the struggle for existence in a better way.

4. It also leads to the existence of new traits.

Mandal's experiments and laws of inheritance

Gregor John Mendel age-appropriately called as the father of genetics.

Mendel conducted classical experiments on the pea plants for the study of the inheritance of traits.

His work watch recognized only in 1900 when Mendel’s laws were rediscovered by Hugo de varies, Carl current, Erich von Tschermak.

 Some terms used by Mendel-

1.Dominant traits-the track which appeared in F1 generation is called a dominant trait. It is denoted by capital letters. E.g. TT

2.Recessive trait-the tracks that do not appear in the F1 generation are called a recessive trait. It is denoted by a small letter. E.g. tt

Why did Mendel select pea plant

Traits Chosen by Mendal

Technique of Mendel

1. Emasculation:- removal of anther before its maturation is called emasculation.

2. Bagging:- the stigma age protected against any foreign pollen with the help of a bag that is called bagging.

Monohybrid cross

Definition:- cross in which inheritance of only one trait is taken into consideration is called a monohybrid cross.

In his first monohybrid cross, Mendel crossed a pure breed tall plant and a pure breed dwarf plant. And he found that in F1 generation only tall plant was born out of this seed, No immediate character mixing or blending was observed by him. Mendel termed tall plants as a dominant and dwarf plant as recessive.

When F1 plants are grown and allowed to self-pollinated he got many seeds and when they are Grown he found that out of total plants three times plants for the tall and one-time plants were dwarf.

The dwarfness which was not visible in the F1 generation was reappeared in the F2 generation.

The phenotypic ratio of a monohybrid cross is 3:1

The genotypic ratio of a monohybrid cross is 1:2:1

Conclusion Of Monohybrid Cross

Law of segregation:- According to this law when a pair of contrasting factor or alleles are brought together in a heterozygous condition, the two remains together without being contaminated, but when gametes are formed from them the two separate out from each other. This is also known as the purity of gametes or Mendel’s first law of heredity

Law of dominance:- When two heterozygous individuals with one or more sets of contrasting characters have crossed the characters that appear in the F1 hybrids are dominant characters and those which do not appear in F1 hybrid are recessive characters.

Dihybrid Cross

Definition:- When the inheritance of two traits is taken into consideration simultaneously the cross is known as a dihybrid cross.

Example:- first trait the shape of the seed in these alternate forms of round and wrinkled.

The second trait the color of seed- alternative forms are yellow and green

2.Round shape is dominant over the wrinkled shape and yellow color is dominant over green color.

3.In F2 generations the seeds were obtained in the ratio 9:3:3:1 is known as the dihybrid ratio.

Conclusion of a dihybrid cross

Law of independent assortment- On the basis of the dihybrid cross, Mendel gave his second law of inheritance. According to this law if we consider the inheritance of two traits simultaneously ,then the segregation of factors during gamete formation and random fusion is independent of each other.

Sex Determination In Human

1. Human beings have 22 pairs of autosomes and one pair of sex chromosomes. All the ova formed by a female are similar in their chromosome type(22+x).therefore, females are homogametic.

2. The male gametes or sperms produced by human males are of two types (22+x) and (22+y).human males are, therefore, heterogametic (male digamety).

The genetic make-up of the sperm that determines the sex of the child.


 The gradual, slow process of changes is called evolution.

The term evolution has been derived from the Latin word “evolves” which means to “unroll” or “unfold”.

The concept of organic evolution was first given by Charles Darwin which simply means “descent with modification”.

Theories of Origin of Life:-

1.Theory of special creation according to this theory.

Christian belief, God created the universe, plants, animals, and human beings in six natural days. Adam and Eve were the first man and woman created by god.

According to Hindu mythology, Lord Brahma created the world with his wish. Manu and Shradha were the first men and women respectively

2. Theory of Spontaneous Generation:- it stated that life originated spontaneously from non-living materials. (abiogenesis).

This idea was disapproved by Louis Pasteur who experimentally proved that life originates from pre-existing life. (biogenesis).

3. Cosmozoic/Panspermia Theory:-

4. Abiogenic/Naturalistic Theory/Chemical Origin Of Life- (A.I.OPARIN and HALDANE) It is now well accepted that life originated from inanimate matter by a series of chemical reactions on the primitive earth.

Major support for this theory was provided by Stanley L.Miller and Harold C.urey in 1953.

Miller and Urey Experiment

1. They designed a glass apparatus and stimulated the conditions that thought to have existed on primitive earth.

2. They circulated a mixture of methane(CH4),ammonia (NH3)and hydrogen (H2), and water vapour in this apparatus.

3. This mixture was maintained at a temperature just below 100degree Celsius.

4. They provided energy for interaction of gases in the form of electric sparks in the gas flask.

5. At the end of a week,15% of the carbon had been converted to simple organic compounds such as amino acids, simple sugars, purines and pyrimidines.

Darwinism /Theory of Natural Selection

It is proposed by Charles Robert Darwin. And he explained in his book the origin of species by means of natural selection.

He went on a voyage on the famous ship H.M.S Beagle.

1. Postulates of Darwinism:-

a. Multiplication of individual of a species in a geometric proportion.

b. Due to geometric multiplication and due to the availability of limited food and space for these individuals the struggle for existence is seen.

c. The operation of natural selection on the existing variability in order to select the best-fitted variations.

d. variations they are the rule of nature and proved to be beneficial for a better existence.

e. Natural selection is the principal element of Darwin’s theory. The principle by which the preservation of useful variation is brought about was called natural selection.

Merits of Darwinism-

The major achievement of Darwin was to recognize one of the major factor in adaptation i.e. natural selection

Demerits of Darwinism-

1. Darwin believed that the natural selection operates on variation but he did not consider the possibility of the origin of new hereditary variations, which are really responsible for the origin of species.

2. Darwin also did not distinguish between hereditary and environmental variations.


Full Name:- Jean Baptiste de Lamarck

Book:- philosophy zoologique

term biology was given by Lamarck.

Concepts of Lamarckism:-

(i) Internal Vital Force:- Some internal forces are present in all the presence of these forces organisms have the tendency to increase the size of organs of entire body.

(ii) Effect of environment and  new needs:- Changing environment gives rise to new needs. New needs or desires produce new structures and change habits of the organisms.

(iii) Use and disuse of Organs:- If an organ is constantly used, it would be better developed whereas disuse of organ results in its degeneration.

(iv) Inheritance of acquired Characters:- during the life time of an organism new characters develop due to internal vital forces, the effect of environment, new needs, and use and disuse of organs.

These acquired characters are inherited from one generation to next.

By continuous inheritance through many generations, these acquired characters tend to make new generations quite different from their ancestors resulting in the formation of new species.

Examples in support of Lamarckism:-

1. Longneck and large fore limbs of Giraffe.

2. Webbed toes of aquatic birds.

3. Disappearance of limbs in snakes.

4. Retractile claws of carnivorous animals.

Criticism in Lamarckism-

August Weismann , A German biologist, was the main opposer of the inheritance of acquired characters. He put forward the “THEORY OF CONTINUITY OF GERMPLASM.”

Factor’s for Evolution

1. Genetic Variation in Population:- Evolution occurs through the accumulation of genetic variations in population over long periods of time. the change in genes occurs in the following ways-

a. Mutations:- Mutations are sudden heritable changes.

b. Gene Recombination:- It adds new alleles and a combination of alleles to the gene is important to process during evolution which causes variations.

c. Gene Migration (gene flow):- If the migrating individuals breed within the new population, the immigrants will add new alleles to the local gene pool of the host population. This is called gene migration.

d. Genetic drift(Sewall wright effect):- a change in frequency of some genes of a small population by chance is called genetic drift.

Effects of genetic drift are most marked in a very small isolated population

2. Isolation:- Isolation is the prevention of mating amongst interbreeding groups due to physical and biotic barriers.

Any factor which prevents interbreeding is known as the isolating mechanism.

Type of Isolation:-

1. Reproductive isolation

2. Geographical isolation

3. Mechanical isolation

4. Psychological isolation

3. Natural selection/selection:- The selection is the process by which those organisms which appear physically, physiologically, and behaviorally better adapted to the environment survive and reproduce, those organisms not so well adapted either fail to reproduce or die.

4.Speciation (origin of new species):- The process of one or more new species from an existing species is called speciation.

Causes of speciation:-

1. Reproductive Isolation

2. geographical Isolation:- It does not play role in the speciation of asexually reproducing animals and self-pollinating plants.            

5. Inherited and Acquired Traits:-

Evidences for Evolution

Evidence of evolution supports the view that the present day diverse forms of living organisms have evolved from common ancestors. These are the followings:-

1. Morphological and anatomical evidence:- This evidence is based on the similarities in the external and internal features of the different kinds of organisms. These features are:-

Homologous Organs:- the organs which perform different functions in different species but have a similar basic structure and similar embryonic origin are called homologous organs.

Homologous organs show the divergent types of evolution.

2. Analogous Organs

The organs that are quite different in fundamental structure and origin but perform similar functions are called analogous organs.

Characters of analogous organs:-

The analogous organs develop from different structures in different environments to perform similar functions.

Examples of analogous organs:-

In animals-

1. wings of birds and wings of a butterfly(insect)- they help these animals in flying, means perform the same function.but their basic structure is quite different.

The wings of birds and bat are another example of analogous organs. The wings of the bat are simply folds of skin, but the wings of birds are modified forelimb.

#the presence of analogous organs provides evidence for convergent evolution or it shows convergent evolution(analogy).

Evidence from fossils/Palaeontological Evidences

The remains or remnants of organisms of remote past are known as fossils.

The fossils provide good evidence for establishing an evolutionary relationship.

Determination of fossils-

1. Relative Method:- The rocks are present in the form of concentric layers. The layer closer to the surface are more recent than deeper is, therefore, reasonable to suppose that the fossils we find closer to the surface are more recent than the fossils we find in deeper means that as we dig deeper, we will find older and older fossils.

2. Carbon:- Dating of rocks-for dating purpose radioactive isotopes are this method ratios of different isotopes like C14 etc are calculated.

Examples of fossils

1. Ammonite:- The ammonites represent fossils of mollusk in the form of a tight spiral shell.

2. Trilobite:- Trilobites are fossils of marine arthropods.

3. Dinosaur skull:- these terrible lizards roamed the earth 250-65 million years was obtained from Narmada valley.

Evolution of eye

In very primitive there is an eyespot to sensitivity to light. From this eyespot, evolved the primitive eye-like structure of flatworm planaria.

The eyes of insects, octopus, and vertebrate animals are altogether different, very well adapted to their mode of life. They have developed through different lines of evolution.

Evolution of feathers

The presence of feathers in some non-flying dinosaurs clearly indicates that feathers were developed in these animals for insulation in cold weather, birds used the feather for flight. It is a piece of evidence in favor of organic evolution because birds have evolved from reptiles.

archaeopteryx is a fossil that represents a link between reptiles and birds.

Artificial selection

Artificial selection is a process by which man selected traits of organisms useful for him.

Artificial selection has led to the domestication of many plants and animals.

Example of artificial selection-

The wild cabbage plant is a good example.

1. Cabbage, kohlrabi, cauliflower, broccoli, kale, etc. have been developed from a common wild ancestor.

2.Different breeds of pigeon-like ponter, fantail, Jacobin have been produced by artificial selection from the wild rock-pigeon.

Difference between natural selection and artificial selection-

The basic differences between natural selection and artificial selection is that in artificial selection.