The following power point presentation discusses about the concepts of Heridity and Evolution. In it, we discuss the laws of Heridity given by Mendel. We also see how Mendel was able to infer and analyze the results he obtained. There after, we study about the structure and function of chromosomes, genes and DNA. Then we discuss about the concept of Evolution, how Evolution resulted in formation of species, we discuss the various theories of Evolution and we track the path of evolution using various references we observe in the living world

1. HEREDity and Evolution
Prepared by
BHAVYA VASHISHT
CLASS X

2. INTRODUCTION
 Living organisms reproduce for existence & continuity of species.
 Produce offspring either through Asexual or Sexual Reproduction.
 Offspring resemble each other & their parents in certain fundamental characters, but
they aren’t exact copies of their parents.
 Organisms reproducing through asexually resemble each other & their single parent
to large extent.
 Organisms reproducing sexually show many distinct variations that are visible
among them & their parents
 Most animals including humans reproduce sexually.
 More differences observed in sexually reproducing species due to various biological
processes occurring at time of reproduction.
 Life originated 3.6 billion years ago. Series of chemical reactions on primitive earth
gave rise to earliest life forms.
 Through continuous & gradual changes (variations) new organisms arose.
 These changes were inherited & have rise to enormous diversity of organisms on
Earth

3. HEREDITY & VARIATIONS
 Members of species show close resemblance in some basic characteristics.
 This happens because all living organisms produce offspring which are similar to
them which attributes to heridity.
 Heredity: Transmission of characteristics (traits/ features) from 1 generation to other.
 Inheritance: Phenomenon by which living organisms transmit their characters from 1
generation to other.
 Asexual reproduction involves 1 parent, so the characters are transmitted from parent
to offspring without changes, only few minor changes arise due to error in DNA
copying or mutations.
 Sexual reproduction involves 2 parents having male and female gamete.
 Gametes produced as result of Meiosis, during which crossing over of chromosomes
& exchange of chromosomal segments takes place.
 Gametes fuse & form zygote. Zygote grows & develops into new individual.
 Information stored in Zygote decides development of individual.

4.  All members particular species are not similar. There are differences in
various characters among various individuals. These differences are
called variations.
 Variation: Any difference in traits between individuals of a species,
caused either through genetic or environmental factors
 Characters/ Traits: recognizable features of an organism
 Example: height, skin color, eye color & shape of nose & skin.
 Variations observed between members of a family are small but
between members of 2 families of a species there are large variation.
HEREDITY & VARIATIONS

5. TYPES OF VARIATIONS
 2 types: Germinal and somatic
 Germinal: Variations caused by differences in no. or structure of chromosome or
genes. They are heritable.
Example: Color of eyes & height.
Play major role in organic evolution.
 Somatic: Variations caused by environmental factors like climate, food supply &
interaction with other organisms. They are non heritable.
 Depending upon utility:
1. Useful variations provide some advantages to individuals to survive in nature &
also help in evolution & artificial selection.
2. Harmful variations are eliminated as the individuals possessing them are unable
to survive
3. Neutral variations may or may not take part in survival of organisms

6. ACCUMULATION OF VARIATIONS
 During reproduction, characters are inherited or transmitted from 1 generation to
other
 There are 2 aspects:
1. Inheritance of basic body structure
2. Inheritance of suitable variations that enable organism for better adaptation
 Variations produced in organisms through reproduction whether they multiply
asexually or sexually.
 Variations produced in organisms during successive generations get accumulated
in organism & pass on to more & more individuals of each generation.
 All variations are not useful for organisms.
 All variations which arise in species do not have chance of survival.
 Environmental factors select advantageous variations, eg: heat resistant character
in bacteria.
 Variations which pass on to next generations are heritable variations & these
ultimately form basis of evolution.
 In different organisms, combination of characters is different, i.e. there are distinct
variations in individuals
 This happens due to chance separation of chromosomes during gametogenesis,
crossing over, meiosis, chance pairing of chromosomes during fertilisation and
mutations.

7. Inheritance of Traits
 Controlled by genetic materials of individuals obtained from parents
 Basic traits are same in all humans & show varied expressions, i.e.
height, skin color, eye color etc.
 Example: The lowest part of ear is called ear lobe. In some, it is
closely attached to the side of head while in others it is not
 Thus, there are 2 variants, free and attached ear lobes in human
population.

8. Rules for inheritance of Traits- Mendel’s
contribution
 Humans are sexually reproducing species. Traits which are inherited in
offspring comes from both parents. Both father & mother contribute
equally to genetic material in child.
 Gregor Johann Mendel worked on garden pea plant (Pisum sativum)
 He observed that pea plants have 7 pairs of contrasting characters or
traits.
 He took pea plants with different characteristics produced progeny
from them & calculated percentage of inheritance.

9. Experimental Material of Mendel
 Mendel selected garden pea plant (Pisum sativum) for his experiment
 Reasons:
1. Plants have short life cycle which makes it possible to study several generations in
short period of time
2. Very easy & convenient to raise, maintain & handle these plants
3. Pea flowers are bisexual, the male & female reproductive organs nature at same
time & ensure self pollination.
4. Artificial cross pollination or hybridisation is possible due to large size of
reproductive organs. Hybridisation achieved by removal of anthers (emasculation)
before pollen grains attain maturity & dusting pistil with pollen grains of desired
characteristics.
5. Plant show several distinct & easily detectable contrasting traits.
6. Large number of seeds are produced in a generation which help in drawing
authentic conclusions.
 Mendel selected 7 visible characters each with 2 contrasting traits. These were
assigned by alphabets, capital alpbhabets for dominant characters, small alphabets
for recessive characters

10. R Y G I T A V
r y g i t a v

11. S.NO. TERMS EXPLANATIONS
1. FACTORS
(GENE)
Unit of DNA on a chromosome which controls the
expression of a character in organism. Mendel called these
units as factors. Later Johannsen gave the term genes
2. ALLELES
(ALLELOMORPHS)
Factors which occur on the same loci (site) in the
homologous
chromosomes and control contrasting characters are
called alleles. For example, Tt, TT or tt.
3. HOMOZYGOUS
(PURE LINE OR
PURE BREEDING)
When an individual contains identical alleles for certain
character on homologous chromosomes, is called
homozygous or pure for that particular character· For
examp1e, TT and tt alleles are homozygous for height
4. HETEROZYGOUS
(HYBRID)
When an individual contains 2 different alleles for a
certain character on homologous chromosomes, it is called
heterozygous or hybrid for that character. For example Tt
alleles are heterozygous for height
5. DOMINANT
ALLELE
Allele which expresses itself in hybrid
6. RECESSIVE
ALLELE
Allele which doesn’t express itself in hybrid & remains
unexpressed due to presence to Dominant allele.
7. GENOTYPE Genetic constitution of any character of an organism

12. S.NO. TERMS EXPLANATIONS
8. PHENOTYPE External or physical appearance of any character of
individual produced by interaction of genes & environment.
9. HOMOLOGOUS
CHROMOSOMES
Pair of exactly similar chromosomes in shape, size and structure
which bears same genes at same loci
10. FIRST FILIAL
GENERATION
(F1 GENERATION)
First generation of offspring (hybrids) produced by a cross
between genetically different individuals called parents
11. SECOND FILIAL
GENERATION
(F2 GENERATION)
Progeny produced by crossing between individuals of F1
Generation.
12. MONOHYBRID
CROSS
Cross between 2 organisms used to study inheritance of
single pair of alleles or single character.
13. DIHYBRID
CROSS
Cross between 2 organisms used to study inheritance of 2
pair of alleles or 2 characters
14. TEST CROSS Cross between an organism of unknown genotype &
homozygous recessive organism.

13. S.NO. TERMS EXPLANATIONS
15. RECIPROCAL
CROSS
Same as original cross, i.e. involves same trait but carried by
sexes opposite to those in original cross
16. CHARACTERS Distinct & well defined morphological & physiological features
of individual. Eg. Height
17. TRAIT Distinguishable expression of a character is called trait, e.g.
tallness & dwarfness are traits of character height

14. MENDEL’S EXPERIMENTAL APPROACH
Selection of pure parent plants
 Mendel selected pure parent plants for a character, i.e. plants
which produce similar traits in every generation.
 E.g.: Tall pea plants, dwarf pea plants, plants with flower in axial
position & plants with flower in terminal position, etc

15. MENDEL’S EXPERIMENTAL APPROACH
Production of F1 generation through hybridisation
 Mendel selected pure plants having alternate traits of a character & performed
reciprocal cross between plants.
E.g. Tall & dwarf plants, round and wrinkled seeds, etc.
 For the cross, 1st he removed anthers from flowers before maturation of stigma &
used these as female parent plants
 Such flowers covered with bags to avoid contamination (Bagging)
 When stigma matured, he collected pollens from other plant (male parent plant) &
dusted on mature stigma.
 Pollinated flowers covered again to avoid further contamination.
 Seeds obtained from this cross collected and sown in next season.
 These plants were hybrid in nature and made F1 Generation.
 This cross involved 2 traits of single character: Monohybrid cross
 Also conducted crosses considering 2 or more contrasting traits of pea plants,
simultaneously: Dihybrid, Trihybrid etc

17. Self breeding of F2 and F3 generation
 Mendel allowed self pollination in plants of F1 generation.
 He collected seeds from these plants and sowed them.
 Plants constituted F2 generation
 Self pollination of F2 plants produced F3 generation.
MENDEL’S EXPERIMENTAL APPROACH

18. MONOHYBRID INHERITANCE
 It is a breeding experiment dealing with single pair of contrasting characters
(Tallness and dwarfness)
 Mendel crossed 2 different pure breed pea plants differing in 1 trait i.e. length of
plant.
 He dusted pollen grains from 1 type of plants over stigma of other type of plants
 He noted occurrence of trait in progeny of succeeding generations.
 He crossed a pure tall plant with pure dwarf plant.
 Offspring produced in F1 generation resembled 1 parent, in F1 generation, only tall
pea plants produced.
 When F1 Tall plants crossed among themselves, F2 generation had both tall & dwarf
plants in ratio 3:1 (75%: 25%)
 Homozygous tall : TT
 Homozygous dwarf: tt
 Heterozygous tall: Tt (Hybrid tall plants)
 Phenotypic Ratio: 3:1
 Genotypic ratio: 1:2:1

20. MONOHYBRID INHERITANCE
 Trait which didn’t appear in F1 generation reappeared in F2 Generation in ratio 3:1
(3 dominant & 1 recessive)
 Mendel concluded that trait which was not found among F1 progeny was not lost but
was suppressed & it reappeared in F2 generation.
 Suppressive trait: dwarfness: Recessive trait
 Expressive trait: Tallness: Dominant trait
 This is called phenomenon/ principle of dominance.
 Mendel suggested that characters are represented by discrete units called factors.
These factors are now called genes
 Traits never mix/ blend together.
 Law of Segregation (Law of purity of gametes): Organism’s characteristics are
determined by internal factors (genes) which occur in pairs and only 1 of these
factors can be present in single gamete and is always pure

21.  Occurs when the F1 generation offspring differ in two traits. It is a cross
between two entities that are heterozygous for two different traits. Mendel
carried out the following experiment for this cross:
 For crossing, he took a pair of contradicting characteristics or traits
 Mendel crossed round-yellow seed and wrinkled-green seed
 In the F1 generation, the outcome was seeds that were round and yellow
 The F1 generation indicated that the round and yellow traits are dominant
while the green colour and the wrinkled shape were recessive traits.
 Self-pollination of F1 progeny resulted in four varying combinations of seeds
in the subsequent generation, the F2 generation.
 The outcome and the dihybrid cross-ratio were – round-yellow, wrinkled-
yellow, wrinkled-green, round-green and the ratio was – 9:3:3:1.
diHYBRID INHERITANCE

24. Interpretations
• In F1 generation, the offspring always show 1 of the parental trait of a
character.
• In F2 generation, offspring produced by self pollination of F1 generation
showed both parental traits
• Mendel termed trait of character which gets expressed in F1 generation as
dominant & other which doesn’t get expressed as recessive.
• In F2 generation, only 1/3rd of F2 plants with dominant trait are true breeding,
rest 2/3rd are not true breeding & resemble F1 hybrids in behavior.
• In Dihyrid cross, Mendel observed 4 types of plants in F2 generation &
concluded that factors of each character assort independently of the other.

25. MENDEL’s Principle of heridity
PAIRED FACTORS
 Paired factors: Every organism contains large number of characters. Character is
represented by 2 factors or alleles.
 Alleles occur on same gene locus on 2 homologous chromosomes
 In parents, factors are present in paired conditions like TT, tt or Tt. At time of gamete
formation, gametes get 1 factor of particular character, i.e. T or t
Parent Gametes
TT
Tt
tt
 On fertilisation, any male gamete fuse with any female gamete, fertilisation is
random but restores the paired condition.
CHARACTER TRAIT FACTOR
HEIGHT TALLNESS TT
DWARFNESS tt
T T
T t
t t

26. MENDEL’s Principle of heridity
LAW OF DOMINANCE
 When 2 dissimilar factors of a character are present in an organism, only 1 expresses
itself in F1 generation and is called dominant factor & other which remains
unexpressed is called recessive factor.
LAW OF SEGREGATION OR PURITY OF GAMETES
 During gamete formation, 2 factors of a character get separated & each gets only 1
factor for the character, gametes are always pure for a particular character.
LAW OF INDEPENDENT ASSORTMENT
 2 or more pairs of contrasting characters are inherited in such a way that a particular
trait is independent of other trait.
 Unit factor (gene) of each character is assorted into gametes independently of the
factor of any other character and gets randomly rearranged in offspring

27. CHROMOSOME, GENE, DNA
 Chromosome: Thread like structures present in nucleus of eukaryotic cells
 Each chromosome contains 2 chromatids
 Chromatids joined at Centromere
 Number of chromosomes in a cell is fixed for each species
 Example: Human- 46 (23 pairs), Fruit fly 16 (8 pairs)
 Both chromatids in a pair of chromosome are homologous.
 This paired condition is Diploid (2n)
 Somatic cells: Diploid (2n)
 Germinal cells/ Gametes: Haploid (n)
 Mendel proposed that factors/ alleles (genes/ segments of DNA) control expression
of trait.
 Each gene occupies specific position on chromosome called its Locus & are
arranged in linear order
 All genes present on chromosome inherited from 1 generation to other.
 Thus genes & chromosomes : Physical Basis of Heridity

28. Structure of chromosome

29.  DNA forms chemical basis of heredity
 DNA is macromolecule made up of large number of nucleotides
 1st identified by Frederick Miescher
 DNA composed of: Nitrogenous Base
Deoxyribose sugar
Phosphate group
 Nucleotide: All 3 are present (Nitrogenous Base, Deoxyribose sugar, Phosphate
group)
 Nucleoside: Lack phosphate
 Nitrogenous Base: 2 types:
 Purines- Adenine (A) and Guanine (G)
 Pyrimidines: Cytosine (C) and Thymine (T)
 DNA transmits genetic information from 1 generation to other. It was proved by
Griffith (1928) and Avery, McLeod & McCarty (1944)
 Gene is segment of DNA on chromosome present at specific position.
Structure of DNA

31. MECHANISM OF HEREDITY
 Traits transmitted from 1 generation to another through genes during sexual
reproduction.
 DNA is information source in cell for making proteins
 Genes determine every character of organism.
 Dominant factor of allele controls formation of functional or structural protein or
enzyme that produces its effect.
 Recessive allele is unable to produce fully effective protein or enzyme resulting in
inability to express the effects in presence of dominant factor.
 In sexual reproduction, both parents contribute equal amount of genetic material to
progeny which determines trait of progeny.

33. SEX DETERMINATION
XX-XY Mechanism
 To understand how the determination of sex happens, we need to know the following
process.
 Humans have 23 pairs of chromosomes. Out of these 23 pairs, 22 pairs are
Autosomes and only one pair is the ‘Sex Chromosome’, which actively takes part in
the process of sex determination.
 Both males and females carry two sets of sex chromosome.
 Male has one X and one Y (XY) sex chromosome in which both are active
 Female has both X (XX) sex chromosome in which one is active.
 The XY sex-determination system is found in humans, mammals, in some insects,
and in few plant species.
 All children will inherit an X chromosome from their mother, despite whether they
are a boy or girl. Thus, the sex of the children will be determined by the type of
chromosome inherited from their father.
 A child who inherits Y chromosome will be a boy and who inherits X chromosome
will be a girl.
 A male semen’s holds approximately 50 percent of Y sperm cells and other 50
percent of X sperm cells. Therefore, there are possibilities that men have three or
more children of a single-gender.

35. SEX DETERMINATION
XX and XO Mechanism
 In some insects, like grasshoppers, the female contains 2 similar sex chromosomes
(XX) but males contain 1 sex chromosome (XO)
ENVIRINMENTAL DETERMINATION OF SEX

36. INHERITANCE OF BLOOD GROUP

37. EVOLUTION
 Gradual changes resulting in formation of new forms from pre-existing forms over a
long period of time
 Organic/ Biological evolution: Evolution in living organisms
 Organic evolution is constant process of gradual changes occurring in organisms
since origin of life that gave rise to enormous variety of complex organisms on earth.
 Occurs due to continuous interaction of environment & genes
 Concept of Evolution 1st given by Charles Darwin in his book The Origin of
species
 Modifications or changes occur in organism due to accumulation of variations.
 Any trait that is acquired by an organism during its lifetime due to any external
conditions is not transmitted to its offspring. These traits are termed as an acquired
trait. Example - the way a person speaks or the skills he/she has. These changes do
not affect the germ cells and so they cannot be passed on from one generation to the
other.
 Any trait that is genetically inherited or passed down from one generation to the next
generation is termed as an inherited trait. Example - the colour of eyes or skin.

38. Situation 1
Situation 2
Situation 3
POPULATION OF RED BEETLES IN GREEN BUSH

40. ORIGIN OF LIFE ON EARTH
 Life is the characteristic that distinguishes living things from non
living things
 All living organisms have some fundamental characteristics (presence
of protoplasm, cellular organisation, nutrition, growth, reproduction
and response to stimuli)
 The inherent capacity of living organisms by which they utilize the
outside materials from taking energy, growth & reproduction in
controlled manner is life

41. ORIGIN OF SOLAR SYSTEM & EARTH
 Origin of life linked with origin of earth.
 Earth originated along solar system after Big Bang.
 Universe originated due to Big Bang i.e. thermonuclear explosion of dense entity
around 15 billion years ago.
 In this explosion, numerous pieces of nebulae were formed.
 These nebulae condensed and organised slowly into galaxies.
 Our galaxy: Milky Way/ Akash Ganga
 Solar system formed around 4.5 billion years ago from huge cloud of cosmic dust &
gas called Solar Nebula
 Our Earth was formed as solid mass (lithosphere) surrounded by gaseous envelope
(Atmosphere).
 Later earth cooled down below 100°C and hydrosphere appeared
 Life appeared 500 million years after formation of Earth, i.e. about 4 billion years
back

42. THEORIES OF ORIGIN
OF LIFE

43. Theory of Special Creation
 Life was created by some supernatural power or God
 Hindu belief: Lord Brahma created world & Manu & Shraddha were
1st man and woman
 Christian belief: God created Adam & Eve and universe along with
plants and animals in 6 days
 Islamic belief: Man was created from clay while his progeny from
quintessence of fluid.
 This theory has no scientific basis

44. Theory of Spontaneous generation
 Life originated from non living things like Mud, Sun, air, water &
decomposing things (abiogenesis)
 Theory was supported by Aristotle
 Louis Pasteur proved experimentally that life originated from pre-
existing life only (biogenesis)

45. COSMOZOIC/ INTERPLANETARY THEORY
 Also called theory of panspermia
 Life originated somewhere else in universe & came to earth from
some heavenly bodies in form of spores or seeds
 Theory put forward by Richter (1865) & supported by Arrhenius
(1908)
 Theory rejected due to presence of very low temperature, Lack of
atmosphere, UV Radiation in space which cant support life

46. NATUralist theory/ abiogenic theory/ Theory of chemical origin of life
 It states that life originated from non-living, inanimate materials through natural
processes and occurrences.
 In 1953, Stanley Miller’s experiment reinforced this theory. He recreated conditions
that were present on earth nearly 4 billion years ago in his experiment.
 He filled ammonia, methane and carbon monoxide-hydrogen and water in a
container to represent the gaseous composition of early earth’s atmosphere.
 Then, he subjected the container to electrical sparks to simulate lightning strikes.
This experiment was done for a week and by the end of it, Miller had noticed an
accumulation of a red-brown substance on the walls of the container.
 When he analyzed the composition of this mysterious substance, he had found that it
contained organic compounds and amino acids which are paramount for life.
 His experiment opened the gates for future scientists to delve deeper into abiogenesis
and today, this is one of the most credible contenders for theories on the origin of
life.

47. SPECIATION
 A group of organisms that are similar in their traits and are capable of breeding
within themselves are termed species.
 The mechanism or process by means of which a new or distant species is formed
from the pre-existing species due to various factors is termed as speciation.
 This process leads to the formation of different species within a population that is not
capable of reproducing among themselves.
 Reproductive isolation prevents population of 2 groups from interbreeding
 Operates through : Temporal barriers
Behavioral barriers
Internal (physiological barrier)
Mechanical barriers
 Due to reproductive isolation, no gene flow occurs between species, breeding
continues within isolated populations. After several 100 years, individuals of these
isolated populations become so different from each other that they are not able to
reproduce with each other even if they meet again.
 This leads to formation of 2 different species from 1 single population.

48. Allopatric speciation
 In this type of species formation, a part of the population becomes geographically
isolated from the main population. The population becomes entirely separated and
finally constitutes a new species. Thus geographic isolation brings about allopatric
speciation
Sympatric evolution
 In this type of species formation, a small segment of the original population becomes
isolated reproductively. As the isolating mechanism comes into force, a new
subspecies emerges. In due course of time a new species is formed. Thus sympatric
speciation is the formation of species within a single population without geographical
isolation. The reproductive isolation brings about sympatric speciation

49. Mechanism ofSPECIATION

50. CAUSES OF SPECIATION
Splitting of Population:
 The populations tends to grow rapidly when there is a favourable environment for
their survival. As a result the population is very large and spread out.
 This makes it difficult for all the members to interact among themselves due to the
geographical isolation, so they tend to reproduce with the local near by population.
And if they are separated by any barrier like a mountain or a river, then they are
isolated further.
 This will cause the gene flow or transfer of genes in a population to reduce and they
may develop into a new or distant species.
Genetic drift:
 The populations undergo a change in their genes for some traits that are specific to
that species due to unprecedented reasons. This leads to them being transferred to
another existing population.
 Example - If there are a population of red and green beetles and the green are
eliminated by being in a forest fire, then the population of only red beetles will
remain.

51. Mutations
 Sudden inheritable changes in genetic constitution.
 After mutation, variations originate which are considered as raw material for
evolution and/or speciation to occur in nature
 Evolution for Darwin was gradual while Hugo de Vries believed that mutations cause
speciation & hence he called it saltation (single step large mutation)
Variations
 The process by which certain populations are selected over others due to their
variations and ability to adapt is termed as natural selection. They lead to the
formation of new species over a period of time.
 Example - In case of a storm, the birds with long or short wings died as compared to
the ones with average wings. So, they were naturally selected to adapt to that
environmental changes and they survive to evolve their species.

52. EVOLUTION AND CLASSIFICATION
 Every species goes through a phase of evolution. The similarities among
organisms that allow them to be grouped are based on the characteristics or
the details of the appearance or behaviour that is seen for a particular form or
a function.
 There are some basic characteristics that are shared by most of the organisms
like, the cell being the fundamental unit of life. But the next level of
grouping or classification may not be common for all the organisms, like the
cell may have a nucleus or not. This classification goes further as whether the
nucleated cells are single celled or multi cellular. This allows a hierarchy to
be created in the evolution process that helps us in the classification of
groups.
 Thus the more common characteristics are shared by two species, the more
closely related they are. The more closely they are related, indicates that they
have had common ancestors recently. Example - In a family, a brother and a
sister are closely related with common ancestors as parents. Now the girl and
her cousin too are related as they common ancestors, grandparents. But they
are distant than her brother as they common ancestors in second generation.
 Thus small group of species with recent common ancestors are built,
followed by distant common ancestors and this goes on backwards in the
evolutionary process.

53. TRACING
EVOLUTION

62. THEORIES OF
EVOLUTION

66. EVOLUTION BY
STAGES

76. THANK YOU