Heredity and Evolution | Heredity and Evolution Class 10 Notes Chapter 9

Notes of heredity and evolution | Heredity and Evolution Class 10 Science Chapter 9 Notes

Heredity and Evolution CBSE Class 10 Science Chapter 9 – Complete explanation Class 10 Science Notes of the chapter “Heredity and Evolution”.

Topics covered in the lesson are Heredity, Darwin Theory, Sex determination, Speciation, Evolution and classification, Important definitions, Theory of evolution. Given here is the complete explanation of the chapter, along with all the important questions and NCERT solutions to book questions have also been provided for the ease of students

Class 10 Science

Chapter 9 – Heredity and Evolution


Heredity and evolution


The Chapter Includes the following topics

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Heredity and Evolution Introduction

As we have studied, reproduction brings about some differences in the offspring. These differences are not easily seen because of similarities of offspring with their parents but there are variations that do exist due to unequal distribution of chromosomes during cell division in sexual reproduction in animals.

As we have always seen that there are a lot of similarities between us and our parents.
Also, we have a lot of dissimilarities between us and our friends or any people who don’t belong to our family.

This is all due to an interesting phenomenon called heredity. So let us first learn what heredity and variation are.

Heredity and Evolution Class 10 Video Explanation




It is the phenomenon of transfer of characters from parents to their offspring.
Variation: The differences shown among individuals of the same species or the differences exhibited by the offspring as compared to the parents are called variation.

Genetics: It is the study of heredity and variation. We all know our body is made up of many cells. In cells, we have a nucleus that has chromatin material which further condenses to form ribbon-shaped ribbon chromosomes.

These chromosomes are made up of DNA and proteins and they also have certain small segments on them that are genes. These genes are responsible for transferring characters from parents to offspring.


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The cells in our body are generally of two types :

  1. Somatic cells (self-replicating)
  2. Sex cells (sperm and egg)

Somatic cells – Content with Examples

Sex Cells – Content with example


He selected pea plant as an experimental plant because :

  1. It has 7 distinct easily contrasting characters
  2. Short life cycle
  3. Easily available
  4. Annual herb
  5. It doesn’t require much care.
  6. It produces a large number of seeds.


He formulated various laws:

  1. Law of Unit Character
  2. Law of dominance
  3. Law of segregation
  4. Law of Independent assortment


Law of unit character: Content with examples

Law of dominance: Content with examples

Law of segregation: Content with examples

Law of independent assortment: Content with examples

Two types of crosses:

1. Monohybrid cross

2. Dihybrid cross


Sex determination

It is the process by which sex of a child is determined. In human beings, a somatic cell contains 23 pairs of chromosomes, i.e. 46 chromosomes. Out of them, 22 pairs have similar chromosomes. These are called autosomes. The chromosomes in the 23rd pair can be different and are called heterosomes. Chromosomes of the 23rd pair are also called sex chromosomes because they determine if a zygote would develop into a male child or a girl child. Let us take and example to understand this-

Father Mother
(parental generation)


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It can be concluded that:

  1. There is a 50-50% chance of a boy or a girl.
  2. The sex of a child depend on the father because he produces different types of gametes.



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Evolution and classification

We have studied that there is an inbuilt tendency in every organism to carry variation because of DNA copying during reproduction. As a result of sexual reproduction, genetic variations give rise to evolution. Some variations are developed during the lifetime of an organism. Such variations are called acquired variations and these variations have no role in evolution.

We can define evolution as :

It is the gradual sequence of changes that took place over the years and lead to the development of entirely new species:
If evolution has taken place there must be proof to it that proves the evolution. We must be thinking that we should have some evidence to prove the occurrence of evolution:


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Different pieces of evidence that prove that evolution takes place are :

Fossils: The dead remains of plants and animals that got buried under the earth over millions of years ago. “The study of fossils is called Paleontology”. Fossil reveals the order of origin i.e.



Archaeopteryx: It is a fossil that resembles birds as well as reptiles. We can say that it is a link between reptiles and birds.

Homologous Organs: They are those which have the same structure but perform different functions. Example- Forelimbs of a man and cow.

Analogous Organs: They are those which have a different structure but perform the same function. Example- wings of birds & insects



Vestigial Organs: They are those which were functional in our ancestors but now they are not functional. Example- nictitating membrane, pinna muscles & vermiform appendix (man).


Embryology: It is the study of the development of an embryo. This study reveals that all organisms of different species that have common ancestors.

Biogenetic Law: According to this law ontogeny recapitulates phylogeny. This means that during the development of embryo complete history is repeated.

Theory of evolution

Lamarck Theory: According to this:

  1. The use or disuse of organs makes the organ efficient or less efficient.
  2. Favorable characteristics are passed on to the next generation.
  3. Accumulation of new characters leads to evolution.


Darwin theory

According to darwin theory :

  1. The population is increasing day by day, but still, the population rate is constant.
  2. Individuals of different species struggle for food, space, and mate.
  3. Those who can struggle to survive, others are eliminated.
  4. Favourable characters pass on to the next generation.
  5. Accumulation of new characters leads to evolution.


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  1. It is the formation of new species from pre-existing ones.
  2. A new species may be formed from the population occurring in different areas of distribution: allopatric speciation.
  3. A new species formed from fragments of the population in the same area due to mutation: sympatric speciation.
  4. Microevolution: It is the formation of species due to small but significant changes like gene mutation and gene recombination.
  5. Macroevolution: It is the development of different genres and taxa.


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Important Definitions

    1. Offspring: The individual organism that results from asexual or sexual reproduction.
    2. Allele: The two contrasting genes corresponding to the same character.
    3. Homogametic: The one which produces the same type of gametes.
    4. Heterogametic: The ones which produce different types of gametes.
    5. Haploid cell: The cell with a half set of chromosomes.
    6. Diploid Cell: The cell with a full set of chromosomes.

  1. Mutation: Sudden change in the hereditary makeup of an organism.
  2. GMO: Genetically modified organisms.
  3. Genetic drift: Random change in the frequency of alleles in a population over successive generations due to error during the DNA copying in gametes.
  4. Inherited traits: The characters that are transferred from parents to offspring.
  5. Acquired traits: traits that are acquired during the life span


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Heredity and Evolution Class 10 NCERT Solutions

1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as

(a) TTWW

(b) TTww

(c) TtWW

(d) TtWw

Ans: TtWw


2. An example of homologous organs is

(a) our arm and a dog’s fore-leg.

(b) our teeth and an elephant’s tusks.

(c) potato and runners of grass.

(d) all of the above.

Ans : (d) All of the above


3. In evolutionary terms, we have more in common with

(a) a Chinese school-boy.

(b) a chimpanzee.

(c) a spider.

(d) a bacterium.

Ans : (a) A Chinese school boy


4. A study found that children with light-colored eyes are likely to have parents with light-colored eyes. On this basis, can we say anything about whether the light eye color trait is dominant or recessive? Why or why not?

Ans: Let us assume children with light colored eyes are: LL or ll or Ll

If the genotype is LL, then


The F1 generation is: LL

If the genotype is ll, the

ll x ll

The F1 generation is: ll

Therefore it can’t be concluded which is dominant.


5. How are the areas of study – evolution, and classification – interlinked?

Ans: The classification involves grouping of similar organisms based on internal and external structures. Two species are more closely related if they have common features .and if two species are closely related it means they have a common ancestor.

This proves that we classify organisms according to their resemblance which is similar to creating an evolutionary tree.


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6. Explain the terms analogous and homologous organs with examples.

Ans: Homologous organs are those which have the same structure but perform different functions. Example- forelimbs of man & cow.
Analogous organs are those which have a different structure but perform the same function. Example- wings of bird & insect

8. Explain the importance of fossils in deciding evolutionary relationships.

Ans: Fossils are the remains of dead plants and animals that existed on earth. They actually represent the ancestors of plants and animals that actually provide the evidence of past organisms and the newly developed better-adapted species.

10. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolutaptations of those organisms that reproduce sexually?

Ans: In sexual reproduction, the two different variations of DNA combine to give rise to new individuals. Therefore sexual reproduction allows more variations and is better adapted whereas asexual reproduction does not allow variation as it produces clones because if there will be more variations then the resultant DNA won’t let it survive. In sexual reproduction, there is no such problem. The environmental conditions keep on changing so for that we need better-adapted species that result from sexual and not asexual reproduction.


11. How is the equal genetic contribution of male and female parents ensured in the progeny?

Ans: By sex determination :

Sex determination :

It is the process by which the sex of a child is determined.

Father Mother
(parental generation)
X, Y X, X

It was concluded:

(a) There is a 50-50% chance of a boy or a girl.

(b) The sex of a child depends on the father because the father produces different types of gametes.


12. Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?

Ans: In species, the variations that offer survival advantage are naturally selected. Individuals adjust to their environment with the help of those selected variations and consequently, these variations are passed to their progeny. The evolution of organisms leads to natural selection. However, there can be some other variations that do not offer a survival advantage and arise only accidentally. Such variations in small populations can change the frequency of some genes even if they are not important for survival. The accidental change in the frequency of genes is called genetic drift.


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