Class 10 Science Notes (Class 10)

CBSE Class 10 Science Chapter 9 Heredity and Evolution

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Heredity and Evolution

Heredity and Evolution

Heredity and Evolution


Heredity is the passing of traits from parents/ancestors to offspring. Heredity occurs through inheritance of genes. Biological inheritance is what is related to heredity.
Study of heredity is termed as Genetics.
Inheritance from a previous generation provides a common basic body design and few changes in it for the next generation. A trait that is genetically passed down from one generation to another is termed as ‘Inherited trait’.
Some of the common examples of inherited traits are hair color (Black, brown, red etc.), eye color (Black, brown, blue), height (tall, short), Ear lobes (free, attached)
You might be wondering how exactly the traits get inherited. To answer this, we will have to look at Mendelian Inheritance in Genetics.
The terms Heredity and Inheritance often sound similar; but they are not the same. Heredity refers to the passing off traits through genes from generation to the next. Whereas, the term Inheritance refers to the passing of property, rights etc.. on death of one generation to the next.

Mendelian Inheritance

Gregor Mendel started studying inheritance in peas. He performed series of experiments with pea plants for 7 long years (1856- 1863). He kept proper count of individuals exhibiting a particular trait in each generation.
Mendel, with his experiments found out the principles based on which it could be guesses how characters get inherited or passed to the offspring. Many rules of heredity were established often termed as “LAWS OF MENDELIAN INHERITANCE”.
Mendel is known as the ‘Father of Genetics’ due to his significant contribution to establish the basic principles in Genetics.

Mendel’s Pea Plant

Mendel chose pea plant for his experiments because of the following reasons:
  • Pea plant has several contrasting characters like height, flower color, seed color & shape
  • Self pollinated plant in nature
  • Cross pollination is easy to be done artificially
  • Short life span
  • Easy to cultivate
Mendel cross-pollinated tall pea plants with dwarf pea plants. (Pollen grains from flowers of tall plants dusted over the stigma of short plants)
Mendel found that the 1st filial generation (F1 generation) consisted of all Tall plants
In the second half of the experiment, he self-pollinated the Tall plants (F1 generation ones). He found that in F2 generation, 75% plants were tall and 25% were short
Mendel’s Conclusion:
The ‘dwarf’ trait was also carried, but remained hidden in F1 generation. This trait got expressed only in F2 generation.

Dominant and Recessive Traits

  • Dominant trait
  1. Trait that gets expressed in the offspring
  2. Takes over the other inherited trait
  3. For example, ‘Tall’ is a dominant trait of pea plants
  • Recessive trait
  1. Trait that remains hidden, dominated by the dominant trait
  2. For example, ‘Dwarf’ is a recessive trait of pea plant
Some other examples of dominant and recessive traits are as follows:
  • ‘Free’ ear lobes is a dominant trait, whereas ‘attached’ ear lobes is a recessive trait
  • ‘ Brown/ Black’ eye colour is a dominant trait, whereas ‘ Blue’ eye colour is a recessive trait

Chromosome n Number during Reproduction

New organisms are formed by sexual reproduction in humans. Normally, all cells in the body are diploid i.e. chromosomes exist in pairs. Humans have 46 chromosomes (23 pairs)
During reproduction, male and female sex cells combine to form the zygote. These male & female sex cells are haploid (have half number of chromosomes, i.e 23 chromosomes).
The specialized male sex cell is Sperm. Sperm has 22 autosomes and 1 sex chromosome.
The specialized female sex cell is Egg. Egg has 22 autosomes and 1 sex chromosome.
During reproduction, the sperm gives its 23 chromosomes (22 autosomes + 1 sex chromosome) and the egg gives its 23 chromosomes (22 autosomes + 1 sex chromosome). These together make the 46 chromosomes of the zygote (child).

Autosomes & Sex Chromosomes

  • Autosomes
  1. Chromosomes other than the sex chromosomes
  2. Exist in pairs, each of which has same form
  3. Control somatic traits
  4. In humans, 22 pairs of autosomes exist
  • Sex chromosomes
  1. Chromosome which determine sex
  2. In humans, 1 pair of sex chromosome exists

Haploid & Diploid Cells

  • Diploid
  1. Cell with two complete sets of chromosomes
  • Haploid
  1. Cell with a single complete set of chromosomes

Normally, all cells in the human body are diploid i.e. chromosomes exist in pairs. Except the male /female sex cells are haploid (have half number of chromosomes, i.e 23 chromosomes).


Homologous & Heterologous Chromosomes

Homologous chromosome:
  • A set of one maternal chromosome and one paternal chromosome that pair up with each other inside a cell
  • Same size & shape
  • Bears corresponding genes governing the same traits
  • Homologous regions code for the same gene
Heterologous chromosome:
  • Differ in shape, size or function
  • Do not belong to the same pair


  • One member of a pair of gene that occupy a given position on a homologous chromosome
  • For example: TT, Tt, tT, tt
Each‘t’ or ‘T’ is an allele (Capital letter denotes Dominant allele, whereas Small letter denotes Recessive allele)

Homozygous & Heterozygous Organisms

  • Homozygous organism
  1. Organism with identical alleles on homologous chromosomes
  2. Examples: TT, tt
  3. If self bred, give rise to offsprings with same traits
  • Heterozygous organism
  1. Organism with different alleles for a character on homologous chromosomes
  2. Also termed as ‘Hybrid’
  3. Examples: Tt, tT


Understanding Mendel’s Experiment

Experiment a)
  • Cross pollinated homozygous tall plants and homozygous dwarf plants
Experiment b)
  • Self pollinated plants of F1 generation

Mendel’s 2nd Experiment with Pea Plants


  • Mendel cross-pollinated pea plants with :
    • Homozygous round & yellow seeds
    • Homozygous wrinkled & green seeds

 In F1 generation, dominant trait (Round & Yellow) got displayed; while the recessive trait (Wrinkled/ Green) remained hidden. This is the Principle of Dominance.

Does that mean the hidden trait is lost or got modified into something else? Let’s look at the F2 generation..
  • Later, Self pollinated F1 generation


In F2 generation, the hidden trait (green/ wrinkled) reappears. This trait [wrinkled(r); green (y)] was present in F1 generation but remained hidden, however retained its identity. 
Different forms of traits retain their identity. This formed the basis of Principle of Segregation.   
In this experiment, Mendel took 2 contrasting characters- colour of seed and shape of seed of the pea plant. It was observed that colour & shape of the seeds were independent of each other.
Though we started with Round-Yellow & Green-Wrinkled combinations, we obtained even Round-Green and Yellow-Wrinkled combinations in F2 generation.

Mendel’s Observations

  • F1 generation displayed only one of the parental trait
  • Hidden trait in F1 generation reappeared unchanged in F2 generation
  • 4 types of plants were obtained in F2 generation in dihybrid cross

Principle of Dominance

In heterozygous organisms, only one out of the two alleles expresses itself (Dominant trait) while the other remains hidden (recessive trait)
Example: In Tt (heterozygous tall plant), T is dominant and t is recessive

Principle of Segregation

Each allele retains its distinct identity, even though they remain together in an individual; they segregate only during gamete formation
Example: In a hybrid tall plant Tt, ‘T’ & ‘t’ segregate only during gamete formation

Principle of Independent Assortment

During gamete formation, segregation of alleles of one pair is independent of the segregation of alleles of the other pair
Example: Self pollination of hybrid plants with Round & Yellow seeds
In this experiment, Mendel took 2 contrasting characters- colour of seed and shape of seed of the pea plant. It was observed that colour & shape of the seeds were independent of each other.
Though we started with Round-Yellow & Green-Wrinkled combinations, we obtained even Round-Green and Yellow-Wrinkled combinations in F2 generation. This shows that the alleles R,r and Y,y segregate independently.


Expression of Traits

Genes control the traits of living organisms.
Mendel’s 1st Experiment with Tall & dwarf plants. Height of the plant depends on the growth hormone, Auxin. If this hormone is more, height is more. The hormone is secreted by respective glands, which in turn is controlled by the proteins.
Gene has the information for synthesis of proteins. If a gene has alteration, enzyme can be less or more efficient which in turn can make the height less or more.

Sex Determination

  • Females have a perfect pair of sex chromosome, XX
  • Males have mismatched pair of sex chromosomes, XY
If the male sex chromosome is X, then the child is Female (XX)
If the male sex chromosome is Y, then the child is Male (XY)


There exists a huge variety of living organisms on Earth. Each reproduces to bring more organisms of its kind on Earth. It is believed that all organisms evolved from a common ancestor.
We will see how reproduction can give rise to variations, which in turn can give rise to new organisms or species. This is Evolution.
Variation during Asexual reproduction
In asexual reproduction, DNA gets copied. During copying, some errors occur (by chance) which give rise to variation in the offspring
Variation during Sexual reproduction
In sexual reproduction, the male sex cell (sperm) and female sex cell (egg) fuse to form the zygote which later develops into a new organism. Sperm and Egg cells are haploid. During reproduction, the sperm gives its 23 chromosomes (22 autosomes + 1 sex chromosome) and the egg gives its 23 chromosomes (22 autosomes + 1 sex chromosome). These together make the 46 chromosomes of the zygote (child).
Therefore, the child tends to get mix traits of both parents- some paternal, some maternal and some new traits. Small new traits in an organism  over a period of several years give rise to a different organism altogether.
What is Evolution?
  • Change in the inherited characteristics of biological populations over successive generations
  • Gradual continuous process in which something changes into a different and usually more complex or better form

Evolution: Darwin’s Finches

Some birds reached the Galapogos islands in the Pacific several years ago by storm. These were seed-eating birds. Due to good climate and no predators, they reproduced fast, and increased in number very fast.
Due to the increase in their population, there was scarcity of food. Every bird wanted to survive.
Due to variation, it was observed that some birds had long beak than others. In sometime, the long-beaked bird started eating worm and hard seeds. So these birds got a survival advantage. The short-beaked birds continued to eat seeds.
Thus, the birds got divided into 2 groups.
Each group reproduced amongst themselves. As a result, different types of finches arose. Currently, there are around 13-15 species of finches on the Galapogos islands. These finches are called Galapogos finches.
They were first collected by Charles Darwin on the Galápagos Islands during the second voyage of the Beagle, hence called Darwin’s finches.

Mechanisms of Evolution

  • Mutation
  1. Change in DNA sequence
  2. Generates variation
  3. Evolution of a single green/blue beetle which later gave rise to more
  • Natural selection
  1. Change in frequency of some genes in a population which give survival advantage
  2. Evolution of finches with larger beaks
  3. Evolution of the green beetles
  • Genetic drift
  1. Change in frequency of some lucky genes in a population even though these do not give any survival advantage
  2. Evolution of the Blue beetles
  • Migration
  1. Movement by organisms from one place to another
  2. Evolution of pink beetles

Acquired and Inherited Traits

  • Acquired traits
  1. Traits acquired by organisms during their lifetime
  2. Not passed from one generation to another
  • Inherited traits
  1. Traits controlled by genes
  2. Passed on from one generation to the next
Some examples:
Muscles of weight-lifters, scars, length of hair are examples of Acquired traits
Natural hair colour, shape of the ear lobe, eye colour and feet shape are examples of Inherited traits

Evidences of Evolution

  1. Following evolutionary relationships, we find all organisms have come from a common ancestor. The more common characteristics, the more closely they are related.
Brothers/ sisters have a lot more similarities than first cousins. Similarly, second cousins share even lesser similarities.
  1. Evidence from Fossils
  2. Evidence from Morphology and Anatomy

Evidence from Fossils

Fossils are preserved remains of living organisms from remote past.
Fossil mainly preserves only a portion of the dead organism (eg: skeleton, bone, teeth etc..)
Fossils may vary from microscopic (single bacterial cell) to dinosaurs.
Layers of fossils are formed one after another over years. Deeper the layer where the fossil is found, older it is.
Composition of the fossils helps in age estimation. Age estimation detects ratios of isotopes of different elements. Each isotope has a specific half-life. For example, C-12, C-14 are isotopes of Carbon.
Knowing the isotopes, we will know their half-lives. With that, we can calculate for how long it has been there, which in turn estimates the age of fossil.
  • Fossils at upper layers are more complex than that at lower layers
  • Fossils records show there is a link between birds & reptiles
  • Fossils records show how evolution occurred in some mammals with time

Evidence from Morphology and Anatomy

  • Comparative embryology of animals
       All animals go through these similar stages of early development of fetus:
       Zygote (single-celled) -> Group of cells -> Embryo (2-layered) -> Embryo (3-layered)
  • Homologous and Analogous organs
  1. Organs with common origin & structure but different functions are termed as Homologous organs
  2. This tells us that there exists an evolutionary relationship between different species, may be they originated from the same species with fore-limbs, gradually got changes as per their needs for survival. So, we can say that they share a common ancestor.
  3. Example: Forelimbs of amphibians, reptiles, birds, mammals
  4. Organs which perform similar function but have different origin & structures are termed as Analogous organs
  5. Example: Wings of bat, birds, insects

Evolution by Stages

  • Evolution by eyes
Eyes are the organs of sight. Eyes are complex in vertebrates like humans. They did not evolve all of a sudden, but evolved gradually. In earlier forms of organisms like euglena, a small eye spot was seen hidden.
Flatworms have eye spots to detect light. These eyespots gave survival advantage to flatworms. Gradually, with time, eyes developed. All vertebrates (mammals) have similar basic structure of eye.
  • Evolution of feathers
Feathers evolved in dinosaurs for insulation against cold weather. Later, birds adopted feathers for flight.
  • Artificial selection
Process of selecting desired traits to breed other plants/animals to get desirable traits is termed as Artificial selection.
Artificial selection
Natural selection
Process conducted by man
Natural phenomenon
Traits selected are beneficial to man
Traits selected are beneficial to species
Less time needed to yield results
Long time is needed for the results

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