Chapter – 8

Heredity

In this post we have given the detailed notes of class 10 Science Chapter 8 (Heredity) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 8: Heredity

Genetics 🧬

• “The branch of biology that studies how hereditary characters are passed on to offspring, and the similarities and variations among generations, is called genetics.”

Heredity

Organisms have a remarkable ability to produce offspring through reproduction. Certain traits are passed on from parents to offspring, generation after generation — these are called hereditary/inherited characters. The study of such inherited characters is called heredity.

Contribution of Gregor Johann Mendel

Gregor Johann Mendel is called the “Father of Genetics” because of his important contributions to this field. He was a friar (monk) from Austria. He carried out numerous experiments on pea plants and, based on them, put forward certain conclusions, which he published in a report in 1866.

Based on his experiments, Mendel arrived at the following conclusions:

• The factor that carries hereditary characters from one generation to the next was called a “factor” — it is now known as a gene.
• In hybrid offspring, this factor remains unchanged (does not blend), so it reappears unaltered in the next generation.

Variation

The differences in body design and DNA seen among different individuals of the same species are called variations. Such variations are inherited from one generation to the next, and are therefore also called genetic/hereditary variations. Some of these variations are present from birth, such as eye colour and hair colour, while variations such as body build and height can develop after birth.

Two Types of Variation

• Somatic (body cell) variation
• Germ cell (reproductive cell) variation

Somatic cell variation:

• This occurs in the body (somatic) cells.
• These are not passed on to the next generation.
• These do not contribute to biological evolution.
• These are also called acquired characters.
• Example: Piercing of ears, cutting the tails of dogs.

Germ cell variation:

• This occurs in the reproductive (germ) cells.
• These are passed on to the next generation.
• These are helpful in biological evolution.
• These are also called inherited/hereditary characters.
• Example: Human hair colour, human height.

Accumulation of Variation during Reproduction

Variations are seen as a result of reproduction, whether the organism reproduces asexually or sexually.

Sexual Reproduction

• The mode of reproduction in which the fusion of two gametes forms a structure (zygote) that gives rise to a new organism is called sexual reproduction. If the two fusing gametes are similar in shape and size, they are called isogametes, and their fusion is called syngamy.

In sexual reproduction:

• Variation is comparatively greater.
• It arises through crossing over, independent assortment of chromosomes, and mutation.

Asexual Reproduction

In most animals, fertilisation (the fusion of sperm and egg) is essential for reproduction. However, some organisms can reproduce without fertilisation — this is called asexual reproduction.

In asexual reproduction:

• Variations are fewer.
• They arise mainly due to small errors during DNA copying.

Advantages of Variation

• Because of the wide range of variation seen in nature, organisms can gain many kinds of advantages.
• Example: Heat-resistant bacteria have a better chance of surviving a sudden rise in temperature than heat-sensitive ones.
• The environment’s role in selecting the more useful variants forms the basis of the process of evolution.

Mendel’s Contribution 🌱

• Mendel put forward some fundamental laws of inheritance.
• Mendel is known as the father of genetics. He studied several contrasting (alternative) traits of the pea plant that are clearly visible — for example, round/wrinkled seeds, tall/dwarf plants, white/violet flowers, etc. He crossed pea plants having contrasting traits, such as tall plants and dwarf plants, and calculated the percentage of tall and dwarf plants in the offspring generation.

Why Mendel Chose the Pea Plant

Mendel selected the pea plant for his experiments because of the following properties:

• Pea plants show clearly visible contrasting alternative traits.
• They have a short life cycle.
• They are normally self-pollinating, but can also be cross-pollinated artificially.
• They produce a large number of seeds in a single generation.

Monohybrid Cross

A cross between two pea plants that differ in a single pair of contrasting traits is called a monohybrid cross. Example: A cross between a tall plant and a dwarf plant.

Observations

• In the first filial generation (F₁), no plant was of intermediate height — all plants were tall. This meant that of the two traits, only one parental trait was expressed.
• In the F₂ generation, 3/4 of the plants were tall and 1/4 were dwarf.
• Phenotypic ratio in F₂ = 3 : 1 (3 tall : 1 dwarf)
• Genotypic ratio in F₂ = 1 : 2 : 1
• The combinations TT, Tt, and tt occur in the ratio 1 : 2 : 1.

Conclusions

• Both TT and Tt plants are tall, whereas tt is a dwarf plant.
• A single copy of T is enough to make the plant tall, whereas for a plant to be dwarf, both copies must be t (i.e. tt).
• Traits like T are called dominant traits, while traits like t are called recessive traits.

Dihybrid Cross

• A cross between two pea plants that differ in two pairs of contrasting traits.
• The results of a dihybrid cross, in which parents differed in two pairs of contrasting traits such as seed colour and seed shape, showed:
  • F₂ round, yellow seeds: 9
  • Round, green seeds: 3
  • Wrinkled, yellow seeds: 3
  • Wrinkled, green seeds: 1
• This shows that the two traits (seed shape and seed colour) are inherited independently of each other.

Laws of Heredity

According to Mendel’s first law, an organism’s heredity is passed on to it through reproduction from its parents — this was demonstrated using the pea plant. If there are two factors for a trait and they are not identical, one factor easily dominates over the other. This is also called the Law of Dominance.

Mendel’s Laws of Inheritance

These laws are as follows:

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

Law of Dominance: When Mendel crossed homozygous plants having different (contrasting) traits, he studied the plants that showed only one of the two traits in this cross. He observed that one (dominant) trait expresses itself while the other (recessive) trait remains hidden. This phenomenon is called dominance, and this law is known as Mendel’s Law of Dominance.

Law of Segregation / Purity of Gametes: During gamete formation, the two alleles of a gene pair separate from each other so that each gamete receives only one allele. Hence, this is called the Law of Segregation.

Gametes are always “pure” for a trait — they carry only one allele for that trait.

Law of Independent Assortment: This law is based on the results of the dihybrid cross. According to this law, in a dihybrid cross, the inheritance of one trait is completely independent of the inheritance of another trait. That is, the alleles of one trait separate and get rearranged independently of the alleles of another trait during gamete formation.

This gives a trait ratio of 9 : 3 : 3 : 1.

Sex Determination

• Each human cell has 23 pairs of chromosomes, of which 22 pairs are called autosomes and the last (23rd) pair is called the sex chromosomes. In the diploid state, the female sex chromosome pair is XX, while the male sex chromosome pair is XY. It is these chromosomes in males that determine the sex of the offspring in humans.

Factors Responsible for Sex Determination

• In some organisms, sex determination depends on the temperature at which the fertilised eggs are incubated. Example: Snails.
• In some organisms, such as humans, sex determination depends on the sex chromosomes — XX (female) and XY (male).

Sex Determination in Humans

• Half the children can be boys and half can be girls. All children, whether boys or girls, inherit an X chromosome from their mother. Hence, the sex of the child depends on what type of chromosome they inherit from their father.
• A child who inherits an X chromosome from the father will be a girl, and a child who inherits a Y chromosome from the father will be a boy.

Evolution — a Brief Overview 🦕

Evolution is the slow, continuous process, beginning billions of years ago, through which new species of organisms have arisen from earlier ones. While the current NCERT syllabus for Class 10 focuses on Heredity, a basic understanding of the related idea of variation leading to evolution is useful for connecting the concepts.

Acquired Traits

Traits that an organism acquires during its own lifetime are called acquired traits. Example: Reduction in the weight of an undernourished beetle.

Properties of acquired traits:

• These traits are gained by organisms during their own lifetime. They do not cause any change in the DNA of the germ cells and are therefore not passed on/transferred to the next generation.
• They are not helpful in biological evolution. Example: Reduction in the weight of an undernourished beetle.

Inherited Traits

Traits that an organism receives from its parents are called inherited traits. Example: Colour of human eyes and hair.

Properties of inherited traits:

• These traits are received by organisms through heredity.
• They occur in the germ cells and are transferred to the next generation.
• They are helpful in biological evolution. Example: Colour of human eyes and hair.

Speciation

The formation of a new species from a pre-existing species is called speciation. For a species to survive in a changing environment, some of its members must adapt through changes in certain external traits, which can eventually give rise to a new species.

How Does Speciation Occur?

• Gene flow: Occurs between two populations that are not completely separated but are only partially isolated from each other.
• Genetic drift: A sudden change in the frequency of genes in successive generations of a population.
• Natural selection: The process by which nature selects and favours organisms that are better adapted.
• Geographical isolation: Caused by barriers such as rivers, mountains, etc., in a population, which prevents interbreeding between the two sub-populations.

Causes of Genetic Drift

• Sufficient change occurring in DNA.
• Change in the number of chromosomes.

Traits (Characteristics)

A description of external form or behaviour is called a trait. In other words, a particular structure or a particular function is called a trait. Examples:

• Having four limbs is a trait.
• Photosynthesis occurring in plants is also a trait.

Homologous Traits/Organs

Traits/organs in different organisms that have basically the same underlying structure, even though their functions may differ, are called homologous organs.

Example: The basic structure of the limbs of birds, reptiles, amphibians, and mammals is the same, although these limbs are used for very different functions in these different vertebrates.

Homologous organs show that these organs share a common origin from a similar ancestor, and they provide evidence for evolution.

Analogous Traits/Organs

Organs that differ in structure and origin, but perform the same function, are called analogous organs.

Example: The wings of a bird and the wings of a bat.

Analogous organs show that organs performing the same function have evolved from different ancestors.

Fossils

“The preserved remains of ancient plants and animals found within rocks are called fossils.” The collection of fossils and their sequence according to age reflects the order in which the process of evolution has occurred over time.

Examples:

• Ammonite — fossil — invertebrate
• Trilobite — fossil — invertebrate
• Knightia — fossil — fish
• Rajasaurus — fossil — dinosaur skull

How Old Are Fossils?

When excavated, fossils found closer to the surface of the earth are found to be more recent than fossils found at deeper levels.

Fossil dating: A technique in which the age of a fossil is determined based on the ratio of different isotopes of an element found in the fossil.

Evolution and Classification

Evolution and classification are closely related.

• The classification of organisms reflects their evolutionary relationships.
• The more characteristics two species share, the closer their relationship.
• The greater the similarities, the more recently they are likely to have shared a common ancestor.
• Similarities among organisms give us the opportunity to group them together and study them systematically.

Stages of Evolution

Evolution occurred gradually, over many generations.

1. Advantage of a trait: For example, the evolution of the eye — the development of complex organs is not possible through a single change in DNA; it happens gradually, over many generations.

• Planaria has a very simple eye.
• Insects have complex (compound) eyes.
• Humans have binocular eyes.

2. Change of function: For example, evolution of feathers — feathers originally evolved for insulation in cold weather, and later also proved useful for flight.

Example: Dinosaurs had feathers but were not capable of flying with them. Birds adapted feathers for the purpose of flying.

Artificial Selection

Very different-looking structures can evolve from a common basic design. About two thousand years ago, humans cultivated wild cabbage as a food plant, and through selection developed various different vegetables from it. This is called artificial selection.

Molecular Phylogeny

• This is based on the idea that changes in DNA during reproduction are the basic events underlying evolution.
• Distantly related organisms will accumulate a greater number of variations in their DNA.

Main Tools Used to Study Human Evolution

• Excavation
• DNA sequencing
• Dating (age determination)
• Study of fossils

Key Points to Remember

• Genetics is the study of heredity and variation — how traits pass from parents to offspring. 🧬
• Gregor Mendel, the “Father of Genetics”, used pea plants to establish the basic laws of inheritance in 1866.
• Variation can be somatic (not inherited) or germinal (inherited); germinal variation is the raw material of evolution.
• Mendel’s monohybrid cross gave a phenotypic ratio of 3:1 and genotypic ratio of 1:2:1 in the F₂ generation.
• Mendel’s dihybrid cross gave a ratio of 9:3:3:1, showing independent inheritance of traits.
• Mendel’s three laws are the Law of Dominance, Law of Segregation, and Law of Independent Assortment.
• In humans, sex is determined by the father’s contribution of an X or Y chromosome; mothers always contribute an X chromosome.
• Homologous organs share structure but not function (indicating common ancestry); analogous organs share function but not structure.
• Fossils and their relative depth/age provide direct evidence for the process and timeline of evolution. 🦴
• Evolution and classification are linked — greater similarity between species indicates a more recent common ancestor.

We hope that class 10 Science Chapter 8 (Heredity) notes in English helped you. If you have any query about class 10 Science Chapter 8 (Heredity) notes in English or about any other notes of class 10 Science in English, so you can comment below. We will reach you as soon as possible…