CBSE Class 10 Science Notes Chapter 8 - Heredity

CBSE Class 10 Science Chapter 8 – Heredity explains how traits are transmitted from parents to offspring and why variations occur in living organisms. This CBSE Class 10 Science notes chapter forms the genetic foundation of biology, covering key concepts such as inheritance patterns, Mendel’s experiments, dominant and recessive traits, genotype–phenotype relationships, and the role of genes and chromosomes.

These CBSE Class 10 Science Notes for Chapter 8 Heredity are designed to simplify complex genetic principles into easy-to-understand explanations, supported by diagrams, definitions, and exam-oriented examples. Students looking for heredity class 10 notes will find these notes highly useful for quick revision, last-minute preparation, and answering application-based questions confidently in the board examination.

1.0Download CBSE Notes for Class 10 Science Chapter 8: Heredity - Free PDF!!

Download CBSE Notes for Class 10 Science Chapter 8: Heredity – Free PDF to understand genes, traits, inheritance patterns, Mendel’s experiments, and variation. These concise, exam-oriented notes follow the latest CBSE syllabus and are ideal for quick revision and board exam preparation.

2.0Heredity: Passing Traits Across Generations

• Heredity is the process through which traits are passed from parents to offspring. 
• This involves the inheritance of genes. 
• These genes are located on chromosomes found in every organism's cell. 
• Sexual reproduction, in particular, creates new combinations of genes, leading to greater variation among offspring.

3.0Gregor Johann Mendel 

• Mendel was an Austrian monk and scientist who first worked on heredity experiments and gave theory in 1866 but people of that time couldn't understand him and rejected his theory. 
• Later in 1900 three different scientists from 3 different places also found the same result. They were Hugo deVries, Tschermak and Correns. 
• They rediscovered the theory of heredity proposed by Mendel. So Mendel is called "Father of genetics".

Advantages of Pisum sativum:

Pairs of allelic characters found in garden pea plant

Laws of Inheritance

1. Law of dominance : In crossing between organisms purely for contrasting characters of a pair, only one character of the pair appears in the F1 generation. This character is termed as dominant while the one which does not express itself in F1 generation is termed as recessive. 
2. Law of segregation : Different alleles or genes of a character remain together in an individual and segregate randomly at the time of gamete formation. This is also known as the Law of purity of gametes. 
3. Law of independent assortment : This law states that when individuals differing in two or more than two pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other. e.g., The inheritance of seed shape character is not related to the seed color character. Rather, the two characters inherit independently of each other.

4.0Sex Determination

• In sexually reproducing species, determining the sex of offspring can vary between species. 
• In humans, sex is determined genetically by specific sex chromosomes. 
• When offspring are conceived, the mother always contributes an X chromosome, while the father can contribute either an X or a Y chromosome. 
• If the child inherits an X chromosome from the father, the child will be female; if the child inherits a Y chromosome, the child will be male.
• In other species, environmental factors such as temperature can determine the sex of offspring, as in certain reptiles. 
• Additionally, some species, like snails, can change their sex based on environmental conditions, demonstrating that sex determination mechanisms vary widely in nature.

5.0Evolution: The Process of Change Over Time

• It is the gradual change in species over generations, driven by genetic variation and natural selection. 
• Variations arise through mutations and gene recombination during reproduction. Some variations may offer an advantage to individuals in a population, helping them survive and reproduce more successfully than others. 
• Over time, these advantageous traits become more common, leading to evolutionary changes in the species.

6.0Natural Selection

• Natural selection, a concept introduced by Charles Darwin, is how organisms better suited to their environment tend to survive and pass on their traits to the next generation. 
• For example, birds with longer beaks may be more successful at accessing food in hard-to-reach places, giving them a survival advantage. 
• Over time, this trait will become more common in the population, as birds with shorter beaks are less likely to survive and reproduce.
• Natural selection gradually accumulates beneficial traits within a population, helping species adapt to changing environments. 
• This process explains how organisms evolve, resulting in the vast diversity of life we see today.

1. Genetic drift 

• The change in the frequency of certain genes in a population over generations or the random changes in the gene frequency occurring by chance alone. 
• The effect of genetic drift is very small in large populations and large in small populations. 
• Genetic drift provides diversity without any adaptation.

2. Speciation 

• This  occurs when populations of a species become reproductively isolated, leading to the emergence of new species. 
• Geographic isolation, such as the formation of mountains or rivers, can separate populations, allowing them to evolve independently. 
• Over time, genetic differences accumulate, and the populations can no longer interbreed, forming distinct species.

3. Adaptation

• Adaptation is how a species becomes better suited to its environment. 
• For example, polar bears have adapted to their cold environment by developing thick fur and fat layers to retain heat. 
• Similarly, desert plants have evolved mechanisms to store water and survive in arid conditions. 
• Adaptations can be physical, such as body structures, or behavioral, such as migration patterns.

4. Homologous organs 

• These are those organs having similar basic structure but have been modified to perform different functions. e.g. forelimbs of reptiles, frogs, lizards, birds and humans are homologous organs. 
• Such homologous characteristics help to identify an evolutionary relationship between apparently different species. 
• This relationship is called divergent evolution or homology.

5. Analogous organs: Those organs which have different origin and structural plan but appear similar and perform similar functions are called analogous organs, while this relationship is called convergent evolution or analogy. e.g. Wings of an insect, bird and bat.
6. Fossils : Fossils are remains or impressions of the hard parts of the extinct organism preserved in the sedimentary rock or other media. 

7.0The Role of Variation in Evolution

• Variation is essential for evolution. 
• In sexually reproducing organisms, variation arises from mixing genes from two parents. 
• Mutations, or random changes in DNA, can also introduce new genetic traits. 
• Variation allows populations to adapt to environmental changes. 
• For instance, if a population of insects is exposed to a new pesticide, some individuals may carry a genetic mutation that makes them resistant to the chemical. 
• These insects are more likely to survive and pass the resistance gene to their offspring. Over time, the population evolves to become resistant to the pesticide.

8.0Detailed CBSE Class 10 Science Chapter 8 – Key Notes

Heredity is the passing of characters from parents to offspring's or from one generation to next.

Note:

• Genetics is a branch of biology which deals with the study of heredity and its variation.

• Sexually reproducing organisms show greater variations which may lead to increased survival of individual. So greater diversity will be generated by sexual reproduction.

• The lowest part of the ear, called the earlobe, is closely attached to the side of the head in some of us, and not in others. Free and attached earlobes are two variants found in human populations.

Terminology used

Alleles

Alternative forms of a gene which are located on same position (loci) on the homologous chromosome are called alleles.

Gene

• Unit of heredity which transfers characters from parents to their offsprings during reproduction is called genes.

• Chromosome is a thread like structure in the nucleus of a cell. It is formed of DNA which carries the genes.

Note:

The differences in the traits or characters among the individuals of a species are called variations. e.g. Plant height – tall and dwarf.

Genotype

The genetic constitution of an organism or the description of genes present in an organism e.g., TT, Tt, tt is called genotype.

Phenotype

External and morphological appearance of an organism for a particular character is called phenotype. e.g., Tall, Dwarf.

Dominant

Any character that appears in the F₁ generation offspring from a cross between parents possessing contrasting characters (such as tallness and dwarfness in pea plants) is called dominant character.

Recessive

Recessive factor or allele is one which is unable to express its effect in the presence of its contrasting factor in heterozygous state.

Homozygous

• In homozygous condition, two factors of a character are same (TT).

• In hybrid/heterozygous condition, two factors of a character are different (Tt).

Mendel’s Experiment

Gregor Johann Mendel (1822 – 1884)

• Mendel was an Austrian monk and scientist who first worked on heredity experiments and gave theory in 1866.
• Later in 1900 three different scientists Hugo de Vries, Tschermark and Carl Correns rediscovered Mendel's laws of inheritance.
• Mendel is called father of genetics.

Mendelian experiments

In 1856-57, he started his historical experiments of heredity on pea (Pisum sativum) plant.

Note:

Mendel said that each character is controlled by a pair of factor which is now known as gene.

Also Read: Mendel's Experiment

9.0Advantage of Pisum sativum

• Pisum sativum is a unique plant selected by Mendel for his experiments which had following importance:

Pairs of allelic characters found in garden pea plant

Monohybrid cross

• It is a cross in which only one character is studied at a time.
• In his first monohybrid cross, Mendel crossed a pure breed tall plant (TT) and a pure breed dwarf plant (tt).
• He found that in F₁ generation only tall plant were born out of these seeds, no intermediate character or mixing or blending was observed by him. Mendel termed tallness as dominant and dwarfness as a recessive character.

IN F₂ GENERATION :

Phenotypic ratio was → 3 : 1
Genotypic ratio for was → 1 : 2 : 1

Mendel’s postulates

• In heterozygous conditions out of the two alleles only one allele is able to express its effect and it is called dominant allele and other which does not show its effect in heterozygous condition is called recessive allele.
• When F₁ plants are grown and allowed to self-pollinate he got many seeds and when they were grown he found that the ratio of tall plants and dwarf plants was 3 : 1 respectively.
• The dwarfness which was not visible in F₁ generation reappeared in F₂ generation.
• The dwarfness which was not visible in F₁ generation reappeared in F₂ generation.

Dihybrid cross

Mendel’s laws of inheritance

Law of dominance

• In crossing between organisms pure for contrasting characters of a pair, only one character of the pair appears in the F₁ generation.
• This character is termed dominant while the one which does not express itself in F₁ generation is termed recessive.

Law of segregation

Different alleles or genes of a character remain together in an individual and segregate randomly at the time of gamete formation. This is also known as the Law of purity of gametes.

Law of independent assortment

• This law states that when individuals differ in two or more than two pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other.
• E.g. The inheritance of seed shape character is not related to the seed colour character. Rather, the two characters inherit independently of each other.

Sex determination

It is the phenomena of determination of sex of an offspring.

Factors responsible for sex determination

1. Environmental
2. Genetic

Sex determination in humans

Chromosomes

In humans, the diploid number of chromosomes is 46, or 23 pairs present in the nuclei of their cells.

1. There are 22 pairs of homologous chromosomes (look alike) called autosomes.
2.  The 23rd pair which differs in males and females are called sex chromosomes or allosomes.

These two chromosomes determine the sex of an individual, and are indicated by the letters X and Y.

The gametes (egg and sperm) have only one set of chromosomes.

• Birth of male and female child has equal %, because X and Y chromosomes are produced in equal number and it all depends on either sperm with X or Y chromosome fuses with egg containing X chromosome.
• Birth of male and female child depends on Y or X chromosomes of male, female always produce X chromosomes, so have no significant role in sex determination of offspring.

10.0Benefits of CBSE Notes Class 10 Science Chapter 8 - Heredity

• Effective Time Management: Studying with notes saves time by allowing you to quickly access and review the essential information.
• Exam Preparation: Notes are specifically tailored to the CBSE Class 10 syllabus, ensuring you are focusing on the relevant content for your Class 10 Science exams.
• Clarity on Diagrams and Processes: Good CBSE Notes often include simplified diagrams and step-by-step explanations of processes like DNA replication or the inheritance of traits.
• Self-Assessment: Reviewing notes can help you identify areas where your understanding is weak, allowing you to focus your study efforts accordingly.
• Building a Strong Foundation: A solid understanding of heredity is fundamental for further studies in biology and related fields, and well-prepared notes contribute to this foundation.

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