Class 10 Science Notes Chapter 8: Heredity

Class 10 science notes chapter 8 heredity explains why children resemble their parents while still having their own unique characteristics. For example, a child may inherit their mother's eye colour or their father's height, yet no two individuals are exactly alike. These similarities and differences are the result of heredity and variation.

These class 10 science notes chapter 8 heredity cover all the important concepts from the chapter in a simple and easy-to-understand manner. You'll learn about Mendel's experiments, dominant and recessive traits, patterns of inheritance, genes, chromosomes and sex determination in humans. A clear understanding of these topics builds a strong foundation in genetics and supports effective board exam preparation.

Topics Covered in Class 10 Science Notes Chapter 8 Heredity

Heredity and Variation	Genes and Their Expression
Variation in Asexual and Sexual Reproduction	Chromosomes
Mendel's Experiments on Inheritance	Role of Both Parents in Heredity
Monohybrid Cross	Inherited Traits and Sex Determination in Humans
Dihybrid Cross	Dominant and Recessive Traits

Class 10 Science Notes Chapter 8 Heredity: Simplified Notes 

Have you ever wondered why children resemble their parents but are never exactly the same? Class 10 science notes chapter 8 heredity explains how traits are inherited from one generation to the next and why variations occur among individuals. These variations play an important role in the survival and evolution of living organisms.

Heredity and Variation

Every living organism produces offspring that resemble their parents. However, offspring are not exact copies because slight differences, known as variations, are introduced during reproduction.

Heredity is the process through which traits are passed from parents to offspring.

Variation refers to the differences observed among individuals of the same species.

Importance of Variation

Variation is essential because it:

• Helps organisms adapt to changing environmental conditions.
• Increases the chances of survival of a species.
• Forms the basis of evolution through natural selection.
• Creates diversity within populations.

Variation in Asexual and Sexual Reproduction

Asexual Reproduction	Sexual Reproduction
One parent is involved.	Two parents are involved.
Offspring are almost identical to the parent.	Offspring show greater variation.
Variations arise mainly due to DNA copying errors.	Variations arise due to recombination of genes from both parents.

Why are Variations Important?

Not every variation helps an organism survive. However, beneficial variations increase the chances of survival under changing environmental conditions. Over many generations, such useful variations become common in a population.

Mendel's Experiments on Inheritance

Gregor Johann Mendel is known as the Father of Genetics because he discovered the basic principles of inheritance through experiments on pea plants.

Why Did Mendel Choose Pea Plants?

Pea plants were suitable because they:

• Grow quickly.
• Produce many offspring.
• Have easily distinguishable contrasting traits.
• Can self-pollinate as well as cross-pollinate.

Contrasting Traits Studied by Mendel

Trait	Contrasting Trait
Tall	Short
Round seeds	Wrinkled seeds
Violet flowers	White flowers
Yellow seeds	Green seeds

Monohybrid Cross

A monohybrid cross studies the inheritance of one pair of contrasting traits, such as tallness and shortness in pea plants.

Mendel's Experiment

Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt).

F₁ Generation

All the offspring in the first filial (F₁) generation were tall (Tt). Although each plant inherited one gene for tallness and one for shortness, only the tall trait was expressed. This showed that the tall trait masked the short trait.

F₂ Generation

When the F₁ plants self-pollinated, the second filial (F₂) generation contained:

• 3 Tall plants
• 1 Short plant

Phenotypic Ratio: 3 : 1 (Tall : Short)

Genotypic Ratio: 1 TT : 2 Tt : 1 tt

This experiment showed that traits are inherited as separate units (genes) and are passed from one generation to the next without blending. It also helped Mendel identify the concepts of dominant and recessive traits.

Dihybrid Cross

A dihybrid cross studies the inheritance of two pairs of contrasting traits at the same time.

Example:

• Tall plant with round seeds
• Short plant with wrinkled seeds

The F₂ generation produced four combinations:

• Tall with round seeds
• Tall with wrinkled seeds
• Short with round seeds
• Short with wrinkled seeds

Law of Independent Assortment

Mendel concluded that different pairs of traits are inherited independently of one another.

This explains why offspring can show new combinations of characteristics not seen together in either parent.

Dominant and Recessive Traits

Mendel observed that when two contrasting traits were inherited together, only one trait was visible in the first generation, while the other remained hidden and appeared again in the next generation. Based on this observation, he classified traits as dominant and recessive.

1. Dominant Trait

A dominant trait is expressed even when only one copy of its gene is present. It masks the expression of the recessive trait in a heterozygous individual.

Examples:

• Tall plants
• Round seeds
• Violet flowers

2. Recessive Trait

A recessive trait is expressed only when both copies of the gene are recessive. It remains hidden in the presence of a dominant gene.

Examples:

• Short plants
• Wrinkled seeds
• White flowers

Difference Between Dominant Trait and Recessive Trait

Dominant Trait	Recessive Trait
Tall	Short
Round seeds	Wrinkled seeds
Violet flowers	White flowers

Key Point: A dominant trait does not mean it is more common or stronger, and a recessive trait does not mean it is weaker. These terms simply describe how traits are expressed when different forms of a gene are inherited together.

Genes and Their Expression

Genes are the basic units of heredity that carry genetic information from parents to offspring. A gene is a specific segment of DNA that contains the instructions for making a particular protein. These proteins play an essential role in the growth, development, and functioning of an organism, ultimately determining its inherited characteristics or traits.

Genes express themselves by directing the production of proteins. These proteins help determine various characteristics of an organism by:

• Building and maintaining body structures.
• Regulating growth and development.
• Controlling metabolic activities.
• Influencing physical traits, such as height, eye colour, and hair type.

For example, a gene controlling plant height produces proteins involved in hormone formation. Efficient protein production leads to taller plants, while altered genes may produce shorter plants.

Chromosomes

Genes are arranged on thread-like structures called chromosomes, which are present inside the nucleus of every cell. These chromosomes carry the hereditary information that is passed from parents to their offspring.

Important Points

• Humans have 46 chromosomes, arranged into 23 pairs.
• One chromosome of each pair is inherited from the mother.
• The other chromosome is inherited from the father.
• During reproduction, each parent contributes one set of chromosomes, ensuring that the offspring receives an equal share of genetic material while maintaining the normal chromosome number.

Role of Both Parents in Heredity 

Sexual reproduction ensures that both parents contribute equally to the genetic makeup of the offspring. This allows the child to inherit characteristics from both the mother and the father.

• The male gamete (sperm) carries one set of chromosomes.
• The female gamete (egg) also carries one set of chromosomes.

During fertilisation, the sperm and egg fuse to form a zygote. The two sets of chromosomes combine, restoring the normal chromosome number. 

As a result, the offspring inherits half of its genetic material from each parent, ensuring equal genetic contribution and the transmission of hereditary traits from both sides of the family.

Inherited Traits

Inherited traits are characteristics that are passed from parents to offspring through genes.

Examples of Inherited Traits

• Eye colour
• Hair colour
• Blood group
• Earlobe type
• Height (partly inherited)
• Skin colour (influenced by multiple genes)

Some traits are inherited directly, while others are also influenced by environmental factors such as nutrition and lifestyle.

How is the Sex of a Child Determined

One of the most interesting aspects of heredity is how the sex of a child is determined. In humans, this is controlled by a special pair of chromosomes called sex chromosomes.

Every human cell contains 46 chromosomes, arranged into 23 pairs. Among these:

• 22 pairs are called autosomes, which control most body characteristics.
• 1 pair consists of sex chromosomes, which determine whether an individual is male or female.

Female	Male
XX	XY

During reproduction, both parents contribute one set of chromosomes to the child.

• Every egg produced by the mother carries 22 autosomes and one X chromosome.
• Sperms produced by the father are of two types:
• 22 autosomes + X chromosome
• 22 autosomes + Y chromosome

At fertilisation, one sperm fuses with the egg to form the zygote.

Father's Sperm	Mother's Egg	Child	Sex
X	X	XX	Girl
Y	X	XY	Boy

If the fertilising sperm carries an X chromosome, the child will have XX chromosomes and will be a girl. If it carries a Y chromosome, the child will have XY chromosomes and will be a boy.

Since the mother's egg always contributes an X chromosome, the sex of the child depends on whether the father's sperm contributes an X or a Y chromosome. Therefore, the father's sperm determines the sex of the child.

Probability of Having a Boy or a Girl

Each fertilisation event is completely random, so there is:

• 50% chance of a girl (XX)
• 50% chance of a boy (XY)

Important Exam Point: The mother does not determine the sex of the child. It is the X- or Y-bearing sperm from the father that determines whether the child will be a boy or a girl. This is a commonly asked concept in CBSE examinations.