NCERT Class 10 Science Chapter 8 – Heredity

NCERT Class 10 Science Chapter 8 explains how traits are passed from parents to offspring. In NCERT Class 10 Science Chapter 8, students learn about heredity, variation, Mendel’s experiments and the mechanism of sex determination in humans. NCERT Class 10 Science Chapter 8 is a core genetics chapter that connects directly with reproduction and evolution.

NCERT Class 10 Science Chapter 8 begins with the concept of Accumulation of Variation during Reproduction. While reproduction creates similar individuals, small changes during DNA copying produce variations. NCERT Class 10 Science Chapter 8 highlights that these variations accumulate over generations and play a major role in evolution.

A central theme of NCERT Class 10 Science Chapter 8 is Heredity, which refers to the transmission of traits through Genes. Genes are segments of DNA located on Chromosomes. In NCERT Class 10 Science Chapter 8, it is explained that each individual inherits two versions of a gene — one from the mother and one from the father.

NCERT Class 10 Science Chapter 8 gives special importance to Gregor Johann Mendel (1822–1884), known as the Father of Genetics. Through experiments on Garden Pea Plants (Pisum sativum), Mendel established laws of inheritance.

Key observations from NCERT Class 10 Science Chapter 8 include:

• Dominant Traits express even in presence of one allele.
• Recessive Traits express only when both alleles are recessive.
• Monohybrid cross shows 3:1 Phenotypic Ratio.
• Dihybrid cross shows 9:3:3:1 Ratio.

NCERT Class 10 Science Chapter 8 also explains how traits get expressed. Genes control the formation of Proteins, and these proteins determine the physical characteristics of organisms.

Another important topic in NCERT Class 10 Science Chapter 8 is Sex Determination. Humans have 23 pairs of chromosomes, including one pair of sex chromosomes. Females have XX, while males have XY chromosomes. NCERT Class 10 Science Chapter 8 clearly states that the father determines the sex of the child.

NCERT Class 10 Science Chapter 8 is highly important for CBSE board exams and foundation-level competitive exams because genetics-based questions are frequently asked.

Students should refer to the official NCERT website at for authentic textbooks and syllabus updates.

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8.1 Accumulation of Variation During Reproduction

• Reproduction produces new individuals that are Similar but not Identical, because small variations arise during DNA Copying.
• In Asexual Reproduction, variations are minimal since offspring inherit almost the same genetic material from one parent.
• If a single bacterium divides repeatedly, the resulting individuals will be very similar with only minor differences.
• In Sexual Reproduction, variation increases significantly because DNA from two different parents combines in the offspring.
• Over successive generations, variations accumulate and create Diversity within a population.
• Not all variations are equally useful; some may help individuals survive better under specific environmental conditions.
• Selection of favourable variations by the environment forms the basis of Evolutionary Processes.

8.2 Heredity

• Heredity refers to the transmission of traits from parents to offspring through genetic material.
• The most obvious outcome of reproduction is generation of individuals with a similar Basic Body Design.
• Traits are controlled by units of inheritance called Genes, which are present on Chromosomes.
• Both father and mother contribute nearly equal amounts of genetic material to the child.
• For each trait, two versions are inherited — one from the Maternal Parent and one from the Paternal Parent.
• The combination of these gene versions determines the characteristics expressed in the offspring.
• Heredity ensures continuity of species while allowing accumulation of variations across generations.

8.2.1 Inherited Traits

• An Inherited Trait is a characteristic that is passed from parents to offspring through Genes.
• A child inherits two versions of each trait, one from the Mother and one from the Father.
• Some traits, such as Free or Attached Earlobes, show visible variations within human populations.
• Inheritance provides a common Basic Body Design, but also introduces subtle differences among individuals.
• Variations that arise due to environmental factors during an individual’s lifetime are Not Inherited.
• Only those traits that are controlled by genes in reproductive cells are passed to the next generation.
• Thus, heredity explains both similarities and differences seen among members of a species.

8.2.2 Rules for the Inheritance of Traits – Mendel’s Contributions

• Gregor Johann Mendel (1822–1884) conducted experiments on Garden Pea Plants (Pisum sativum) and laid the foundation of modern genetics.
• He selected contrasting traits such as Tall/Short Plants and Round/Wrinkled Seeds for his experiments.
• When a Tall (TT) plant was crossed with a Short (tt) plant, all F1 Generation plants were tall, showing that tallness is a Dominant Trait.
• When F1 plants were self-pollinated, the F2 Generation showed a ratio of 3 Tall : 1 Short, indicating reappearance of the recessive trait.
• Mendel proposed that traits are controlled by pairs of factors (now called Genes) and each parent contributes one factor.
• A trait expressed in the presence of its alternative form is called Dominant, while the one that remains unexpressed is Recessive.
• In experiments involving two traits, Mendel observed Independent Assortment, where traits are inherited independently in a 9:3:3:1 Ratio in F2 generation.
• Mendel’s laws explain how traits are inherited through generations and form the basis of Genetics.

8.2.3 How do these Traits get Expressed?

• Traits are controlled by Genes, which are segments of DNA present on chromosomes.
• Genes carry information for making specific Proteins, and these proteins determine the characteristics of an organism.
• A change in the DNA sequence of a gene can alter the protein formed, leading to variation in a trait.
• For example, in pea plants, the gene for tallness produces a growth hormone, resulting in Tall Plants.
• If the gene is altered and does not produce sufficient growth hormone, the plant remains Short.
• When two versions of a gene are present, the Dominant Gene expresses itself even if only one copy is present.
• The Recessive Gene is expressed only when both gene copies are recessive.
• Thus, traits are expressed through interaction between genes and the proteins they produce.

8.2.4 Sex Determination

• In human beings, sex is determined genetically by a pair of Sex Chromosomes.
• Humans have 23 pairs of chromosomes, out of which 22 pairs are autosomes and one pair is the sex chromosome.
• Females have two similar sex chromosomes (XX), while males have one X and one shorter Y chromosome (XY).
• During reproduction, each parent contributes one sex chromosome to the child through their gametes.
• All ova produced by the mother carry only the X chromosome.
• Sperms produced by the father may carry either X or Y chromosome.
• If a sperm carrying X fertilises the egg, the child will be XX (Girl); if it carries Y, the child will be XY (Boy).
• Thus, the Father determines the Sex of the Child, not the mother.

Exam Oriented Facts

• Heredity is the transmission of traits from parents to offspring through Genes.
• Basic body design is inherited, but small changes during DNA Copying produce Variations.
• Accumulation of variations across generations leads to Diversity in populations.
• Environmental selection of favourable variations forms the basis of Evolution.

• A Gene is a segment of DNA that controls a specific trait.
• Genes are located on Chromosomes present in the nucleus.
• Humans have 23 pairs of Chromosomes (22 pairs of autosomes + 1 pair of sex chromosomes).
• Each trait is controlled by two versions (alleles), one inherited from each parent.
• Gregor Johann Mendel (1822–1884) is known as the Father of Genetics.
• He performed experiments on Garden Pea Plants (Pisum sativum).
• Monohybrid cross (Tall × Short) produced 3:1 Phenotypic Ratio in F2 generation.
• Genotypic ratio in monohybrid cross is 1:2:1 (TT : Tt : tt).
• Dominant Trait expresses even in presence of one allele (T).
• Recessive Trait expresses only when both alleles are recessive (tt).
• Dihybrid cross showed 9:3:3:1 Ratio, proving Law of Independent Assortment.
• Mendel proposed that traits are controlled by pairs of Factors (Genes).
• Example: Tallness (T) is dominant over Shortness (t).
• In peas, Round Seeds (R) are dominant over Wrinkled Seeds (r).
• Dominant traits mask expression of recessive traits in heterozygous condition.
• Genes control production of Proteins, which determine traits.
• A functional gene produces specific proteins leading to expression of the trait.
• Mutation or alteration in gene may affect protein formation and trait expression.
• Asexual reproduction produces minimal variation.
• Sexual reproduction increases variation due to combination of genes from two parents.
• Variation improves chances of survival under changing environmental conditions.
• Females: XX
• Males: XY
• All ova carry X chromosome.
• Sperms carry either X or Y chromosome.
• Child inheriting XX = Girl.
• Child inheriting XY = Boy.
• Therefore, Father determines the sex of the child.
• Monohybrid cross phenotypic ratio: 3:1
• Monohybrid cross genotypic ratio: 1:2:1
• Dihybrid cross phenotypic ratio: 9:3:3:1

NCERT Class 10 Science Chapter 8 – Heredity builds a strong foundation in genetics and inheritance. A thorough understanding of NCERT Class 10 Science Chapter 8 helps students master concepts such as dominant and recessive traits, Mendel’s laws and sex determination.

NCERT Class 10 Science Chapter 8 is crucial for CBSE board preparation and forms the base for advanced biology topics in higher classes and competitive exams.

Continue reading NCERT Class 10 Science Chapter 9 – Light: Reflection and Refraction for physics concepts in optics.

FAQs

Q1. What is NCERT Class 10 Science Chapter 8 about?
NCERT Class 10 Science Chapter 8 explains heredity, variation, Mendel’s laws and sex determination.

Q2. Who is the father of genetics in NCERT Class 10 Science Chapter 8?
Gregor Johann Mendel.

Q3. What is the 3:1 ratio in NCERT Class 10 Science Chapter 8?
It is the phenotypic ratio observed in a monohybrid cross.

Q4. How many chromosome pairs do humans have according to NCERT Class 10 Science Chapter 8?
Humans have 23 pairs of chromosomes.

Q5. Why is NCERT Class 10 Science Chapter 8 important for exams?
NCERT Class 10 Science Chapter 8 is important because genetics and inheritance are high-weightage topics in CBSE exams.

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