NCERT Science Notes - Class 10
Chapter 8 - Heredity

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 10 | Science | Chapter - 8 | Heredity. In this chapter, students delve into the fascinating world of genetic inheritance. It explores how traits are passed down from parents to offspring through genes, highlighting key concepts like dominant and recessive alleles, genotype and phenotype, and Mendelian laws of inheritance. By understanding heredity, students gain insight into genetic diversity, the basis for evolution, and the role of DNA in transmitting characteristics. This chapter also touches on the significance of genetic variations and mutations in shaping life as we know it, making it a cornerstone for comprehending biological inheritance and evolution.

NCERT Science Notes - Class 8 Chapter 9 - Friction notes ajs, cbse notes class 10 ajslearning, cbse notes ajs, ajs notes class 10, ajslearning, ajs chalo seekhen

NOTES

8.0 - Introduction

Reproductive Processes and New Individuals:
- Reproduction results in new individuals who are similar to their parents but show subtle differences.
Variation in Asexual and Sexual Reproduction:
- Asexual Reproduction:
  - Produces some variation, although minimal, due to genetic copying errors or environmental factors.
- Sexual Reproduction:
  - Maximizes variations through the combination of genes from two parents.
  - This process increases the chances of successful adaptations within a species.
Observation of Variations:
- Example in Plants:
  - In crops like sugarcane, where reproduction often occurs through cloning, individuals show very little variation.
- Example in Animals and Humans:
  - In sexually reproducing organisms, distinct variations can be seen among individuals, leading to diversity within the species.
Study Focus:
- This chapter explores the mechanisms that create and pass on variations, leading to diversity within a population.

8.1 - Accumulation of Variation During Reproduction

Inheritance and Variation:
- Each generation inherits a basic body design from the previous one, along with minor changes.
- When the new generation reproduces, it passes on inherited differences and also introduces additional variations.
Asexual Reproduction:
- Example: Bacteria – If one bacterium divides, resulting in two, and then each divides again, the four bacteria are nearly identical.
- Minor Differences: Small inaccuracies in DNA copying introduce minor variations between the offspring.
Sexual Reproduction:
- Greater Diversity: Combines genetic material from two parents, creating more significant diversity in offspring than asexual reproduction.
Survival and Variations:
- Not all variations are equally beneficial for survival.
- Example: Heat-resistant bacteria have an advantage during a heat wave.
- Environmental Selection: Variants that are better adapted to the environment are more likely to survive, forming the foundation for evolutionary processes.

Summary: During reproduction, both inherited and newly created variations accumulate, especially in sexually reproducing organisms. These variations can influence an individual’s ability to survive environmental challenges, with favorable traits being selected over time. This selection of traits forms the basis of evolution.

8.2 - Heredity

The primary outcome of reproduction is creating new individuals with similar characteristics to their parents. The rules of heredity guide how these traits are inherited from one generation to the next. Let’s explore how traits are passed down.

8.2.1 - Inherited Traits

Definition: Inherited traits are characteristics passed from parents to offspring, leading to similarities within a species.
Human Variation: While a child possesses basic human features, they do not look exactly like their parents, showing natural variation within human populations.

Activity 8.1: Observing Earlobes

Objective: Identify students with either free or attached earlobes and record the percentages for each type.
Procedure:
- Observe the earlobes of all students and classify them as free or attached (refer to Fig. 8.2).
- Calculate the percentage of students with each type of earlobe.
Extension: Check the earlobe type of each student’s parents.
Analysis: Correlate students’ earlobe types with those of their parents to infer a pattern or rule for the inheritance of earlobe types.

Suggested Outcome

Based on the observed correlation, students may suggest that certain earlobe types are inherited from parents, hinting at a genetic pattern, where free or attached earlobes may follow a specific rule of inheritance.

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

The inheritance of traits follows specific patterns, largely influenced by the equal genetic contribution from both the mother and the father. Each parent provides one set of genetic information, meaning that each child has two versions (or alleles) for each trait—one from each parent.

Gregor Mendel’s Work on Inheritance

Mendel’s Discovery: Gregor Mendel, through his experiments on pea plants, discovered foundational rules for how traits are passed down. His work, done over a century ago, provided insights into the mechanisms of inheritance.
Key Insight: Traits in offspring are influenced by the combination of alleles received from both parents.
Impact on Observed Traits: Mendel observed that the trait visible in an offspring (phenotype) depends on which version (dominant or recessive) of each allele is present.

Mendel’s research laid the groundwork for understanding genetic inheritance, explaining why certain traits appear in offspring based on the combination and dominance of alleles received from each parent. This inheritance pattern has since been observed widely across different organisms, including humans.

Gregor Johann Mendel (1822–1884)

Background:

Mendel was trained in a monastery and later pursued science and mathematics at the University of Vienna. Although he didn’t succeed in obtaining a teaching certificate, his passion for science remained. Returning to his monastery, Mendel began studying inheritance through pea plants, a project that would eventually shape genetics.

Experiments on Pea Plants:

Mendel studied several contrasting characteristics in garden peas, such as:
- Seed shape: round vs. wrinkled
- Plant height: tall vs. short
- Flower color: white vs. violet

Method and Findings:

Crossing Plants: Mendel crossed plants with different traits, such as tall and short plants, and recorded the traits of the offspring (F1 generation).
First Generation (F1): In the F1 generation, all plants were tall, indicating that only one parental trait was visible. No intermediate height (like "medium-height") was observed.
Second Generation (F2): When F1 plants were self-pollinated, a mix of tall and short plants appeared in the F2 generation, with approximately three-quarters tall and one-quarter short.

Conclusion:

Inheritance Factors (Genes): Mendel proposed that traits are controlled by two copies of a factor, now known as genes.
Dominant and Recessive Traits: In F1 plants, the tallness trait was dominant (visible), while shortness was recessive (hidden but inherited).

Mendel’s Laws of Inheritance were derived from his careful tracking and mathematical analysis of these patterns, laying the foundation for modern genetics.

Activity 8.2

Experiment to Confirm 1:2:1 Ratio in F2 Generation: To confirm that the F2 generation has a 1:2:1 ratio of TT (homozygous tall), Tt (heterozygous tall), and tt (homozygous short) plants, we could:

Self-pollinate F1 Plants: Allow the F1 generation (all Tt) to self-pollinate.
Observe F2 Phenotypes: Count and classify the tall and short plants in the F2 generation.
Genotype Analysis: Perform genetic analysis on the F2 plants to confirm the 1:2:1 ratio of TT, Tt, and tt plants.

In this experiment:

TT and Tt plants exhibit the tall trait (dominant).
tt plants exhibit the short trait (recessive).

Determining Dominant and Recessive Traits:

Dominant traits (like T for tallness) only need one allele to be expressed in the phenotype.
Recessive traits (like t for shortness) require both alleles to be present.

Inheritance of Multiple Traits (Dihybrid Cross): When two characteristics are studied (e.g., plant height and seed shape), Mendelian experiments reveal that traits like tallness and round seeds are dominant, as seen when a tall, round-seeded plant is crossed with a short, wrinkled-seeded plant. All F1 progeny are tall with round seeds.

F2 Generation Observations:

F2 progeny can show a mix of traits, including new combinations:
- Tall plants with round seeds
- Tall plants with wrinkled seeds
- Short plants with round seeds
- Short plants with wrinkled seeds

Conclusion: This experiment demonstrates independent assortment, where traits such as height and seed shape are inherited independently, creating diverse combinations in F2 offspring. Mendel’s findings emphasize how traits recombine to produce varied genetic outcomes in subsequent generations.

8.2.3 - How do these Traits get Expressed?

Mechanism of Heredity:

DNA as Information Source: DNA within cells provides instructions for synthesizing proteins. A gene is a segment of DNA that encodes information to create a specific protein.
Role of Proteins: Proteins, particularly enzymes, influence traits by controlling biochemical processes. For instance, a plant's height is impacted by the hormone levels that regulate growth, and the production of these hormones depends on the activity of enzymes.
Example - Tall and Short Plants:
- If the enzyme responsible for hormone production functions efficiently, more growth hormone is produced, leading to tall plants.
- If a mutation reduces enzyme efficiency, less hormone is produced, resulting in short plants.

Inheritance of Traits:

During sexual reproduction, each parent contributes one set of genes to the progeny, resulting in two sets of genes (one from each parent).
Thus, each characteristic is determined by two copies of the same gene—one from each parent.

Formation of Germ Cells:

Germ cells (reproductive cells) carry only one set of genes to ensure that offspring inherit one set from each parent.
This single set of genes in germ cells allows for independent inheritance of traits, as seen in Mendel's experiments where traits like round (R) and yellow (y) seeds are inherited separately, supporting the principle of independent assortment.

Explanation of Inheritance and Sex Determination

Chromosomes and Inheritance:

Chromosome Structure: Each gene set exists on separate pieces of DNA called chromosomes, not as one continuous thread. In sexually reproducing organisms, each cell has two copies of each chromosome—one from each parent.
Germ Cell Formation: During reproduction, germ cells (sperm and egg) take one chromosome from each pair, which can be either the mother’s or father’s chromosome. When these germ cells combine during fertilization, they restore the normal chromosome number in the offspring, ensuring species’ DNA stability.
This system of inheritance aligns with Mendel’s observations on the segregation and independent assortment of traits.

8.2.4 - Sex Determination

Variety in Sex Determination Mechanisms:
- Some species, like reptiles, depend on environmental factors (e.g., temperature) to determine the sex of offspring.
- Other species, such as snails, can change sex based on environmental conditions, meaning sex isn’t always genetically fixed.
Human Sex Determination:
- In humans, sex is genetically determined by the inheritance of sex chromosomes. Humans have 23 pairs of chromosomes: 22 pairs are similar, while the 23rd pair differs between sexes.
- Sex Chromosome Pairing:
  - Females: XX chromosome pair.
  - Males: XY chromosome pair, with the Y chromosome being shorter.
- Inheritance Pattern:
  - Every child inherits one X chromosome from their mother.
  - The father’s contribution determines the child’s sex:
    - An X chromosome from the father results in a girl (XX).
    - A Y chromosome from the father results in a boy (XY).

Thus, the sex of a human child depends on whether they inherit an X or Y chromosome from the father, explaining the 1:1 ratio of male to female offspring in humans.

NCERT Science Notes - Class 10 | Chapter 8 - Heredity

NCERT Science Notes - Class 9 | Chapter 12 - Improvement in Food Resources

NOTES

8.0 - Introduction