Chapter 8: HEREDITY
1. Introduction
Heredity is the process through which parents pass on their traits or characteristics to their offspring. These traits can be physical, like eye color or height, or functional, like blood group or ability to roll the tongue.
While we inherit most features from our parents, we are not their exact copies because of variation — small differences in genes that make each person unique.
2. Basic Terms Related to Heredity
Term | Meaning |
Trait | Any characteristic or feature of an organism (e.g. tallness, eye color). |
Inherited Traits | Traits that are passed from parents to offspring through genes. |
Variation | The differences seen among individuals of the same species. |
Gene | A small unit of DNA that controls a specific trait. |
Chromosome | Thread-like structures in the nucleus made up of DNA; they carry genes. |
DNA (Deoxyribonucleic acid) | The molecule that carries genetic information. |
Alleles | Different forms of the same gene (for example, tall or short height). |
Genotype | The genetic makeup of an organism (TT, Tt, or tt). |
Phenotype | The observable trait (tall or short). |
Dominant trait | Trait that appears even when only one copy of the gene is present (e.g., tallness). |
Recessive trait | Trait that appears only when both copies of the gene are the same (e.g., shortness). |
3. Mendel’s Experiments
The father of genetics, Gregor Johann Mendel, performed experiments on pea plants (Pisum sativum) to study how traits are inherited.
Why Pea Plant?
Mendel chose pea plants because:
- They have clearly visible contrasting traits (like tall/short, round/wrinkled seeds).
- They can be easily self- or cross-pollinated.
- They grow and reproduce quickly.
4. Mendel’s Monohybrid Cross
Monohybrid cross means studying the inheritance of a single trait.
Example:
He crossed a pure tall (TT) pea plant with a pure short (tt) pea plant.
Parent generation (P):
- Tall (TT) × Short (tt)
F₁ generation:
- All plants were Tall (Tt) — the tall trait dominated.
F₂ generation (self-pollination of F₁):
- Ratio of Tall : Short = 3 : 1
Conclusion:
- Each trait is controlled by a pair of factors (genes).
- One factor can mask (dominate) the other — this is the Law of Dominance.
5. Mendel’s Dihybrid Cross
Dihybrid cross studies the inheritance of two traits at the same time.
Example: Seed color (yellow/green) and seed shape (round/wrinkled).
Parents:
- Round Yellow (RRYY) × Wrinkled Green (rryy)
F₁ generation: All Round Yellow (RrYy)
F₂ generation: Shows 4 combinations —
Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green = 9 : 3 : 3 : 1
Conclusion:
Traits are independently inherited — this is the Law of Independent Assortment.
6. Mendel’s Laws of Inheritance
- Law of Dominance
- One factor in a pair may dominate the other.
- Example: T (tall) dominates over t (short).
- Law of Segregation
- The two factors (genes) separate during gamete formation, so each gamete carries only one.
- Law of Independent Assortment
Traits of different characters are inherited independently.
7. Expression of Traits – How Genes Work
Traits are controlled by genes present on chromosomes, made of DNA.
- A segment of DNA acts as instructions for making a specific protein.
- These proteins control the expression of a trait.
For example:
A gene for tallness controls the production of a growth-related protein — if the gene is functional → tall plant; if defective → short plant.
8. How Do Traits Get Expressed?
- Genes are part of DNA that carry information for protein synthesis.
- Proteins decide how the cell functions and hence determine the trait.
- The environment can also influence how a trait appears.
Example: Identical plants may grow differently in poor soil.
9. Inheritance of Traits in Humans
Examples of inherited traits:
- Free or attached earlobes
- Tongue rolling ability
- Widow’s peak hairline
- Dimple presence
Each trait depends on two genes — one from each parent.
10. Sex Determination in Humans
Humans have 23 pairs of chromosomes (46 total).
- 22 pairs are autosomes (non-sex chromosomes).
- 1 pair is sex chromosomes – X and Y.
Males → XY
Females → XX
- The mother always contributes X
- The father contributes either X or Y.
If:
- X (father) + X (mother) → Girl (XX)
- Y (father) + X (mother) → Boy (XY)
Therefore, the father determines the sex of the child, not the mother.
11. Difference Between Inherited and Acquired Traits
Inherited Traits | Acquired Traits |
Passed from parents through genes. | Developed during lifetime due to environment or practice. |
Found in DNA. | Not found in DNA. |
Example: Eye color, blood group. | Example: Muscle growth by exercise, speaking English. |
Can be passed to next generation. | Cannot be inherited. |
12. Importance of Heredity
- Maintains continuity of species.
- Explains how traits pass from one generation to another.
- Helps in studying diseases that run in families.
- Useful in agriculture and animal breeding (to improve varieties).
13. Key Terms Summary
Term | Definition |
Heredity | Transfer of traits from parents to offspring. |
Gene | Unit of heredity controlling a trait. |
Chromosome | Thread-like DNA structure carrying genes. |
DNA | Molecule carrying genetic information. |
Dominant gene | Gene that masks another gene. |
Recessive gene | Gene that is masked by dominant gene. |
Alleles | Two alternate forms of a gene. |
Phenotype | Observable characteristic. |
Genotype | Genetic constitution of an organism. |
Variation | Differences among individuals. |
14. Summary of Mendel’s Observations
Cross | Traits Studied | F₁ Result | F₂ Ratio | Law Involved |
Monohybrid | Tall/Short | All Tall | 3:1 | Law of Dominance |
Dihybrid | Round/Yellow | All Round Yellow | 9:3:3:1 | Law of Independent Assortment |
15. Quick Recap
- Heredity explains why children resemble their parents.
- Genes control traits; they exist in pairs.
- One gene can dominate (dominant/recessive).
- Mendel discovered the basic laws of inheritance.
- Variations are essential for evolution.
- Father’s sperm determines the baby’s gender.
- Inherited traits differ from acquired ones.
16. Important Questions for Revision
- What is heredity?
- Who is known as the father of genetics?
- What are Mendel’s laws of inheritance?
- Explain the difference between dominant and recessive traits.
- How is the sex of a child determined in humans?
- What is the difference between inherited and acquired traits?
- Why is variation important in living organisms?
- What is a dihybrid cross? What ratio is obtained?
✅ In short:
Heredity is nature’s way of ensuring continuity, while still allowing enough variation to keep life adaptable and diverse.