Heredity — Class 10 Science
"From your eye colour to your blood type — DNA from your parents writes the story of who you are."
1. About the Chapter
This chapter explores HEREDITY — how characteristics pass from parents to offspring through GENES.
2025-26 Note
The chapter previously also covered EVOLUTION, but the Evolution section was removed in the rationalised syllabus. Class 10 now focuses on heredity only. Evolution is covered later.
Topics
- Inherited vs acquired characteristics
- Mendel's experiments
- Dominant and recessive traits
- Genes and chromosomes
- Variations and mutations
- Sex determination in humans
2. Inherited Traits
Definition
Characteristics passed from PARENTS to OFFSPRING through GENES.
Examples
- Eye colour
- Hair colour and texture
- Blood type
- Height (genetic component)
- Skin tone
- Tongue rolling ability
- Earlobe attachment (attached vs free)
Acquired vs Inherited
- ACQUIRED: from environment/experience (not passed on)
- Examples: tan from sunbathing, scars, learning a language
- INHERITED: through genes (can be passed on)
- Examples: eye colour, blood type
Why It Matters
Only INHERITED traits can be passed to the NEXT GENERATION. Acquired traits cannot.
3. Genes and Chromosomes
Genes
- Basic units of inheritance
- Made of DNA
- Carry instructions for traits
- Each gene controls a specific trait (or part of one)
Chromosomes
- Long strands of DNA + protein
- Located in cell nucleus
- Humans have 23 PAIRS = 46 chromosomes
- One chromosome from EACH parent
Alleles
Different versions of the SAME gene.
- Each person has TWO alleles per gene (one from each parent)
- Example: Eye colour gene — brown allele or blue allele
4. Mendel's Experiments (Father of Genetics)
Who Was Mendel?
Gregor Mendel (1822-1884) — Austrian monk and scientist. Worked with PEA PLANTS in monastery garden.
His Work
Studied 7 traits in peas, including:
- Stem height (tall vs dwarf)
- Seed shape (round vs wrinkled)
- Seed colour (yellow vs green)
- Flower colour (purple vs white)
Key Discovery
Some traits are DOMINANT (shown when present), others are RECESSIVE (hidden if dominant present).
Mendel's Laws
Law of Dominance: When two contrasting traits are present, only the DOMINANT one is expressed.
Law of Segregation: When gametes form, the two alleles for a trait SEPARATE. Each gamete carries only ONE allele.
Law of Independent Assortment: Different traits are inherited INDEPENDENTLY.
Punnett Square
A diagram showing possible genetic combinations.
Example: Tall (T, dominant) crossed with dwarf (t, recessive).
TT × tt → All offspring Tt (TALL — dominant expressed)
Then if Tt × Tt:
- 25% TT (tall)
- 50% Tt (tall, but carry dwarf gene)
- 25% tt (dwarf — recessive shown!)
This gives the famous 3:1 ratio in F2 generation.
5. Inheritance of Traits (Detailed Example)
Brown vs Blue Eyes
- Brown (B) is dominant
- Blue (b) is recessive
Parents: Bb (brown-eyed carrier) × Bb (brown-eyed carrier)
Punnett Square:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Results:
- 25% BB (brown)
- 50% Bb (brown, carrier)
- 25% bb (blue!)
That's why two brown-eyed parents can have a blue-eyed child!
Blood Type Inheritance
Multiple alleles: A, B, O.
- A and B are co-dominant
- O is recessive
If parents are AB and OO:
- Children can be: AO (type A) or BO (type B)
6. Sex Determination in Humans
Chromosomes
- Humans have 23 pairs (46 total)
- 22 pairs of AUTOSOMES (same in both sexes)
- 1 pair of SEX CHROMOSOMES
Sex Chromosomes
- Females: XX
- Males: XY
How Sex is Determined
Mother gives: X (only X, both her chromosomes are X) Father gives: X or Y (he has one of each)
- If father gives X → child XX → FEMALE
- If father gives Y → child XY → MALE
Sex of the child is determined by the FATHER's chromosome.
(This is why blaming the mother for not having a boy is biologically wrong.)
Probability
50% chance of boy or girl — depends on which sperm fertilises the egg.
7. Worked Examples
Example 1: Inherited vs Acquired
Classify:
- Tan from sun — ACQUIRED
- Eye colour — INHERITED
- Knowledge of Hindi — ACQUIRED
- Blood type — INHERITED
Example 2: Mendel
A pure tall plant (TT) is crossed with pure dwarf (tt). What is F1 generation?
- All Tt — tall (because T is dominant)
Example 3: F2 generation
The F1 Tt is self-pollinated. What is F2?
- 25% TT (tall)
- 50% Tt (tall)
- 25% tt (dwarf)
- 3:1 ratio of tall:dwarf
Example 4: Sex Determination
Why is the father responsible for child's sex?
- Mother always gives X.
- Father gives X or Y.
- X from father → girl (XX)
- Y from father → boy (XY)
- So FATHER's chromosome determines sex.
8. Common Mistakes
-
Dominant = better
- WRONG. Dominant just means MORE LIKELY TO BE EXPRESSED. Not 'better'. Recessive traits aren't 'worse'.
-
All inherited traits visible
- WRONG. Recessive traits can be hidden as 'carriers' (Bb).
-
Mother determines child's sex
- WRONG. FATHER determines sex (his X or Y).
-
Chromosomes = genes
- Chromosomes CONTAIN many genes. Like a book vs words.
-
All cells have 46 chromosomes
- SOMATIC cells: 46. GAMETES (sperm/egg): 23 (half).
9. Indian Context
Indian Genetics Research
- Indian Statistical Institute
- National Centre for Cell Science
- Many genetic research institutes
Indian Famous Geneticists
- G.N. Ramachandran: protein structure (DNA related)
- Hargobind Khorana: Nobel 1968 (deciphered genetic code)
- Vidita Vaidya: brain genetics
Indian Genetic Diversity
- India has high genetic diversity due to history of migrations
- Many distinctive genetic groups
- Research helps with personalised medicine
10. Conclusion
Heredity is the FOUNDATION of biology:
- Genes carry inherited information
- Mendel discovered laws of inheritance
- Dominant/Recessive traits explain observations
- Sex chromosomes determine male/female (XX/XY)
- Father determines sex of child
Master:
- Mendel's laws
- Punnett squares (3:1 ratio)
- Dominant vs recessive
- Sex determination
- Inherited vs acquired
This is essential for Class 11-12 biology and ALL medical fields.
Genetics: the code of life. You are the latest chapter in 4 billion years of writing.
