Heredity — Class 10 Science
What CBSE examines here (2026-27). This is now a 6-page chapter with two in-text question sets (6 questions) and one end-of-chapter Exercise (4 questions, not 12 as some older manifests claim) — the Evolution half of the old "Heredity and Evolution" chapter is gone entirely (no natural selection, speciation, fossils, or human evolution anywhere in the current text). What's also gone, more surprisingly: the current chapter never uses the words "allele," "genotype," "phenotype," "Punnett square," "homozygous/heterozygous," or the names "Law of Dominance," "Law of Segregation," "Law of Independent Assortment." It teaches the same ideas, but entirely through Mendel's own pea-plant data (using notation like TT, Tt, tt directly) and plain narrative — no formal vocabulary layer. That vocabulary is accurate and standard elsewhere (Class 11, NEET), so it's kept in the appendix, but it isn't this chapter's own language.
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
- A section of DNA that carries the information for making one protein is called the gene for that protein
- Genes control traits by controlling the proteins (often enzymes) that produce them — e.g. an efficient enzyme making more of a height-promoting hormone means a taller plant
Chromosomes
- Each gene set is carried not as one long thread of DNA but as separate, independent pieces, each called a chromosome
- Each cell has two copies of every chromosome — one each from the male and female parent
- Each germ cell (sperm/egg) carries only one chromosome from each pair; combining two germ cells restores the normal two-copy number in the offspring
This chapter doesn't put a number on how many chromosome pairs humans have overall, or name the non-sex-chromosome pairs — it just says there are "22 such pairs" of ordinarily-matched chromosomes, plus one pair (the sex chromosomes) that doesn't always match. It also never uses the word "allele" — it just says each trait has "two copies" or "two versions" of the controlling gene.
4. Mendel's Experiments (Father of Genetics)
Who Was Mendel?
Gregor Mendel (1822-1884) — educated at a monastery, studied science and mathematics at the University of Vienna, then grew peas at his monastery. He was the first to actually count how many individuals showed each trait in every generation.
His Work
Crossed pea plants with contrasting characters — round/wrinkled seeds, tall/short plants, white/violet flowers, and more.
What the tall × short cross showed
Crossing a tall plant with a short plant gave an F1 generation that was all tall — no "medium-height" plants, so only one parental trait showed, not a blend. Self-pollinating those F1 tall plants gave an F2 generation where one quarter were short — so the shortness trait had been inherited all along in the F1 plants, just not expressed. This is why Mendel proposed that each plant carries two copies of the factor controlling a trait: TT and Tt are both tall, only tt is short. Traits like T (expressed even with one copy) are called dominant; traits like t (needing both copies) are called recessive.
What the two-trait cross showed
Crossing a tall, round-seeded plant (dominant traits) with a short, wrinkled-seeded plant gave an F1 that was all tall with round seeds. Self-pollinating that F1 gave an F2 with new combinations too — some tall/wrinkled and some short/round plants appeared, alongside the two parental combinations. Mendel's actual seed-shape/seed-colour cross recorded 556 F2 seeds: 315 round-yellow, 108 round-green, 101 wrinkled-yellow, 32 wrinkled-green — close to a 9:3:3:1 ratio. This showed that the two traits (shape and colour) are inherited independently of each other, because each germ cell takes just one chromosome from each pair, and the two traits sit on different chromosome pairs.
5. Sex Determination in Humans
Chromosomes
- The chapter states humans have "22 such pairs" of ordinarily-matched chromosomes (commonly called autosomes elsewhere, though not in this chapter), plus one further pair — the sex chromosomes — that isn't always a matching pair
- Together that's 23 pairs (46 chromosomes), though the chapter doesn't state that total explicitly
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.
6. 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.
7. 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 in a plant carrying one dominant and one recessive copy (Tt) — the recessive trait only reappears in later generations.
-
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).
8. 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
9. 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.
Appendix — beyond the current chapter
Not this chapter's own vocabulary, but standard, correct genetics terms used in more detailed treatments (Class 11, NEET prep) for the same ideas this chapter teaches.
| Term used elsewhere | What this chapter calls the same idea |
|---|---|
| Allele | A "copy" or "version" of a gene (e.g. T and t) |
| Genotype | The gene combination itself (e.g. Tt) |
| Phenotype | The trait actually shown (e.g. tall) |
| Homozygous / heterozygous | Having two identical copies (TT, tt) / two different copies (Tt) |
| Punnett square | The grid Mendel's crosses are shown in (Fig. 8.3, Fig. 8.5) — never named as such |
| Autosome | One of the "22 such pairs" of ordinarily-matched chromosomes |
| Law of Dominance | Why only the T trait shows in a Tt plant |
| Law of Segregation | Why each germ cell carries only one copy of each gene, not both |
| Law of Independent Assortment | Why seed shape and seed colour are inherited independently in the two-trait cross |
Also beyond this chapter (removed in rationalisation, not brought back): natural selection, speciation, fossil evidence, and human evolution — all previously part of a combined "Heredity and Evolution" chapter. This chapter is heredity only.
