Heredity Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 8 notes: heredity, Mendel's monohybrid and dihybrid crosses, dominant and recessive traits, genotype vs phenotype, and sex determination.
Heredity — Class 10 Science Notes
Chapter Snapshot
Heredity is the passing of traits (characteristics) from parents to offspring. This chapter explains how traits are inherited through Mendel's experiments with pea plants — dominant and recessive traits, the monohybrid cross (3:1) and dihybrid cross (9:3:3:1) — the difference between genotype and phenotype, and how sex is determined in humans.
Board relevance: almost always gives a cross (Punnett square) numerical and a sex-determination question. This is a scoring chapter if you master the crosses.
Syllabus note (rationalised): the Evolution portion (Darwin, natural selection, speciation, fossils, human evolution) has been removed. The current chapter is only about heredity.
Key Concepts & Definitions
Heredity — the transmission of characters from parents to offspring. Variation — differences among individuals of a species.
Gene — a unit of DNA on a chromosome that controls a particular trait. Genes occur in pairs; the two forms of a gene are called alleles (e.g. T for tall and t for short).
- Dominant allele/trait — expressed even when present in a single copy (written in capital, e.g. T).
- Recessive allele/trait — expressed only when both alleles are recessive (small letter, e.g. t).
- Homozygous — both alleles identical (TT or tt). Heterozygous — two different alleles (Tt).
- Genotype — the genetic makeup (TT, Tt, tt). Phenotype — the visible characteristic (tall, short).
Gregor Mendel — the "father of genetics"; he experimented on garden pea (Pisum sativum) plants, studying contrasting traits (tall/short, round/wrinkled seeds).
Mendel's Monohybrid Cross
A monohybrid cross studies one pair of contrasting traits. Cross a pure tall (TT) with a pure short (tt) pea plant:
F1 generation: all Tt → all tall (the recessive short trait is hidden).
F2 generation (self-pollinate F1, Tt × Tt): use a Punnett square —
T t
T TT Tt
t Tt tt
- Genotypes: 1 TT : 2 Tt : 1 tt (ratio 1:2:1).
- Phenotypes: 3 tall : 1 short (ratio 3:1).
This shows that traits are controlled by pairs of alleles, one inherited from each parent, and that a recessive trait can reappear in the F2 generation.
Law of segregation: the two alleles of a trait separate (segregate) during the formation of gametes, so each gamete carries only one allele.
Mendel's Dihybrid Cross
A dihybrid cross studies two pairs of traits together — e.g. seed shape (round R dominant / wrinkled r) and colour (yellow Y dominant / green y).
- Cross RRYY (round yellow) × rryy (wrinkled green). F1: all RrYy (round yellow).
- F2 phenotypic ratio: 9 : 3 : 3 : 1 — 9 round-yellow, 3 round-green, 3 wrinkled-yellow, 1 wrinkled-green.
New combinations (round-green, wrinkled-yellow) appear, showing that the two traits are inherited independently.
Law of independent assortment: the alleles of different traits are inherited independently of one another.
Sex Determination in Humans
Humans have 23 pairs of chromosomes, including one pair of sex chromosomes:
- Females: XX • Males: XY
- All eggs carry an X chromosome.
- Sperm carry either an X or a Y chromosome.
- If an X-sperm fertilises the egg → XX → girl; if a Y-sperm → XY → boy.
So the father's sperm determines the sex of the child — the mother always contributes an X. There is roughly a 50:50 chance of a boy or girl.
Acquired vs Inherited Traits
- Inherited traits — carried in the genes and passed to offspring (e.g. eye colour, free/attached earlobes).
- Acquired traits — developed during a lifetime and not passed on, because they affect only body (somatic) cells, not the reproductive (germ) cells (e.g. muscles from exercise, a cut finger, learned skills).
This is why traits an organism gains in its lifetime do not change the DNA of its offspring. For example, if a group of beetles is repeatedly cut, their offspring are still born with full-sized bodies — the injury never enters the germ cells, so it cannot be inherited. Only changes in the genes (DNA) of the reproductive cells can be passed to the next generation, and it is these inherited variations that heredity is concerned with.
Key Facts
Quick facts boards ask directly:
Topic Fact to remember
Father of genetics Gregor Mendel
Plant Mendel used Garden pea (Pisum sativum)
Dominant / recessive symbol Capital (T) / small (t) letter
F1 of TT × tt All tall (Tt)
Monohybrid F2 phenotype 3 : 1
Monohybrid F2 genotype 1 : 2 : 1
Dihybrid F2 phenotype 9 : 3 : 3 : 1
Law: alleles separate in gametes Law of segregation
Law: traits inherited independently Independent assortment
Female sex chromosomes XX
Male sex chromosomes XY
Sex of child decided by Father (X or Y sperm)
Human chromosome pairs 23
Acquired traits NOT inherited
Two definitions to quote: Genotype — the genetic constitution (allele combination) of an organism. Phenotype — the observable characteristics resulting from that genotype.
Important Question Patterns
1. Monohybrid cross (3 marks): work a TT × tt cross to F2 with a Punnett square; give genotypic (1:2:1) and phenotypic (3:1) ratios.
2. Dihybrid cross (2–3 marks): state the 9:3:3:1 ratio; explain what "new combinations" prove (independent assortment).
3. Genotype vs phenotype (2 marks): distinguish them; why Tt and TT look the same; dominant vs recessive.
4. Sex determination (2–3 marks): explain with a chromosome cross that the father determines the child's sex; XX vs XY.
5. Acquired vs inherited (2 marks): why acquired traits are not inherited; classify given examples.
⚡ Quick Revision
- Heredity = traits passed parent → offspring, via genes (alleles: dominant capital, recessive small).
- Genotype = allele makeup (TT/Tt/tt); phenotype = appearance (tall/short). Homozygous (TT, tt) vs heterozygous (Tt).
- Monohybrid (Tt × Tt): phenotype 3:1, genotype 1:2:1. F1 of TT × tt is all tall (Tt).
- Dihybrid F2: phenotype 9:3:3:1 → traits inherited independently.
- Laws: segregation (alleles separate into gametes); independent assortment (different traits inherited independently).
- Sex determination: female XX, male XY; egg always X, sperm X or Y → father determines sex.
- Acquired traits are NOT inherited (they don't reach the germ cells).
- Rationalised syllabus: evolution removed — heredity only.
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