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Heredity — Class 10 Notes

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Biology · Ch 8Class 10 · CBSE 2026–27Medium weightage

Heredity

What the board actually asks: Mendel's monohybrid and dihybrid pea-plant crosses (dominant vs recessive traits, the 3:1 and 9:3:3:1 F2 ratios), how genes carried on chromosomes are inherited equally from both parents and expressed, and sex determination in human beings (XX/XY) - examined through Punnett-square crosses, 'is the trait dominant or recessive?' reasoning, and 'how is the sex of the child determined?' questions.

NCERT VERBATIM sourced, word-for-wordAUTHORED LazyTopper teaching scaffolding
The Big Idea AUTHORED

Traits pass from parents to offspring by predictable rules - Mendel worked them out with pea plants.

A child carries the basic body design of its parents yet is never identical to them, because each parent passes on one copy of every gene. Mendel's crosses with garden peas revealed the rules of this inheritance: some traits are dominant and hide a recessive partner, each trait is inherited independently of the others, and the F2 generation reappears in fixed proportions - 3:1 for one trait and 9:3:3:1 for two. The same 'two copies of every gene, carried on chromosomes' logic also explains how the sex of a human child is fixed by the X or Y chromosome it inherits from its father.

Formulae
Monohybrid ratio
= 3 tall : 1 short (the plants you see are in the ratio 3 : 1; the gene pairs are in the ratio 1 : 2 : 1)
Tip: 3 tall : 1 short; 1 TT : 2 Tt : 1 tt.
Dihybrid ratio
9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green (9:3:3:1)
Tip: Order: round-yellow 9, round-green 3, wrinkled-yellow 3, wrinkled-green 1.
Human sex chromosomes
Female = (all eggs X) · Male = (sperm X or Y) the sex is decided by whether the sperm carries X or Y
Tip: The sex is decided by the X or Y chromosome in the sperm; it is chance, not choice.
Gene copies
Two copies of every gene per body cell (one from each parent); each germ cell carries only one copy
Inherited Traits
A child gets DNA from both parents, so each trait has two versions, for example free or attached earlobes.
Mendel's Experiments with Garden Peas
Mendel worked with garden pea plants and contrasting visible traits: round or wrinkled seeds, tall or short plants, white or violet flowers.
Dominant and Recessive Traits
A trait that shows in the first generation (F₁) is dominant; the trait that is hidden is recessive. Using T for tall and t for short: TT and Tt plants are tall, and only tt is short.
Monohybrid Cross: F₁ and F₂ Step by Step
Tall (TT) × short (tt): all F₁ plants are Tt and tall. Self-pollinating F₁ gives F₂ with TT, Tt, Tt and tt: three tall plants to one short plant.
Independent Inheritance of Two Traits
Cross a tall plant with round seeds with a short plant with wrinkled seeds: all F₁ plants are tall with round seeds. In F₂ there are also new combinations: tall plants with wrinkled seeds and short plants with round seeds. This shows that height and seed shape are inherited independently.
Human Chromosomes: 22 Pairs + Sex Chromosomes
There are 22 pairs of ordinary chromosomes and one pair of sex chromosomes: XX in females and XY in males. Every child gets an X from the mother. Each child has a 50% chance of being a boy and a 50% chance of being a girl.

Shortcuts & memory tricks →

Definitions the board rewards NCERT VERBATIM

State these in NCERT’s exact words

Heredity
NCERT verbatim — memorise this exact wording· Ch 8,
“The rules of heredity determine the process by which traits and characteristics are reliably inherited.”
What it means: The set of rules by which characteristics pass reliably from parents to their offspring.
Rules for the inheritance of traits
NCERT verbatim — memorise this exact wording· Ch 8,
“The rules for inheritance of such traits in human beings are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child.”
What it means: Because the father and mother each give an equal share of genetic material, every trait comes in two versions - one from each parent.
Gene
NCERT verbatim — memorise this exact wording· Ch 8,
“A section of DNA that provides information for one protein is called the gene for that protein.”
What it means: A stretch of DNA that carries the instructions for making one particular protein.
Dominant and recessive traits
NCERT verbatim — memorise this exact wording· Ch 8,
“Traits like ‘T’ are called dominant traits, while those that behave like ‘t’ are called recessive traits.”
What it means: A dominant trait shows up even when only one copy is present; a recessive trait appears only when both copies are the recessive form.
Genes control characteristics
NCERT verbatim — memorise this exact wording· Ch 8,
“Thus, genes control characteristics, or traits.”
What it means: Genes work by making proteins (such as enzymes), and those proteins decide the visible traits of the organism.
Chromosome
NCERT verbatim — memorise this exact wording· Ch 8,
“This is explained by the fact that each gene set is present, not as a single long thread of DNA, but as separate independent pieces, each called a chromosome.”
What it means: Each set of genes is carried as separate thread-like pieces called chromosomes; body cells have them in pairs, one of each pair coming from each parent.
Sex determination in humans
NCERT verbatim — memorise this exact wording· Ch 8,
“Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.”
What it means: Human females carry two X chromosomes; males carry one X and one Y - this difference decides the sex of a child.
Key terms — NCERT verbatim (state word-for-word)
Genes (Mendel's factors)
This led Mendel to propose that two copies of factor (now called genes) controlling traits are present in sexually reproducing organism. (Ch 8, )
Independent inheritance of traits
Thus, the tall/short trait and the round seed/wrinkled seed trait are independently inherited. (Ch 8, )
Germ cell - one gene set
For this mechanism to work, each germ cell must have only one gene set. (Ch 8, )
Restoring the chromosome number
When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species. (Ch 8, )
Must-know concepts AUTHORED FRAMING

…and where they’re tested

Inherited traits and variation
A child shares the basic body design of its parents but is never identical to them, and sexually reproducing populations show a great deal of variation. This is because each individual receives one of the two copies of every gene from each parent. Working out which traits reappear in the offspring, and in what proportion, is how the rules of inheritance were discovered.
NCERT Fig 8.2Free and attached earlobes
(a) A free earlobe and (b) an attached earlobe - the lowest part of the ear is closely attached to the side of the head in some people and hangs free in others. Free and attached earlobes are two inherited variants found in human populations.
why offspring resemble yet differ from their parents; free vs attached earlobes as an inherited human trait.
Mendel's monohybrid cross - dominant, recessive and 3:1
Crossing a pure tall pea plant (TT) with a short one (tt) gives an F1 that is entirely tall (Tt) - only the dominant trait is seen. Self-pollinating the F1 gives an F2 in which tall and short reappear in a 3:1 phenotype ratio (genotypes TT:Tt:tt = 1:2:1), proving the recessive 'short' factor was hidden in the F1, not lost.
NCERT Fig 8.3Inheritance of traits over two generations (monohybrid cross)
Top row (F1): Tall (TT) x Short (tt) gives all tall offspring (Tt). Bottom row (F2): F1 x F1 (Tt x Tt) gives Tall (TT), Tall (Tt), Tall (Tt) and Short (tt) - a 3 tall : 1 short ratio.
3-mark: a monohybrid cross showing a dominant trait masking a recessive one; the 3:1 F2 phenotype (1:2:1 genotype) ratio.
Dihybrid cross and independent inheritance - 9:3:3:1
When two traits are followed together - seed shape (round/wrinkled) and seed colour (yellow/green) - the F1 shows both dominant traits (round, yellow), and the F2 gives four phenotypes in a 9:3:3:1 ratio. New combinations such as round-yellow and wrinkled-green appear because the two traits are inherited independently of each other.
NCERT Fig 8.5Independent inheritance of two separate traits, shape and colour of seeds
RR yy (round, green) x rr YY (wrinkled, yellow) gives an F1 that is all Rr Yy (round, yellow); the F2 Punnett square gives 315 round-yellow (9) : 108 round-green (3) : 101 wrinkled-yellow (3) : 32 wrinkled-green (1) out of 556 seeds - a 9:3:3:1 ratio.
how two traits are inherited independently; the 9:3:3:1 F2 dihybrid ratio; the appearance of new trait combinations in the F2.
How traits are expressed - genes, proteins and chromosomes
A gene is a section of DNA that codes for one protein; proteins (for example an enzyme that controls a growth hormone) determine a characteristic such as plant height. Because every organism carries two copies of each gene - one from each parent - packed on paired chromosomes, and each germ cell carries only one copy, both parents contribute equally to every trait, and fertilisation restores the paired chromosome number.
genes control characteristics; the role of chromosomes; how the equal genetic contribution of both parents is ensured in the progeny.
Sex determination in human beings (XX / XY)
Of the 23 chromosome pairs in a human cell, 22 are matched; the 23rd is the pair of sex chromosomes. Females are XX and males are XY. Every egg carries an X, while sperm carry either X or Y - so a child's sex depends on the father: an X-bearing sperm gives a girl (XX), a Y-bearing sperm gives a boy (XY).
NCERT Fig 8.6Sex determination in human beings
Male (XY) and Female (XX). Gametes: the mother gives only X eggs, the father gives X or Y sperm; the resulting zygotes are XX (a girl) or XY (a boy) - so the father's chromosome decides the sex.
3/5-mark: how the sex of a child is determined in human beings; why the father's chromosome decides the sex.
Worked examples — real solved problems NCERT VERBATIM

Solved exactly as the source does them

Every problem is a real sourced item, quoted verbatim with its page; the step-by-step decoding and mark-logic are LazyTopper-authored coaching.

Example Fig. 8.3 walkthrough,
“[authored prompt] Trace a monohybrid cross: a pure tall pea plant (TT) crossed with a short one (tt), through the F1 and F2 generations.”
Parental cross (P).Cross a homozygous tall plant (TT) with a homozygous short plant (tt). Each parent passes on one factor, so every gamete carries either T or t.
1 mark
F1 generation.All F1 plants are Tt and are tall, because a single copy of the dominant 'T' is enough to make the plant tall - the recessive 't' is present but not expressed.
1 mark
F2 generation.Self-pollinating the F1 (Tt x Tt) gives genotypes TT : Tt : tt in a 1:2:1 ratio; since TT and Tt are both tall and only tt is short, the phenotype ratio is 3 tall : 1 short.
1 mark
Mark-scheme logic (authored): CBSE 3-mark - 1 mark for the parental genotypes and gametes, 1 for the all-tall Tt F1 (dominant masking recessive), and 1 for the F2 3:1 phenotype (1:2:1 genotype) ratio.
Example Fig. 8.5 walkthrough,
“[authored prompt] Trace a dihybrid cross between a round-green-seeded plant (RRyy) and a wrinkled-yellow-seeded plant (rrYY) to the F2 generation.”
Example Section 8.2.3 walkthrough,
“[authored prompt] Explain how a gene decides whether a pea plant is tall or short.”
Example Fig. 8.6 walkthrough,
“[authored prompt] Trace how the sex of a human child is determined, using the parents' sex chromosomes.”
Problem-solving strategies AUTHORED
Cross question?Write the parents' genotypes and their gametes first, then combine them in a Punnett square - marks are given for the gametes and the F2 ratio, not just the final answer.
'Dominant or recessive?' asked?The trait that appears in the F1 (and in three-quarters of the F2) is dominant; the one that disappears in the F1 and returns in one-quarter of the F2 is recessive.
Ratio to remember?One trait gives 3:1 in the F2; two traits give 9:3:3:1. Reaching these ratios is what shows the cross has been done correctly.
Sex-determination question?State that the mother is XX (all eggs carry X) and the father is XY (sperm carry X or Y), so it is the father's chromosome that fixes the child's sex - a common 3-mark point.
Common mistakes that cost marks

Where students lose easy marks

Writing that the F1 is a 'mixture' or a 'medium-height' plant. In a monohybrid cross the F1 shows only the dominant trait - there are no blended halfway forms.
Confusing genotype with phenotype: the F2 genotype ratio is 1:2:1 (TT:Tt:tt) but the phenotype ratio is 3:1 (tall:short). Quoting 3:1 as the genotype ratio loses the mark.
Saying the mother decides the child's sex. The egg always carries X; it is the father's X- or Y-bearing sperm that determines the sex.
Forgetting that a recessive trait is hidden, not lost, in the F1 - it reappears in the F2, which is the whole point of Mendel's experiment.
Formulae
Monohybrid ratio
= 3 tall : 1 short (the plants you see are in the ratio 3 : 1; the gene pairs are in the ratio 1 : 2 : 1)
Tip: 3 tall : 1 short; 1 TT : 2 Tt : 1 tt.
Dihybrid ratio
9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green (9:3:3:1)
Tip: Order: round-yellow 9, round-green 3, wrinkled-yellow 3, wrinkled-green 1.
Human sex chromosomes
Female = (all eggs X) · Male = (sperm X or Y) the sex is decided by whether the sperm carries X or Y
Tip: The sex is decided by the X or Y chromosome in the sperm; it is chance, not choice.
Gene copies
Two copies of every gene per body cell (one from each parent); each germ cell carries only one copy
Inherited Traits
A child gets DNA from both parents, so each trait has two versions, for example free or attached earlobes.
Mendel's Experiments with Garden Peas
Mendel worked with garden pea plants and contrasting visible traits: round or wrinkled seeds, tall or short plants, white or violet flowers.
Dominant and Recessive Traits
A trait that shows in the first generation (F₁) is dominant; the trait that is hidden is recessive. Using T for tall and t for short: TT and Tt plants are tall, and only tt is short.
Monohybrid Cross: F₁ and F₂ Step by Step
Tall (TT) × short (tt): all F₁ plants are Tt and tall. Self-pollinating F₁ gives F₂ with TT, Tt, Tt and tt: three tall plants to one short plant.
Independent Inheritance of Two Traits
Cross a tall plant with round seeds with a short plant with wrinkled seeds: all F₁ plants are tall with round seeds. In F₂ there are also new combinations: tall plants with wrinkled seeds and short plants with round seeds. This shows that height and seed shape are inherited independently.
Human Chromosomes: 22 Pairs + Sex Chromosomes
There are 22 pairs of ordinary chromosomes and one pair of sex chromosomes: XX in females and XY in males. Every child gets an X from the mother. Each child has a 50% chance of being a boy and a 50% chance of being a girl.
Shortcuts & Tricks
Capital letter = dominant; small = recessive; a recessive trait shows only with both copies small.
MCQ / check only: don't write this as working
Monohybrid F₂: 3:1 and 1:2:1. Dihybrid F₂: 9:3:3:1.
MCQ / check only: don't write this as working
Pure dominant × pure recessive: F₁ is 100% dominant-looking.
MCQ / check only: don't write this as working
Every egg carries X; the sperm carries X or Y. The X or Y in the sperm decides the sex: chance, not choice.
MCQ / check only: don't write this as working
Write parents' genes, gametes, then F₁ and F₂ in a Punnett square, with ratios.
Safe in written answers
Memory Tricks
Bada T chalta hai (dominant), chhota t chhupa rehta hai (recessive).
T dominant; t recessive; TT, Tt tall; tt short.
Teen–ek dikhta hai; ek–do–ek andar ka asli.
Monohybrid F₂. 3 tall : 1 short visible; 1 TT : 2 Tt : 1 tt genes.
Nau, teen, teen, ek: Gol-peela, Gol-hara, Jhurri-peela, Jhurri-hara.
Dihybrid F₂ order. 9 round-yellow · 3 round-green · 3 wrinkled-yellow · 1 wrinkled-green.
F₁ mein sab Lambe: ek trait dikha, dusra chhupa.
F₁ result. TT × tt → all tall (tallness dominant).
Maa sirf X deti hai; shukranu ka X ya Y tay karta hai (chance, choice nahi): X → beti, Y → beta.
Sex determination. XX female (all eggs X); XY male; sperm with X → girl, sperm with Y → boy.
22 jodi + ek jodi sex-chromosome (X/Y) ki.
Human chromosomes. 22 pairs of ordinary chromosomes plus one pair of sex chromosomes (XX or XY).
Mendel ka Matar: Gol–Jhurri, Lamba–Chhota, Safed–Baingani.
Contrasting traits. round/wrinkled seeds; tall/short plants; white/violet flowers.
Har gene ki do copy: ek Maa se, ek Papa se; germ cell mein sirf ek.
two copies per body cell; one per germ cell.
Naye jodi: Lamba-Jhurri, Chhota-Gol.
Independent inheritance. tall-wrinkled and short-round appear in F₂.
Mindmap
      • A child shares its parents' body design but is not identical
      • Both parents contribute practically equal genetic material
      • Two copies (versions) of every gene per trait - one from each parent
      • Free vs attached earlobes - an inherited human trait
      • Tall (TT) short (tt) all tall (Tt)
      • Dominant trait masks the recessive one
      • : 3 tall : 1 short (genotype 1:2:1)
      • Recessive factor hidden in , reappears in
      • Round-green (RRyy) x wrinkled-yellow (rrYY)
      • all round and yellow (Rr Yy)
      • : 9:3:3:1 - four phenotypes
      • New combinations traits inherited independently
      • Gene = a section of DNA coding for one protein
      • Proteins/enzymes control characteristics (e.g. plant height)
      • Genes carried on paired chromosomes
      • Each germ cell carries one copy; fertilisation restores the pair
      • 22 matched pairs + 1 sex-chromosome pair
      • Female XX (all X eggs), Male XY (X or Y sperm)
      • Father's chromosome decides sex: X girl, Y boy
      • Don't say the mother decides the sex

The diagrams you must be able to read and reproduce for Heredity - each Mendelian cross and the human sex-determination cross. Learn the genotypes and the F2 ratios in the exact NCERT form.

NCERT Fig 8.3Inheritance of traits over two generations (monohybrid cross)
Top row (F1): Tall (TT) x Short (tt) gives all tall offspring (Tt). Bottom row (F2): F1 x F1 (Tt x Tt) gives Tall (TT), Tall (Tt), Tall (Tt) and Short (tt) - a 3 tall : 1 short ratio.
NCERT Fig 8.5Independent inheritance of two separate traits, shape and colour of seeds
RR yy (round, green) x rr YY (wrinkled, yellow) gives an F1 that is all Rr Yy (round, yellow); the F2 Punnett square gives 315 round-yellow (9) : 108 round-green (3) : 101 wrinkled-yellow (3) : 32 wrinkled-green (1) out of 556 seeds - a 9:3:3:1 ratio.
NCERT Fig 8.6Sex determination in human beings
Male (XY) and Female (XX). Gametes: the mother gives only X eggs, the father gives X or Y sperm; the resulting zygotes are XX (a girl) or XY (a boy) - so the father's chromosome decides the sex.
NCERT Fig 8.2Free and attached earlobes
(a) A free earlobe and (b) an attached earlobe - the lowest part of the ear is closely attached to the side of the head in some people and hangs free in others. Free and attached earlobes are two inherited variants found in human populations.

3 competency questions from the CBSE board pattern, with the full step-marked solution for each — the marks are shown per step, as the official scheme awards them.

4 marksSection E

Ram and Asha were a married couple with four children — one had sickle cell anaemia; others showed no symptoms. Ram and Asha showed no symptoms. The trait is not sex-linked. (a) Is the disease caused by a dominant or recessive trait? Why? (b) If the affected child married a person without a mutation in the sickle cell gene, what percentage of their children would have sickle cell anaemia? Show the cross. OR (b) Identify the genetic composition of the sickle cell trait in Asha and Ram and use that to predict the genetic composition in the other four children who did not show symptoms.

Step-marked solution
  1. [1 mark] (a) The disease is caused by a recessive trait — both parents are unaffected yet a child is affected, so the trait appears only when both recessive alleles come together.
  2. [1 mark] (a) Therefore both Ram and Asha must be heterozygous carriers (Ss); the trait skips generations, which is characteristic of recessive inheritance.
  3. [1 mark] (b) Affected child (ss) × person with no mutation (SS): the cross ss × SS gives all offspring Ss ⟹ 0% affected, 100% carriers.
  4. [1 mark] (b OR) Ram (Ss) × Asha (Ss) → 1 SS : 2 Ss : 1 ss; the three unaffected children are either SS or Ss, while the affected child is ss.
2 marksSection B

A species of mammal has two phenotypes, white fur and brown fur. Table 1 shows the parent genotypes: WW = white fur, Wb = white fur, bb = brown fur. Draw a Punnett square to show how an individual with white fur and an individual with brown fur produce a generation, F1, all with white fur.

Figure from the question paper for: A species of mammal has two phenotypes, white fur and brown fur. Table 1 shows the parent
Step-marked solution
  1. [1 mark] Select the parents bb (brown) and WW (white) so that all F1 are white.
  2. [1 mark] The Punnett square gives four Wb crosses — all offspring are Wb (white fur).
3 marksSection C

A plant breeder crosses a PA PN plant with a PA PA plant. Predict, using a genetic diagram, the proportion of pure breeding plants in the offspring.

Step-marked solution
  1. [1 mark] Set up the genetic diagram with the gametes separated into column and row headings: parent 1 gametes PA and PN; parent 2 gametes PA and PA.
  2. [1 mark] Complete the four genotype crosses in the grid: PA PA, PA PA, PN PA, PN PA.
  3. [1 mark] Conclude that 50% of the offspring are PA PA, i.e. half are pure breeding.
Practise Heredity competency questions

Read NCERT Chapter 8 on LazyTopper (jesc108)

Official NCERT chapter PDF (jesc108) — ncert.nic.in

Sources: NCERT 2026-27 (tap to view)
Source ledger PROVENANCE

Source edition: NCERT Reprint 2026-27, Class 10 Science, Ch 8. One row per sourced item — every verbatim, example and NCERT figure traces to its page.

ItemTypeSource
HeredityverbatimNCERT 2026-27 Ch8
Rules for the inheritance of traitsverbatimNCERT 2026-27 Ch8
GeneverbatimNCERT 2026-27 Ch8
Dominant and recessive traitsverbatimNCERT 2026-27 Ch8
Genes control characteristicsverbatimNCERT 2026-27 Ch8
ChromosomeverbatimNCERT 2026-27 Ch8
Sex determination in humansverbatimNCERT 2026-27 Ch8
Genes (Mendel's factors)verbatimNCERT 2026-27 Ch8
Independent inheritance of traitsverbatimNCERT 2026-27 Ch8
Germ cell - one gene setverbatimNCERT 2026-27 Ch8
Restoring the chromosome numberverbatimNCERT 2026-27 Ch8
Monohybrid cross - tall x short pea plantsexampleNCERT 2026-27 Ch8 Fig 8.3
Dihybrid cross - seed shape and colour (9:3:3:1)exampleNCERT 2026-27 Ch8 Fig 8.5
How a gene decides tall vs shortexampleNCERT 2026-27 Ch8 Sec 8.2.3
Sex determination in human beingsexampleNCERT 2026-27 Ch8 Fig 8.6
NCERT Fig 8.2figureNCERT 2026-27 Ch8 8.2
NCERT Fig 8.3figureNCERT 2026-27 Ch8 8.3
NCERT Fig 8.5figureNCERT 2026-27 Ch8 8.5
NCERT Fig 8.6figureNCERT 2026-27 Ch8 8.6

Heredity · spec-rendered note · NCERT Reprint 2026-27, Class 10 Science, Ch 8

Practise writing answers the way examiners mark them → Guide