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Class 10 Heredity notes: NCERT Science Chapter 8 revision

CMDLearn · 3 November 2026

These are short revision notes for Chapter 8 of the NCERT Class 10 Science textbook (Reprint 2026-27). The chapter is six printed pages long (pages 128 to 133), and almost every line of it can turn into a board question. The notes follow the book's sections, give the key words as a list, describe one diagram you should be able to draw, and end with the mistakes students make most often.

Old name, new name. Older books and many websites call this chapter "Heredity and Evolution". In the current book it is just "Heredity": it has no section on evolution, fossils or speciation. If a set of notes teaches those, it was written for the old book.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts Heredity in Unit II, World of Living, worth 25 of the 80 theory marks, together with Life Processes, Control and Coordination, and How do Organisms Reproduce. CBSE gives marks per unit, not per chapter, so no one can honestly tell you an exact mark count for Heredity alone. For this chapter the unit lists Mendel's contribution and the laws of inheritance, with a brief introduction to sex determination. More on the units: CBSE Class 10 Science syllabus 2026-27.

1. Variation builds up over generations (section 8.1)

• Each new generation gets the basic body design of its parents, plus small changes of its own.
• Those changes are passed on, so the differences pile up generation after generation (Fig. 8.1 shows one organism giving two, then four, each a little different).
• In asexual reproduction the only source of change is small errors when DNA is copied, so the offspring are very alike. The book's example: bacteria dividing twice give four almost identical bacteria.
• The book contrasts a sugarcane field, where the plants differ very little, with humans and other animals that reproduce sexually, where the differences are easy to see.
• Sexual reproduction produces much more variety.
• Not every variant survives equally well. The environment picks out some variants (a bacterium that tolerates heat does better in a heat wave), and that selection is the basis of evolution.

2. Inherited traits and Mendel's single-trait cross (sections 8.2.1 and 8.2.2)

• A trait is a feature such as free or attached earlobes (Fig. 8.2), plant height or seed shape.
• The father and the mother give the child practically equal amounts of genetic material, so every trait has two versions in each child, one from each parent.
• Mendel grew garden peas and, unlike earlier workers, counted how many plants showed each trait in every generation. He used pairs of contrasting characters: round or wrinkled seeds, tall or short plants, violet or white flowers.
• Tall crossed with short: every plant in the first generation (F1) was tall. There were no in-between, medium-height plants.
• F1 plants self-pollinated: in the second generation (F2), one plant in four was short. So the "short" version had been carried by the F1 plants all along without showing.
• Mendel's explanation: each plant carries two copies of a factor, today called a gene, for each trait. The two copies may be the same or different.
• With T for tall and t for short: TT and Tt plants are tall, and only tt is short. One T is enough for tallness.
• A version that shows even when only one copy is present (T) is dominant. One that shows only when both copies are that version (t) is recessive.
• The F2 plants come in the gene combinations TT, Tt and tt in the ratio 1:2:1, which gives 3 tall : 1 short. Fig. 8.4 shows the same 3:1 pattern for violet and white flowers.

3. Two traits at once: independent inheritance (section 8.2.2)

• Cross a pea plant with round, green seeds (RRyy) with one with wrinkled, yellow seeds (rrYY). Every F1 seed is round and yellow (RrYy): round and yellow are the dominant versions.
• Let the F1 self-pollinate. The F2 has all four mixtures of shape and colour: the parents' round-green and wrinkled-yellow, the F1's round-yellow, and wrinkled-green, which no plant before it showed.
• Mendel's counts in Fig. 8.5: 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green, out of 556 seeds. That is close to 9 : 3 : 3 : 1 (sixteenths).
• Mixtures the parents never had (round-yellow, wrinkled-green) prove that the gene for seed shape and the gene for seed colour are inherited independently of each other. The text makes the same point for tall/short and round/wrinkled.

4. How a gene shows up as a trait (section 8.2.3)

• The cell's DNA carries the information for making proteins. The stretch of DNA that codes for one protein is the gene for that protein.
• The book's chain for plant height: gene → enzyme → amount of growth hormone → height. An efficient enzyme makes plenty of hormone and a tall plant. An altered gene gives a less efficient enzyme, less hormone and a short plant.
• Both parents add equally to the offspring's DNA, so each pea plant holds two sets of genes, and each germ cell must hold one set.
• The genes are not one long thread of DNA but separate pieces, the chromosomes. Body cells have chromosomes in pairs, one of each pair from each parent. A germ cell takes one chromosome from every pair, and it can be either the mother's or the father's copy. That is what lets R and y travel separately.
• When two germ cells fuse, the full number of chromosomes comes back, which keeps the species' DNA stable.

5. Sex determination (section 8.2.4)

• Species do this in different ways. In a few reptiles the egg's incubation temperature decides male or female. Snails can change sex, so for them sex is not fixed by genes.
• In humans sex is largely decided by genes.
• Humans have 22 pairs of ordinary chromosomes plus one pair of sex chromosomes.
• Women carry XX, a matched pair. Men carry XY: one normal-sized X and a short Y.
• Every child receives an X from the mother. A child who gets X from the father is a girl (XX); a child who gets Y from the father is a boy (XY). So it is the father's chromosome that settles the child's sex, and on average half the children are boys and half are girls (Fig. 8.6).

Key words at a glance

• Variation: the differences between individuals of the same species.
• Heredity: the passing of traits from parents to offspring by fixed rules.
• Gene: the section of DNA that carries the information for one protein.
• Dominant trait: the version that shows when only one copy is present.
• Recessive trait: the version that shows only when both copies are that version.
• F1, F2: the first and second generations of offspring after the parent cross.
• Chromosome: one of the separate pieces of DNA that together make up a gene set.
• Germ cell: a reproductive cell, carrying a single set of chromosomes.
• Sex chromosomes: the X and Y pair; XX in women, XY in men.

The diagram to know: a single-trait cross, drawn as a grid

Board papers often ask you to show a monohybrid cross. Here is how to draw it so a marker can follow every step.

1. Parents (P): write "Tall (TT) × Short (tt)" at the top. Under each parent, circle its gametes: T from the tall plant, t from the short one.
2. F1: join the gametes with arrows to one box labelled Tt, all tall.
3. Selfing: write "F1 × F1: Tt × Tt" under it.
4. Grid: draw a 2 × 2 grid. Put the gametes of one F1 plant (T, t) along the top and the other's (T, t) down the side.
5. Fill the four boxes: TT, Tt, Tt and tt.
6. Label the result under the grid: gene combinations 1 TT : 2 Tt : 1 tt; plants 3 tall : 1 short.

Mark which letter is dominant (capital T) and give the grid a title. The sex-determination diagram works the same way: gametes X, X from the mother and X, Y from the father give two XX boxes (girls) and two XY boxes (boys).

Mistakes that cost marks

• Mixing up the ratios. 3:1 is the F2 ratio of plants you can see (tall : short); 1:2:1 is the ratio of gene combinations (TT : Tt : tt). 9:3:3:1 belongs only to a cross involving two traits.
• Saying the short trait was "lost" in F1. It was carried, just not shown; that is why it comes back in F2.
• Thinking a tall plant must be TT. Tt plants are tall as well.
• Saying the mother decides the child's sex. She always passes an X; the father's X or Y decides.
• Writing "dominant means more common". Dominance is about which version shows in a Tt individual, not about how many people have it.
• Revising evolution from old notes. The current chapter is "Heredity" only; check what your book contains.

Revise with questions

Test yourself next: Class 10 Heredity, 8 practice questions with answers. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

Where CMDLearn fits

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Sources

All checked on 3 November 2026.

• NCERT, Science, Textbook for Class X, Chapter 8 "Heredity" (Reprint 2026-27), pages 128-133: sections 8.1, 8.2.1 to 8.2.4, Figures 8.1 to 8.6, the Mendel box and "What you have learnt". ncert.nic.in/textbook/pdf/jesc108.pdf
• CBSE, Science curriculum, Class X, 2026-27 (course structure and Unit II): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, section numbers, figure numbers and Mendel's seed counts are from the documents above. The wording of these notes, the diagram steps and the list of mistakes are our own.

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