Heredity and Evolution

Two tall pea plants produce a short offspring. Has inheritance failed? The surprise disappears when we distinguish the alleles a plant carries from the trait we can see. Heredity is about the transmission of biological information; a visible resemblance is only one piece of evidence.

These original Class 10 Science notes follow NCERT Chapter 8, Heredity, Reprint 2026–27, with a separately identified Evolution extension from CBSE's current reading material. The older “heredity-and-evolution” address is retained so existing links keep working.

Read the assessment scope carefully. CBSE's 2026–27 Science syllabus lists Heredity under Unit II and identifies Evolution as formative learning. Its later Note for Teachers uses the broader wording “Heredity and Evolution” for material not assessed in the year-end examination. Because those passages differ in scope, this page does not promise a board-exam allocation for Heredity. Use the official syllabus, pages 5–6, and your school's current assessment guidance. Formative learning still belongs in the course; it is not the same as deleting the science.

Introduction

Heredity is the transmission of genetic information from parents to offspring. Variation means differences among individuals, including differences between offspring and their parents. Genetics studies inheritance and variation. A trait is a characteristic, such as a pea seed's shape; traits can reflect genetic factors, environmental influences, or both.

Where variation comes from

DNA copying preserves much of an organism's information, but copying is not perfectly error-free. A change in DNA sequence is a mutation. In asexually reproducing lineages, inherited copying changes can accumulate even when individuals initially look very similar. Asexual reproduction therefore does not mean that all descendants remain genetically identical forever.

Sexual reproduction adds new combinations of existing genetic variants. Chromosome assortment and the exchange of DNA during meiosis, followed by fertilisation, reshuffle information from two parents. Recombination rearranges variants; mutation can introduce a new variant. Neither process deliberately creates the exact adaptation an organism will need.

A variation's effect depends on the circumstances. Heat tolerance may improve a bacterium's survival in a hot environment; another difference may have little effect or be harmful. Variation can increase a population's capacity to persist through change, but it does not guarantee that every individual or every species survives.

Can a trait's present frequency tell us exactly when it arose?

No. In a simplified asexual-lineage exercise, a more widespread trait might have had longer to spread if reproduction and survival were otherwise comparable. But selection, migration and chance can also change frequencies. Finding a trait in 60% rather than 10% of a population is not, by itself, a reliable date. State the assumptions behind the inference.

Some important terms

Keep the physical material, the information and the observed outcome distinct. DNA is a molecule; a chromosome packages a long DNA molecule with proteins; a gene is a functional unit of hereditary information within DNA. Many genes provide instructions for proteins, while some produce functional RNA. “One gene equals one visible trait” is too simple.

• Alleles: alternative versions at a genetic location. In the diploid pea examples here, a plant has two alleles at the locus being studied, one inherited from each parent.
• Genotype: the allele combination at the locus or loci being discussed, such as TT, Tt or tt. Those letters describe a model; they are not chemical labels written inside the plant.
• Phenotype: an observable or measurable characteristic, such as tall or short growth under the stated conditions. It can include biochemical traits, not only outward appearance.
• Homozygous: the two alleles at a locus are the same, as in TT or tt. Heterozygous: they differ, as in Tt.
• Dominant and recessive: a relationship between alleles and the phenotype being considered. In the complete-dominance pea model, Tt is tall like TT, while tt is short. The recessive allele remains present in Tt and can be transmitted.
• Homologous chromosomes: a corresponding pair carrying the same kinds of genes at corresponding positions, though their alleles can differ. One member of the pair comes from each parent.
• Gamete: a reproductive cell carrying one chromosome set. Fertilisation joins two gametes and restores two sets in the zygote.

Dominant does not mean stronger, better, more common or more likely to enter a gamete. In this model a Tt plant transmits T and t with equal probability. Also, dominance is not defined simply by “whatever happens to appear in the first generation”; it is the relationship revealed by an appropriate cross and its offspring.

Mendel and his experiments on inheritance

Gregor Johann Mendel investigated inheritance using garden peas, Pisum sativum. His important contribution was a testable explanation of patterns across generations, supported by counting offspring rather than relying on a few striking resemblances. The selected traits included plant height, flower colour and seed shape and colour.

Peas offered distinguishable character variants and many offspring. They normally self-pollinate, while controlled cross-pollination allows a researcher to choose the two parents. True-breeding lines make the starting conditions clearer: for the character under study, they consistently produce the same phenotype when self-pollinated.

P denotes the parental generation. Their offspring form F1, the first filial generation. Offspring produced from F1 plants form F2. For the crosses below, assume diploid inheritance, ordinary allele segregation, complete dominance of the specified allele, and comparable survival of the resulting genotypes. These assumptions explain when the familiar ratios apply.

Why was the second generation more informative than the first?

If all F1 plants look tall, at least two explanations are imaginable: the short-associated information was lost, or it remains present without producing short growth. Its reappearance in F2 supports the second explanation. Observing another generation distinguishes hypotheses that the F1 appearance alone cannot separate.

Monohybrid cross

A monohybrid analysis follows one character with contrasting variants. Use T for the tall-associated allele and t for the short-associated allele in the stated pea model. Crossing true-breeding TT × tt gives Tt offspring: the TT parent supplies T, the tt parent supplies t, and all F1 plants are tall under this model.

Now self-pollinate an F1 plant: Tt × Tt. Each parent contributes T or t with probability 1/2. Combining one contribution from each gives four equally likely allele-pair outcomes: TT, Tt, tT and tt. Tt and tT are the same genotype, written in opposite parental order.

• Genotype probabilities: TT = 1/4, Tt = 1/2, tt = 1/4. The expected genotypic ratio is 1:2:1.
• Phenotype probabilities: tall = 3/4 and short = 1/4. The expected phenotypic ratio is 3:1.
• Segregation: the two alleles separate during gamete formation, so a gamete receives one allele at this locus, not both.

A Punnett square displays possible combinations and their probabilities. It is not a tray containing the next four seeds. A small family of plants may depart substantially from the expected ratio; even a large sample need not match it exactly. One short offspring after three tall offspring is not “owed” by inheritance.

Same appearance, different genotype

A tall plant can be TT or Tt. Under the model, crossing an unknown tall plant with tt distinguishes the predicted outcomes: TT × tt gives all Tt, while Tt × tt gives Tt and tt with probabilities 1/2 each. This is a test cross. In a finite sample, all-tall offspring do not prove TT with certainty, because a Tt parent could also produce that result by chance.

Self-pollination is another informative comparison: TT predicts only tall descendants; Tt predicts a mixture with an expected 3:1 ratio; tt predicts only short descendants. Such evidence can test the proposed 1:2:1 genotype mixture in F2. Genotyping or sufficiently informative offspring data is needed because appearance alone combines TT and Tt in one category.

Worked prediction: how many short plants should we expect among 80 offspring of Tt × Tt?

The model gives a short-plant probability of 1/4, so the expected count is 80 × 1/4 = 20. “Expected” is a statistical prediction, not a guarantee. A measured count such as 18 would need to be considered with sample variation and the experiment's assumptions; it does not automatically disprove inheritance.

Dihybrid cross

A dihybrid analysis tracks two characters. Follow the seed example in NCERT Figure 8.5: R produces round seeds in RR and Rr; rr produces wrinkled seeds. Y produces yellow seeds in YY and Yy; yy produces green seeds. Begin with RRyy × rrYY. The parental gametes are Ry and rY, so F1 plants are RrYy and produce round, yellow seeds.

For RrYy × RrYy, if the two loci assort independently, each parent produces RY, Ry, rY and ry gametes with probability 1/4 each. Each gamete has one allele for shape and one for colour. Combining four possible gametes from each parent creates sixteen equally likely pairings.

• Round and yellow: 3/4 × 3/4 = 9/16.
• Round and green: 3/4 × 1/4 = 3/16.
• Wrinkled and yellow: 1/4 × 3/4 = 3/16.
• Wrinkled and green: 1/4 × 1/4 = 1/16.

The expected phenotypic ratio is therefore 9:3:3:1. New combinations can appear because the alleles controlling the two characters are not forced to travel together in their original parental combinations.

Prediction versus actual counts

NCERT's figure reports 315 round-yellow, 108 round-green, 101 wrinkled-yellow and 32 wrinkled-green seeds, a total of 556. These observed counts approximate the model's ratio; they are not literally 9, 3, 3 and 1. For example, 9/16 of 556 is 312.75: an expected value can be fractional even though every actual seed count is a whole number.

Independent assortment is not a claim that every pair of genes always behaves independently. Genes close together on the same chromosome can be linked and inherited together more often. The four equal gamete probabilities and 9:3:3:1 result belong to the stated model; departures can prompt examination of linkage, survival differences, sampling or other assumptions.

Can a gamete from RrYy contain RR?

Not in the ordinary haploid model being used. RR would contain two alleles at the shape locus and none specified at the colour locus. The four appropriate possibilities are RY, Ry, rY and ry. Write the gametes before filling the square; that catches many errors.

How do traits get expressed?

Genes influence traits through their products and through the regulation of cellular processes. In a protein-coding example, the information in DNA is used to make RNA, which is used to build a protein. In this example, an enzyme is a protein that helps a chemical reaction proceed. Changing a relevant gene can change the enzyme's activity and, in turn, an observable feature.

NCERT uses a simplified plant-height pathway: a gene influences an enzyme involved in producing a growth hormone; altered enzyme activity can change hormone production and growth. This connects DNA → gene product → cellular process → trait. It does not mean that all plant height depends on one gene, or that unlimited hormone always makes a plant taller. Nutrition, other genes and environmental conditions also matter.

Why the chromosome number does not double every generation

In the diploid life cycle, meiosis produces cells with one chromosome set, and fertilisation brings two such sets together. In the usual human chromosome count, an egg contributes 23 chromosomes and a sperm contributes 23; the zygote has 46, arranged in 23 pairs. This is a statement about chromosome-set contributions, not a claim that the egg and sperm have the same size or provide equal amounts of cytoplasm.

At an ordinary paired nuclear-gene locus, this gives the offspring an allele from each parent. It explains why an offspring can combine parental variants while maintaining the species' usual chromosome number. Most human body cells have this diploid set; mature red blood cells lack a nucleus, and gametes carry one set, so “every cell has 46 chromosomes” is inaccurate.

Sex determination

Different species use different developmental systems. In some reptiles, incubation temperature influences sexual development. Humans use a system in which sex chromosomes and genes play major roles; the X and Y chromosomes are part of a larger developmental process.

In the usual school model, an XX parent produces X-bearing eggs, while an XY parent produces X-bearing or Y-bearing sperm. An X-bearing sperm joining an X-bearing egg produces XX; a Y-bearing sperm joining an X-bearing egg produces XY. XX is usually associated with female development and XY with male development.

The model predicts roughly equal probabilities of these chromosome combinations, not an obligation for each family to contain equal numbers of boys and girls. The outcome of an earlier fertilisation does not make a particular sperm type due next time. Neither parent consciously chooses the fertilising sperm, and blaming a mother for a child's sex has no scientific basis.

Actual human sex development includes variation and depends on other genes and developmental processes as well. The NHGRI X-chromosome explanation makes that complexity explicit. The simplified XX/XY diagram teaches chromosome transmission; it is not a complete description of every person or a test of gender identity.

What does a chromosome diagram establish, and what does it leave out?

It can show which chromosome combinations follow from the stated gametes and assumptions. It does not establish a guaranteed sequence of births, a person's identity, or the full course of sexual development. Keep the model's useful prediction separate from claims it was not designed to make.

Reasoning with inheritance evidence

Real human characteristics should not be treated as if they were automatically Mendel's selected pea traits. Eye colour involves several genes, so the familiar “brown always dominates blue” classroom rule is inadequate. A resemblance between parents and children is evidence worth investigating; it does not alone identify an allele or establish dominance.

The same care applies to earlobes. A picture can help describe an observed shape, but sorting ears into two boxes does not establish a one-gene rule. Use supplied observations or fictional plant crosses here, rather than collecting classmates' family, medical or genetic information. An ethically designed project states its question, sample, observations and model limits.

A fictional tall, violet-flowered pea crossed with ttww gives only violet flowers and about half short plants. Explain a consistent parent genotype.

Under the stated two-locus model, TtWW is consistent: Tt × tt gives half tall and half short, while WW × ww gives all Ww, violet-flowered offspring. A finite sample of all violet flowers does not by itself prove WW with absolute certainty; the familiar answer assumes the observed pattern represents the underlying model probabilities.

Does one A-blood-group parent, one O parent and an O child prove which blood-group allele is dominant?

Not from those three phenotype observations alone, without an inheritance model and other evidence. Under the established ABO model, an A parent can carry an O-associated allele, but invoking that known model is different from discovering dominance from this one family. A conclusion must not secretly assume the fact it is supposed to establish.

Design a responsible investigation of an unfamiliar animal coat trait.

Use an existing, ethically collected dataset or a fictional dataset with known parentage and enough offspring. Define the phenotype categories, propose competing inheritance models and compare their predictions with observations across generations. Account for age and environment. Do not breed or manipulate animals merely to complete a school worksheet, and do not infer a universal rule from a few similar-looking animals.

Evolution

Formative extension: this and the next two sections connect heredity to the Evolution material supplied by CBSE for 2026–27. They are distinguished from the current six-page NCERT Heredity chapter and carry no year-end exam guarantee.

Biological evolution concerns changes in inherited characteristics of populations across generations. Changes in allele frequencies provide one way to study it. Evolution is not restricted to the origin of a new species, does not always require millions of years, and is not a ladder from “primitive” organisms to humans.

Selection, chance and environmental effects

CBSE's beetle stories compare three explanations for a changed population. First, a heritable colour can reduce predation in a particular setting; survivors then leave more offspring, so the variant can become more common. This is natural selection. Reproductive success matters, not merely physical strength. The environment does not instruct an individual beetle to mutate into the needed colour.

Second, a chance event can leave a small, unrepresentative set of survivors, changing allele frequencies without a colour-related advantage. This is genetic drift, often especially consequential in small populations. Both selection and drift can operate in real populations; the stories isolate them to make the comparison clear.

Third, poor nutrition can reduce body mass even when the population's inherited variants have not changed. If food returns and growth recovers, that observation alone does not demonstrate evolutionary change. An inherited difference and an environmental effect can produce superficially similar changes in appearance.

Acquired and inherited traits

A scar or a change in muscle size does not rewrite the corresponding inherited DNA information in gametes. Such a bodily change is not automatically transmitted as the same trait to offspring. For a genetic change to pass through sexual reproduction, it must enter the reproductive lineage. Mutations can also occur in body cells, so “all genetic variation happens only in gametes” is wrong.

The useful school distinction is between a change acquired by an individual and a heritable change that can persist through reproduction. Avoid turning it into the sweeping claim that parental environments can never influence offspring in any way. The simple models here track inherited DNA variants; they do not cover every developmental or epigenetic effect.

Speciation

Formative extension. Speciation is the formation of new species. In the sexually reproducing beetle model, reduced exchange between populations can allow differences to accumulate. Mutation supplies variation; selection and drift can change its distribution; reproductive barriers may eventually limit successful interbreeding.

Gene flow is the movement of genetic variants between populations through reproduction. It often keeps populations genetically connected, rather than automatically creating a new species. A river or mountain can reduce contact, but geographical separation alone does not prove reproductive isolation. Populations may still interbreed if they meet again.

The interbreeding criterion is useful for this model but cannot be applied in the same way to asexual organisms or every fossil. Likewise, a self-pollinating plant already has limited mating exchange, so the role of an additional physical barrier differs from that in a freely interbreeding population. Ask what process actually limits genetic exchange in the organism being studied.

Evolution and classification

Formative extension. Classification can represent shared ancestry, but a superficial resemblance is not enough to establish the nearest relationship. Researchers compare patterns across anatomy, development, fossils and DNA. A shared derived feature can be informative; similar functions can also evolve independently.

Homology and analogy depend on what is compared

Vertebrate forelimbs share an inherited underlying structural plan, even when used for running, swimming or flight. Such corresponding structures are homologous. Bat and bird forelimb bones are homologous as modified vertebrate forelimbs. Their wings as independently evolved flight adaptations are analogous: similar function does not mean flight was inherited from their shared forelimbed ancestor.

Thus, “bat and bird wings are different, so they have no shared structural ancestry” is misleading. Specify the level of comparison. An insect wing and a vertebrate forelimb wing offer another example of flight structures with different anatomical origins.

Fossils and dating

Fossils are preserved remains, impressions or traces of past organisms, including evidence of their activity. They help reconstruct earlier life and changing environments. Fossil anatomy can preserve combinations of features that clarify relationships; one specimen does not necessarily identify the direct ancestor of a living species.

In an undisturbed sedimentary sequence, lower layers are generally older than layers above them. Folding, faulting, erosion and redeposition can complicate that relationship. Numerical dating uses a method suited to the material and age range, sometimes dating associated rock layers rather than the fossil itself. Radiocarbon dating works for suitable relatively recent organic material, roughly within 50,000 years; it cannot date a dinosaur fossil tens of millions of years old. The Smithsonian dating guide explains why different clocks are needed.

Stages, changed functions and artificial selection

A complex feature need not appear fully formed in one mutation. Intermediate features can have useful functions, and an existing feature can later serve another role. Feathers on non-flying dinosaurs show why the history of feathers cannot be reduced to an immediate need for flight. Evolution does not plan ahead towards a final design.

Artificial selection provides an observable connection between inherited variation and human choices. Repeatedly selecting plants with particular inherited features helped produce cultivated forms such as cabbage, kale, broccoli, cauliflower and kohlrabi from the wild-cabbage lineage. This is selection across generations, not one plant deciding to turn into several vegetables.

Explaining evolution after life exists is also different from explaining the first origin of life. Experiments producing organic molecules under proposed early-Earth conditions are evidence about chemical possibilities; making such molecules is not the same as creating a living cell.

Human evolution is a branching history

All living humans belong to Homo sapiens. Fossils and genetic evidence support our species' African origins and a history of dispersal and mixing. Living chimpanzees are our relatives, not our ancestors; both lineages share earlier ancestors. Neither modern humans nor present-day bacteria are an unfinished step waiting to become another living species.

Comparing DNA helps reconstruct relationships, but “more differences always means exactly this many years” is not a universal clock. Researchers must account for which sequences are compared, how they change and independent evidence. The Smithsonian Human Origins evidence collection brings genetics, fossils, dating and behaviour together. Use it to ask what each kind of evidence can establish, rather than to rank living human groups as more or less evolved.

A river separates two beetle populations. What evidence would strengthen a speciation claim?

Evidence of lasting reproductive barriers, together with inherited divergence, is more informative than the existence of the river alone. In the sexual-species model, ask whether successful interbreeding remains possible when contact occurs. A physical barrier can start a process without proving that the process is complete.

Why it still matters

Inheritance models teach a powerful habit: separate an observation, a model and a prediction. A phenotype does not reveal every allele. A probability does not promise a particular family or seed tray. A population trend does not by itself identify its cause. These distinctions are useful in plant breeding, biological research and the interpretation of genetic claims.

For a small source investigation, compare the current NCERT pea-cross figure with the National Library of Medicine's account of eye colour. List one reason the selected pea trait fits a simple model and one reason human eye colour needs a more complex explanation. The aim is to improve a model when evidence requires it, not to discard useful models or force every trait into them.

• Alleles are versions; genotype and phenotype are different kinds of description.
• Segregation gives a gamete one allele at a locus in the model.
• 1:2:1 and 3:1 describe different outcomes; 9:3:3:1 needs independent assortment and the other stated assumptions.
• Expected ratios concern probability, not a guaranteed sequence.
• Genes, environment and development can all matter to a trait.
• Selection, drift and environmental effects are different explanations to test.
• Evolution concerns populations and branching relationships, not a ladder of worth.

Connect this chapter with How Do Organisms Reproduce?, revisit Control and Coordination for hormones and responses, or browse the Class 10 Science collection.

Sources

• NCERT Science, Chapter 8: Heredity, Reprint 2026–27 — complete chapter, crosses, reported seed counts and exercises.
• CBSE Science syllabus 2026–27 — topic list, assessment wording and teacher note.
• CBSE Class X Science Reading Material 2026–27, Unit 2 — formative Evolution extension. The filename contains IX, but the document itself is labelled Class X.
• NHGRI Talking Glossary — genes, alleles, gametes, dominance and linkage.
• NHGRI: X chromosome — chromosome transmission and the complexity of human sexual development.
• MedlinePlus Genetics: eye colour — why the simple one-gene explanation is insufficient.
• Smithsonian Human Origins: dating — relative sequences, numerical dates and method limits.
• Smithsonian: One Species, Living Worldwide — shared human species and African origins; the written page corrects the older date in its embedded video.