BharatNotes Why this chapter matters for UPSC: This is Chapter 8, "Heredity", of NCERT's Class X Science (Reprint 2026-27). It explains how variation builds up, how Mendel counted traits in pea plants, how genes work through proteins, and how sex is decided in human beings. Until the 2020-21 edition the chapter was called "Heredity and Evolution" and went on to natural selection, speciation, fossils and human evolution. Those sections are not in the current book; this page keeps them, clearly labelled, because general-science questions on genetics and evolution draw on both.
Contemporary hook: On 9 January 2025 the Ministry of Science and Technology announced that the whole-genome data of 10,000 Indians, gathered under the GenomeIndia Project, was archived at the Indian Biological Data Centre (IBDC) and open to researchers. Every genome in that collection is the "gene set" this chapter describes, two copies of each chromosome, one from each parent.
🧠 First Principles — Read This First
- Copying is never exact. Each generation inherits the parents' design plus small new differences. Asexual reproduction adds a few; sexual reproduction adds many more.
- Two copies, one expressed. A sexually reproducing organism carries two copies of each gene, one from each parent. If they differ, the dominant one shows.
- Genes work through proteins. A gene is the stretch of DNA that codes for one protein; the protein (often an enzyme) shapes the trait.
- Chromosomes travel separately. Because each germ cell takes one chromosome of every pair at random, traits on different chromosomes are inherited independently.
- Selection works on variation. The environment favours some variants; over generations the frequency of genes in a population changes. That, in NCERT's 2020-21 text, is evolution.
PART 1 — Quick Reference
Table 1: Variation across generations (8.1)
| Point | What NCERT says |
|---|---|
| Inheritance gives | A common basic body design and subtle changes in it |
| Asexual reproduction | Four bacteria from two divisions are very similar; "only very minor differences between them, generated due to small inaccuracies in DNA copying" |
| Sexual reproduction | "Even greater diversity" is generated |
| Field example | A sugarcane field shows very little variation; sexually reproducing animals, including humans, show distinct variation |
| Survival | Variants do not have equal chances; heat-tolerant bacteria survive a heat wave better |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27, section 8.1 and Fig. 8.1 (pp. 128-129).
Table 2: Mendel's single-trait cross (tall × short pea)
| Generation | Genetic make-up | What is seen |
|---|---|---|
| Parents | TT (tall) × tt (short) | Tall and short |
| F1 | All Tt | All tall: "no halfway characteristics", no "medium-height" plants |
| F2 (F1 self-pollinated) | TT : Tt : tt = 1 : 2 : 1 | Three tall to one short: "one quarter of them are short" |
| Rule | A single T is enough for tallness; both copies must be t for shortness | T is dominant, t recessive |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27, section 8.2.2, Fig. 8.3 and Activity 8.2 (p. 130).
Table 3: Mendel's two-trait cross (Fig. 8.5)
| Item | Value |
|---|---|
| Parents | RRyy (round, green) × rrYY (wrinkled, yellow) |
| F1 | RrYy, all round and yellow |
| F2 counts | 315 round yellow; 108 round green; 101 wrinkled yellow; 32 wrinkled green |
| Total | 556 seeds |
| Ratio | 9 : 3 : 3 : 1 |
| Conclusion | Seed shape and seed colour are inherited independently; new combinations appear in F2 |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27, Fig. 8.5 (p. 131).
Table 4: From gene to trait (8.2.3)
| Step | NCERT's tallness example |
|---|---|
| DNA | The information source for making proteins in the cell |
| Gene | "A section of DNA that provides information for one protein" |
| Protein | An enzyme needed to make a growth hormone |
| Efficient enzyme | More hormone; tall plant |
| Altered gene, less efficient enzyme | Less hormone; short plant |
| Conclusion | "Thus, genes control characteristics, or traits." |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27, section 8.2.3 (p. 131).
Table 5: How sex is decided (8.2.4)
| Organism | Basis | NCERT's detail |
|---|---|---|
| A few reptiles | Environment | Temperature at which fertilised eggs are kept decides male or female |
| Snails | Not genetic | Individuals can change sex |
| Human beings | "Largely genetically determined" | 22 pairs of chromosomes plus one pair of sex chromosomes |
| Women | XX | A perfect pair |
| Men | XY | One normal-sized X and a short Y |
| The child | Decided by the father | X from the father gives a girl, Y a boy; half the children boys, half girls |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27, section 8.2.4 and Fig. 8.6 (p. 132).
Table 6: Evidence for evolution in the 2020-21 edition (retained content)
| Line of evidence | NCERT's example |
|---|---|
| Homologous organs | Four limbs in mammals, birds, reptiles and amphibians: same basic design, different uses |
| Analogous organs | Wings of a bat (skin folds between elongated fingers) and of a bird (feathers along the arm): same use, different origin |
| Fossils | Preserved traces of organisms; age by depth of the layer or by ratios of isotopes |
| Evolution by stages | Planaria's eye-spots; feathers first for warmth, later for flight |
| Artificial selection | Wild cabbage bred into cabbage, broccoli, cauliflower, kohlrabi and kale |
| Molecular phylogeny | More distantly related organisms carry more differences in their DNA |
Source: NCERT, Science Class X, 2020-21 edition, ch. 9, sections 9.5.1-9.5.3 and the box "Molecular phylogeny" (pp. 152-156).
Table 7: NCERT lines to read with care
| Line in the book | Why to read it with care |
|---|---|
| "Selection of variants by environmental factors forms the basis for evolutionary processes, as we will discuss in later sections." (p. 129) | The later sections on evolution are no longer in the book; the sentence points to nothing |
| The figure beside Fig. 8.5 carries its own caption "Figure 9.5" (p. 131) | The old figure number, left inside the image from the 2020-21 edition, where the chapter was 9 |
| "We have disscussed this issue in the previous Chapter." (p. 131) | Misspelt "discussed"; the previous chapter is Chapter 7 on reproduction |
| Text describes tall/round × short/wrinkled plants; Fig. 8.5 shows seed shape × seed colour (p. 131) | Two different two-trait crosses share one passage; the numbers 315/108/101/32 belong to the figure's cross |
| Sex in humans "largely genetically determined" (p. 132) | NCERT's own qualifier; the rest of the section treats it as decided by the X or Y from the father |
| Class X (2020-21): "A couple of hundred thousand years ago, some of our ancestors left Africa" (p. 157). Class XII: "During ice age between 75,000-10,000 years ago modern Homo sapiens arose" | The two NCERT books give different time-frames; learn each in its own context and do not merge them |
Source: NCERT, Science Class X, ch. 8, Reprint 2026-27 (pp. 129-132); 2020-21 edition, ch. 9 (p. 157); NCERT, Biology Class XII, ch. 6 "Evolution", section 6.9.
PART 2 — Concepts & Narrative
Accumulation of variation (8.1)
Reproduction gives a new generation the parents' body design with small differences. When that generation reproduces in turn, its offspring carry both the inherited differences and new ones (NCERT's Fig. 8.1). In asexual reproduction the new differences come only from slight errors in copying DNA, so the four descendants of one bacterium are very similar. Sexual reproduction mixes the DNA of two parents, so diversity grows much faster.
Variants do not all fare equally. NCERT returns to its heat-wave example from the previous chapter: bacteria able to withstand heat survive better. The in-text question that follows asks which of two traits in an asexual population, A in 10% and B in 60%, is likely to have arisen earlier. The reasoning: in an asexual line a new variant spreads only by descent, so the more widespread trait, B, has probably had more generations to spread.
Inherited traits and Mendel (8.2.1-8.2.2)
A child has all the basic features of a human being but does not look exactly like either parent. NCERT's Activity 8.1 asks students to sort classmates by free or attached earlobes (Fig. 8.2) and compare them with their parents to guess a rule.
The rule rests on one fact: both father and mother contribute "practically equal amounts of genetic material" to the child, so there are two versions of every trait. Which one shows?
Mendel crossed pea plants with contrasting characters: round or wrinkled seeds, tall or short plants, white or violet flowers. In the tall × short cross the first generation (F1) was all tall. When F1 plants were self-pollinated, a quarter of the second generation (F2) was short. So shortness had been carried, hidden, in the F1. Mendel proposed that "two copies of factor (now called genes) controlling traits are present in sexually reproducing organism". They may be identical (TT, tt) or different (Tt).
Dominant and recessive. NCERT: "a single copy of 'T' is enough to make the plant tall, while both copies have to be 't' for the plant to be short. Traits like 'T' are called dominant traits, while those that behave like 't' are called recessive traits." A recessive trait shows only when both copies are recessive.
Activity 8.2 asks how to confirm the 1:2:1 ratio of TT, Tt and tt in the F2. One way: self-pollinate each tall F2 plant. A TT plant gives only tall offspring; a Tt plant gives tall and short in a 3:1 ratio. About one-third of the tall plants should breed true.
Who was Mendel? NCERT's box: Gregor Johann Mendel (1822-1884) was educated in a monastery and studied science and mathematics at the University of Vienna. He failed the examinations for a teaching certificate, went back to the monastery and grew peas. Others had studied inheritance in peas before him; Mendel "was the first one to keep count of individuals exhibiting a particular trait in each generation", and counting led him to the laws of inheritance.
Two traits at once. In the two-trait cross of Fig. 8.5, round seeds (R) and yellow seeds (Y) are dominant. The F1 is RrYy. In the F2, besides the parental types, two new combinations appear: round green and wrinkled yellow. The counts, 315, 108, 101 and 32 out of 556, are close to 9:3:3:1. So the factor for seed shape and the factor for seed colour separate and recombine on their own: they are inherited independently.
The blood-group question (p. 133). A man of blood group A and a woman of group O have a daughter of group O. Can we tell which trait is dominant? No. If A were dominant, the father could carry one hidden O and pass it on. If O were dominant, the daughter could have taken the O from her mother. One family is consistent with both rules, so more crosses are needed.
How traits get expressed (8.2.3)
Cellular DNA holds the information for making proteins. "A section of DNA that provides information for one protein is called the gene for that protein." NCERT links gene to trait through plant height: height depends on how much growth hormone a plant makes; that depends on an enzyme; and the enzyme's efficiency depends on its gene. An alteration that makes the enzyme less efficient gives less hormone and a short plant.
If both parents contribute equally, each pea plant must have two sets of all genes, and each germ cell only one set. How does a germ cell pick one set? Not as one long thread of DNA, for then R and y, which came together from one parent, would always be inherited together and Fig. 8.5's new combinations could not appear. Instead each gene set is in "separate independent pieces, each called a chromosome".
Chromosome. One of the separate pieces in which a cell's gene set is carried. Every body cell has two copies of each chromosome, one from each parent. Every germ cell takes one chromosome from each pair, of either maternal or paternal origin. When two germ cells fuse, the normal number is restored, "ensuring the stability of the DNA of the species". This is the mechanism behind Mendel's results, and it is used by all sexually reproducing organisms.
Sex determination (8.2.4)
Species use very different strategies. In a few reptiles the temperature at which eggs are kept decides whether the young are male or female. Snails can change sex, "indicating that sex is not genetically determined". In human beings sex is "largely genetically determined".
Most human chromosomes come as a maternal and a paternal copy, 22 such pairs. The sex chromosomes are the odd pair: women have a perfect pair, XX; men a mismatched pair, a normal-sized X and a short Y. Every child receives an X from the mother. The father passes either an X (girl) or a Y (boy), so "half the children will be boys and half will be girls". The sex of the child is decided by what it inherits from the father.
Did the 2020-21 edition differ?
Yes. In the 2020-21 edition this was Chapter 9, "Heredity and Evolution" (pp. 142-159). In Reprint 2026-27 it is Chapter 8, "Heredity" (pp. 128-133). The heredity sections carry over, renumbered from 9.1-9.2 to 8.1-8.2. The evolution sections 9.3 to 9.6 (evolution, speciation, evolution and classification, evolution and progress, human evolution) and the exercises on them are not in the current book. They are summarised below as retained content.
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.
Retained content: Evolution (2020-21 edition, ch. 9, pp. 147-158; not in Reprint 2026-27)
An illustration with beetles (9.3.1). NCERT imagines twelve red beetles living in bushes with green leaves. Copying errors produce one green beetle; its descendants are hidden from crows and spread. Here "natural selection is exerted by the crows", giving an adaptation to the environment. In a second story a blue beetle appears, and an elephant happens to stamp on most of the red ones, so blue becomes common by accident. In a third, a plant disease cuts the beetles' food and their weight falls, but nothing changes in their genes. NCERT's lesson from the first two: "the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution."
Natural selection and genetic drift. Natural selection changes gene frequencies because some variants survive and reproduce better (the green beetles hidden from crows). Genetic drift changes them by chance. NCERT: "accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. This is the notion of genetic drift, which provides diversity without any adaptations."
Acquired and inherited traits (9.3.2). The starved beetles of the third story show why some changes do not count. Their "low weight is not a trait that can be inherited". NCERT's general rule: "Change in non-reproductive tissues cannot be passed on to the DNA of the germ cells. Therefore the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution." Its example: cutting off the tails of mice by surgery for generations does not produce tailless mice.
Darwin, and the origin of life. NCERT's box on Charles Robert Darwin (1809-1882) says he set out on a five-year voyage at 22 to South America and the islands off its coast, and argued that evolution took place by natural selection; "He did not know the mechanism whereby variations arose", and he and his Austrian contemporary Mendel did not know of each other's work. NCERT adds that Darwin also studied the role of earthworms in soil fertility. A further box notes that the theory of evolution says nothing about how life began. On that question NCERT cites "J.B.S. Haldane, a British scientist (who became a citizen of India later)", who "suggested in 1929 that life must have developed from the simple inorganic molecules". In 1953 Stanley L. Miller and Harold C. Urey built an atmosphere like the early earth's (ammonia, methane and hydrogen sulphide, no oxygen) over water kept just below 100°C, and passed sparks through it to simulate lightning. "At the end of a week, 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules."
Speciation (9.4). Small changes within a population are micro-evolution. A new species arises when a population splits into two that can no longer interbreed. NCERT's route: a population is divided, for example by a large river; gene flow between the halves falls; each half accumulates its own changes through genetic drift and natural selection (eagles may wipe out crows in one place but not the other). Over generations the two become "incapable of reproducing with each other even if they happen to meet". A change in the number of chromosomes, or a change in behaviour such as green females mating only with green males, can complete the split.
Homologous or analogous? NCERT's test is origin, not use. The forelimbs of mammals, birds, reptiles and amphibians share one basic design though they do different jobs: homologous, a sign of common descent. The wings of bats and birds do the same job but are built differently; bats' wings are skin stretched between long fingers, birds' a feathery covering along the arm. "They look similar because they have a common use for flying, but their origins are not common": analogous.
Evolution and classification (9.5). Classification groups organisms by characteristics, details of form or function, arranged in a hierarchy: cell as the basic unit, a nucleus or not, one cell or many, photosynthesis or not, skeleton inside or outside. "The more characteristics two species will have in common, the more closely they are related." So the "classification of species is in fact a reflection of their evolutionary relationship", just as brothers share parents and first cousins share grandparents.
Fossils (9.5.2). An insect caught in hot mud leaves an impression that hardens. "All such preserved traces of living organisms are called fossils." NCERT's Fig. 9.10 shows a dinosaur skull fossil found in the Narmada valley. A fossil's age can be judged relatively, since deeper layers are older, or by measuring the ratios of different isotopes of the same element in it.
Evolution by stages (9.5.3). Complex organs need not appear in one step. Planaria has simple eye-spots that detect light; eyes are built so differently in insects, octopus and vertebrates that NCERT suggests separate evolutionary origins. Feathers may first have given insulation in cold weather and only later helped flight; some dinosaurs had feathers, and NCERT notes that birds are closely related to reptiles. Selection by people also shapes species. From wild cabbage, farmers over more than two thousand years bred very different vegetables, by "artificial selection rather than natural selection":
| Vegetable | Selected for |
|---|---|
| Cabbage | Very short distances between leaves |
| Broccoli | Arrested flower development |
| Cauliflower | Sterile flowers |
| Kohlrabi | Swollen parts |
| Kale | Slightly larger leaves |
Source: NCERT, Science Class X, 2020-21 edition, ch. 9, section 9.5.3 and Fig. 9.13 (pp. 155-156).
Comparing DNA is the newest tool. Changes in DNA accumulate over generations, so organisms more distantly related carry more differences; the tree built this way matches the classification taught in Class IX.
Evolution is not progress; human evolution (9.6-9.6.1). NCERT is blunt: "it is not true that human beings have evolved from chimpanzees. Rather, both human beings and chimpanzees have a common ancestor a long time ago." Older forms do not vanish; bacteria still thrive in hot springs, deep-sea thermal vents and Antarctic ice. "Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection", and "human beings are not the pinnacle of evolution". On human diversity: "there is no biological basis to the notion of human races. All humans are a single species." Genetic evidence traces us to Africa. "A couple of hundred thousand years ago, some of our ancestors left Africa while others stayed on." Those who left spread through West Asia to Central Asia, Eurasia, South Asia and East Asia, down through Indonesia and the Philippines to Australia, and across the Bering land bridge to the Americas, moving forwards and backwards rather than in a straight line (Fig. 9.14 contrasts evolution as a ladder with evolution as a branching tree).
Beyond the textbook: what Class XII Biology adds
Class XII's chapter "Evolution" picks up where the old Class X text stopped.
- Embryos. Ernst Haeckel (spelt "Heckel" in NCERT) proposed that all vertebrate embryos, human ones included, develop a row of vestigial gill slits behind the head, functional only in fish. NCERT XII records that this was disproved by Karl Ernst von Baer, who "noted that embryos never pass through the adult stages of other animals".
- Human ancestors. NCERT XII: "The Neanderthal man with a brain size of 1400cc lived in near east and central Asia between 1,00,000-40,000 years back", and "Homo sapiens arose in Africa and moved across continents".
Source: NCERT, Biology, Textbook for Class XII, ch. 6 "Evolution", sections 6.3 and 6.9.
Beyond the textbook: GenomeIndia (January 2025)
This chapter's "gene set" has been read for Indians on a national scale.
- What: On 9 January 2025 the Ministry of Science and Technology announced that "The entire collection of 10,000 whole genome samples" had been archived at the Indian Biological Data Centre (IBDC), and launched the Framework for Exchange of Data Protocols (FeED) and the IBDC portals to share it with researchers. It named a future target of "sequencing 10 million genomes".
- Who: The Prime Minister's remarks the same day said the project was approved five years earlier and drew on "more than 20 leading research institutions in the country, such as IISc, IITs, CSIR, and BRIC". A later release (30 April 2025) describes GenomeIndia as "funded by the Department of Biotechnology of the Central Government".
- The count: The same April release reports usable sequence files for 9,772 samples, so 10,000 is the number sequenced, not the number of usable genomes.
- Prelims trap: GenomeIndia is an Indian project, approved about five years before January 2025 and funded by the Department of Biotechnology; the PIB releases do not describe it as part of the international Human Genome Project.
Source: PIB (Ministry of Science and Technology), 9 January 2025 and 30 April 2025; PIB (Prime Minister's Office), 9 January 2025.
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — Dominant and recessive, the 3:1 and 9:3:3:1 ratios, XX/XY, homologous versus analogous, genetic drift, acquired versus inherited traits.
- GS3 (science and technology) — Genomics and biotechnology: GenomeIndia, biological data sharing, the gene-to-protein link behind genetic disease and crop breeding.
- GS1 (Indian society) — NCERT's line that "All humans are a single species" with no biological basis for races; the father's chromosome, not the mother, decides a child's sex, an argument against blaming women for daughters.
Past questions on these themes: Prelims 2026 set a statement question on the Genome India Project: whether it is part of the Human Genome Project, who funds it, and what it aims to build. No Mains question in the bank is set directly on heredity or evolution.
Frames for Mains Answers
1. Heredity and society. Use 8.2.4: a child's sex is decided by the X or Y chromosome from the father. Pair it with the sex-ratio material of the previous chapter for questions on son preference.
2. Why genomics matters for India. Begin with the mechanism (genes code for proteins; an altered gene gives an altered protein and trait), then the national data set (10,000 genomes, IBDC, FeED), then uses: disease research and data sharing, with safeguards for privacy.
3. Evolution is not a ladder. For an essay or ethics frame, NCERT's own words serve: evolution is "the generation of diversity and the shaping of the diversity by environmental selection", and all humans are one species.
Exam Strategy
Prelims fact-traps:
- F1 of tall × short = all tall; F2 = 3 tall : 1 short, genotype 1 TT : 2 Tt : 1 tt.
- Two-trait F2 = 9:3:3:1 (315, 108, 101, 32 of 556 in NCERT's figure).
- A gene = DNA for one protein (NCERT), not for a whole trait.
- Humans: 22 pairs plus XX or XY; the father decides the sex.
- Snails can change sex; in a few reptiles temperature decides it.
- Bird and bat wings = analogous; forelimbs of vertebrates = homologous.
- Genetic drift = chance, no adaptation; natural selection = adaptation.
- Humans did not evolve from chimpanzees: common ancestor.
- Miller and Urey, 1953; Haldane, 1929.
- GenomeIndia: DBT-funded; 10,000 genomes; PIB does not describe it as part of the Human Genome Project.
Mains: Build answers on the mechanism first (two copies, dominance, independent inheritance, chromosomes), then apply it.
Practice Questions
Question 1 is the NCERT exercise MCQ of Reprint 2026-27; questions 2-3 are exercise MCQs of the 2020-21 edition. Practice (UPSC-pattern, not past papers): questions 4-10.
1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c). All progeny violet means the tall parent had two copies of W; almost half short means it carried one t (NCERT Answers, p. 218).
2. (2020-21 edition) An example of homologous organs is
(a) our arm and a dog's fore-leg.
(b) our teeth and an elephant's tusks.
(c) potato and runners of grass.
(d) all of the above.
Answer: (d) (NCERT Answers, 2020-21 edition, p. 281).
3. (2020-21 edition) In evolutionary terms, we have more in common with
(a) a Chinese school-boy.
(b) a chimpanzee.
(c) a spider.
(d) a bacterium.
Answer: (a). All humans are a single species (NCERT Answers, 2020-21 edition, p. 281).
4. In NCERT's two-trait cross (Fig. 8.5), how many of the 556 F2 seeds were wrinkled and green?
(a) 32
(b) 101
(c) 108
(d) 315
Answer: (a). The double-recessive class is the smallest, the "1" of 9:3:3:1.
5. Consider the following statements about sex determination as described by NCERT:
1. In a few reptiles, the temperature at which fertilised eggs are kept decides the sex.
2. Human beings have 22 pairs of chromosomes with a maternal and a paternal copy, besides the sex chromosomes.
3. The sex of a human child is decided by the chromosome inherited from the mother.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). Every child gets an X from the mother; the father's X or Y decides.
6. The wings of a bat and the wings of a bird are
(a) homologous, since both are forelimbs used for flight.
(b) analogous, since they serve the same use but are built differently.
(c) vestigial organs.
(d) evidence that birds evolved from bats.
Answer: (b). Bat wings are skin between elongated fingers, bird wings feathers along the arm.
7. With reference to the GenomeIndia data released in January 2025, consider the following statements:
1. The genome data is archived at the Indian Biological Data Centre (IBDC).
2. The project is funded by the Department of Biotechnology.
3. The project was approved in 2023.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). In January 2025 the Prime Minister said the project had been approved five years earlier.
8. According to NCERT, which of the following changes can be passed on to the next generation?
(a) Low body weight of beetles starved by a plant disease
(b) Absence of tails in mice whose tails were cut off
(c) A change in DNA copied into the germ cells
(d) Skills learnt by an individual during its lifetime
Answer: (c). Changes in non-reproductive tissues cannot reach the DNA of the germ cells.
9. Which of the following can contribute to the formation of a new species, as described in NCERT's 2020-21 text?
1. A geographical barrier that reduces gene flow
2. Genetic drift
3. Natural selection acting differently on the separated populations
4. Inheritance of characters acquired during an individual's lifetime
Select the correct answer:
(a) 1, 2 and 3 only
(b) 1 and 4 only
(c) 2, 3 and 4 only
(d) 1, 2, 3 and 4
Answer: (a). Acquired traits are not inherited (section 9.3.2).
10. NCERT says evolution "should not be equated with 'progress'". Explain this with NCERT's examples, and say what it implies for the idea of human races. (150 words)
Approach: the common ancestor of humans and chimpanzees; bacteria still thriving in extreme habitats; diversity shaped by environmental selection; ladder versus tree; all humans one species, out of Africa.
📦 Revision Capsule
Hard Facts
- Mendel (1822-1884): garden peas; first to count individuals with each trait in each generation.
- Tall × short: F1 all tall; F2 one quarter short; genotypes 1 TT : 2 Tt : 1 tt.
- Two-trait F2: 315 / 108 / 101 / 32 of 556 = 9:3:3:1.
- Gene = section of DNA for one protein.
- Humans: 22 pairs + XX (women) or XY (men); father decides the child's sex.
- Haldane 1929; Miller and Urey 1953 (15% of methane's carbon into simple compounds including amino acids in a week).
- GenomeIndia: 10,000 genomes archived at IBDC (9 Jan 2025); DBT-funded; 9,772 samples' files usable (30 Apr 2025).
Core Concepts
- Variation builds up over generations; sexual reproduction multiplies it.
- Two copies of each gene; dominant shows, recessive hides.
- Genes on separate chromosomes are inherited independently.
- Gene → enzyme → hormone → height.
- Natural selection adapts; genetic drift changes gene frequency by chance.
- Acquired traits are not inherited.
- Speciation needs reduced gene flow plus divergence.
- Evolution is branching, not progress.
Confused Pairs
- Homologous (same origin, different use) vs analogous (same use, different origin).
- Natural selection vs artificial selection (wild cabbage).
- Genetic drift vs natural selection.
- Acquired vs inherited traits.
- Class X (2020-21) "a couple of hundred thousand years ago" left Africa vs Class XII 75,000-10,000 years ago, modern Homo sapiens arose.
Data Points
- 3 : 1 and 1 : 2 : 1 (one trait); 9 : 3 : 3 : 1 (two traits).
- 556 seeds in NCERT's figure.
- 10,000 genomes; target 10 million (Ministry of Science and Technology, 9 Jan 2025).
PYQ Pattern
- Prelims 2026: statement question on the Genome India Project (part of the Human Genome Project? funded by DBT? aim of the project).
- No Mains question in the bank matches this chapter directly.
Sources
- NCERT, Science, Textbook for Class X, ch. 8 "Heredity", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, Answers, Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021, ch. 9 "Heredity and Evolution" and Answers — Wayback Machine.
- NCERT, Biology, Textbook for Class XII, ch. 6 "Evolution" — ncert.nic.in PDF.
- PIB (Ministry of Science and Technology), "India Takes a Giant Leap in Genomics: Launch of Indian Genomic Data Set & IBDC Portals to Empower Global Research", 9 January 2025 — pib.gov.in.
- PIB (Prime Minister's Office), "English rendering of PM's remarks at the start of GenomeIndia Project", 9 January 2025 — pib.gov.in.
- PIB (Ministry of Science and Technology), "GenomeIndia", 30 April 2025 — pib.gov.in.
