NCERT Textbook Questions & Answers + Important Extra Questions
Section A: In-Text Questions
If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer
Trait B is likely to have arisen earlier. In asexual reproduction, traits spread as the population grows from one generation to the next. A trait that has been present for a longer time would have spread to a larger proportion of the population. Since trait B is found in 60% of the population (compared to only 10% for trait A), it has had more time to spread, indicating it arose earlier.
How does the creation of variations in a species promote survival?
Answer
Variations are differences in traits among individuals of the same species. They promote survival because:
(i) When the environment changes dramatically (e.g., severe temperature change, new disease), not all individuals are affected equally — some variants may be better suited to survive in the new conditions.
(ii) These surviving variants can reproduce and pass on their advantageous traits to the next generation, preventing the species from going extinct.
(iii) Without variation, the entire population would be equally vulnerable to any environmental change, risking complete extinction.
Example: If a population of bacteria has some heat-resistant variants and a sudden heat wave occurs, those variants will survive while the rest perish — the species continues.
How do Mendel's experiments show that traits may be dominant or recessive?
Answer
Mendel crossed pure tall pea plants (TT) with pure dwarf pea plants (tt):
F₁ generation: All plants were tall — the tall trait appeared while the dwarf trait seemed to disappear. This shows that tallness is dominant over dwarfness.
F₂ generation (self-pollination of F₁): Both tall and dwarf plants appeared in a 3:1 ratio (3 tall : 1 dwarf).
This proves that:
• The dwarf trait was not lost in F₁ — it was merely hidden (recessive) in the presence of the dominant tall trait.
• The F₁ plants were Tt (heterozygous) — carrying one dominant (T) and one recessive (t) allele.
• The dominant trait expresses itself even when only one copy is present; the recessive trait only appears when both copies are recessive (tt).
How do Mendel's experiments show that traits are inherited independently?
Answer
Mendel performed a dihybrid cross — crossing plants with two contrasting pairs of traits simultaneously. He crossed round yellow-seeded plants (RRYY) with wrinkled green-seeded plants (rryy):
F₁: All plants had round yellow seeds (both traits dominant).
F₂: Four types appeared — Round Yellow, Round Green, Wrinkled Yellow, Wrinkled Green — in a 9:3:3:1 ratio.
The appearance of new combinations (round green and wrinkled yellow) that did not exist in either parent proved that seed shape and seed colour are inherited independently of each other. If they were linked, only the parental combinations would appear. This is known as Mendel's Law of Independent Assortment.
A man with blood group A marries a woman with blood group O. Their daughter has blood group O. Is this information enough to tell you which of the traits — blood group A or O — is dominant? Why or why not?
Answer
No, this information alone is not sufficient to conclusively determine dominance. Here's why:
The father has blood group A, but he could be either homozygous (I^A I^A) or heterozygous (I^A i). The mother has blood group O, so she is ii (homozygous recessive).
If the father is I^A i, the possible offspring are I^A i (blood group A) and ii (blood group O). The daughter being O (ii) simply means she inherited the recessive allele from each parent.
We know from genetics that A is dominant over O, but from this single cross, we can only confirm that the father must be heterozygous (I^A i). We cannot determine dominance from just one family's data without additional crosses.
How is the sex of the child determined in human beings?
Answer
In humans, sex is determined by the sex chromosomes:
• Females have two X chromosomes (XX)
• Males have one X and one Y chromosome (XY)
During gamete formation:
• The mother (XX) produces eggs that all contain one X chromosome
• The father (XY) produces sperm — half carry X and half carry Y
At fertilisation:
• If an X-bearing sperm fertilises the egg
⇒ XX
⇒ Girl
• If a Y-bearing sperm fertilises the egg
⇒ XY
⇒ Boy
Therefore, it is the father's sperm that determines the sex of the child, not the mother. There is an equal probability (50:50) of having a boy or a girl.
What are the different ways in which individuals with a particular trait may increase in a population?
Answer
Three main ways:
(i) Natural selection: If a trait provides a survival advantage in a given environment, individuals with that trait live longer, reproduce more, and pass the trait to more offspring. Over generations, the proportion of individuals with that trait increases. Example: Dark-coloured moths increasing during industrial pollution.
(ii) Genetic drift: In small populations, random events (accidents, natural disasters) can eliminate some individuals irrespective of their traits. By chance, a particular trait may become more or less common — this is called genetic drift. It is especially significant in small, isolated populations.
(iii) Favourable environmental conditions: When resources are abundant, individuals with certain traits may reproduce disproportionately, increasing the frequency of those traits.
Why are traits acquired during the lifetime of an individual not inherited?
Answer
Acquired traits are changes that occur in the body (somatic) cells during an individual's lifetime due to environmental factors, lifestyle, or practice. These changes do not alter the DNA in the reproductive (germ) cells. Since only genetic information present in germ cells (sperm and eggs) is passed to offspring, changes in body cells are not inherited.
Example: A person who builds muscles through exercise will have muscular children only if the children also exercise — the muscle growth is not coded in the parent's DNA and cannot be passed on. Similarly, if a mouse's tail is cut, its offspring will still be born with tails.
Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
Answer
Very small populations face serious genetic problems:
(i) Reduced genetic diversity: With fewer individuals, there are fewer different gene variants (alleles) in the population. This limits the range of variations available for adaptation.
(ii) Inbreeding: In small populations, closely related individuals are forced to mate, leading to inbreeding depression — offspring are more likely to inherit harmful recessive traits, leading to genetic disorders, reduced fertility, and weakened immunity.
(iii) Genetic drift: Random events can have disproportionately large effects on gene frequencies in small populations, potentially eliminating important alleles by chance.
(iv) Reduced adaptability: With fewer variations, the species has a lower chance of surviving future environmental changes, increasing extinction risk.
What factors could lead to the rise of a new species?
Answer
A new species (speciation) can arise through the combination of several factors:
(i) Geographical isolation: Physical barriers (mountains, rivers, oceans) separate a population into groups that can no longer interbreed.
(ii) Genetic drift: Random changes in allele frequencies in small, isolated populations lead to genetic differences over time.
(iii) Natural selection: Different environmental conditions in different locations favour different traits in the separated populations.
(iv) Accumulated variations: Over many generations, genetic differences accumulate to the point where the two groups can no longer interbreed even if brought together — they have become separate species.
(v) Reproductive isolation: Differences in mating behaviour, timing, or physical incompatibility prevent interbreeding.
Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
Answer
No, geographical isolation will not be a major factor for self-pollinating plant species. In self-pollination, the pollen from a flower fertilises the egg of the same flower or the same plant — there is no gene flow between different individuals even when they grow together. Since each plant effectively reproduces independently with its own genes, physical separation does not create any additional genetic barrier. Variations in self-pollinating plants arise mainly through mutations, not through reproductive isolation.
Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
Answer
No, geographical isolation is not a significant factor for asexually reproducing organisms. In asexual reproduction, there is no mating or gene exchange between individuals — new organisms are clones of the parent. Since there is no sexual recombination to generate new gene combinations, geographic separation does not prevent any gene flow (because there was none to begin with). Variations in asexual organisms arise only through random mutations, and these spread clonally, independent of geographical barriers.
How do fossils help in determining evolutionary relationships?
Answer
Fossils are preserved remains or impressions of organisms that lived in the past. They provide evidence for evolution by:
(i) Showing that simpler organisms existed earlier and more complex ones evolved later — establishing a timeline of increasing complexity.
(ii) Revealing transitional forms — organisms with features of two different groups (e.g., Archaeopteryx had features of both reptiles and birds), proving evolutionary links between groups.
(iii) Fossil dating (using rock layer depth and radioactive dating) tells us when different organisms existed and in what sequence.
(iv) Comparing fossil organisms with living organisms helps trace how specific body structures changed over millions of years.
What evidence do we have for the origin of life from inanimate matter?
Answer
In 1953, Stanley Miller and Harold Urey performed a landmark experiment. They recreated conditions thought to exist on early Earth:
• Mixed gases: methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapour in a closed flask
• Applied electric sparks (simulating lightning) at 800°C for about a week
• Result: Simple amino acids (building blocks of proteins) and other organic molecules were synthesised from these inorganic gases.
This experiment proved that the basic molecules of life could have formed spontaneously from non-living matter under the conditions of primitive Earth — supporting the chemical evolution theory of the origin of life.
Section B: NCERT Exercise Questions
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:
Answer
(a) TTWW (b) TTww (c) TtWW (d) TtWw
Ans. (c) TtWW. Since all offspring have violet flowers, the tall parent must be homozygous dominant for flower colour (WW). Since half the offspring are short, the tall parent must be heterozygous for height (Tt). Cross: TtWW × ttww
⇒ offspring: TtWw (tall violet) and ttWw (short violet) in roughly 1:1 ratio for height, all violet for colour.
Q2. 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
Ans. (d) All of the above. Homologous organs have the same structural origin but may perform different functions. Our arm and a dog's leg both evolved from the same ancestral limb; our teeth and elephant tusks are both modified teeth; potato (underground stem) and grass runners (horizontal stem) are both modified stems.
Q3. In evolutionary terms, we have more in common with
(a) a Chinese school-boy (b) a chimpanzee (c) a spider (d) a bacterium
Ans. (a) A Chinese school-boy. All human beings belong to the same species (Homo sapiens), regardless of race or nationality. We share the maximum genetic similarity with other humans. A chimpanzee is our closest animal relative (~98.7% DNA similarity), but another human shares >99.9% of our DNA.
A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?
Answer
We cannot conclusively determine dominance from this observation alone. If light eye colour were recessive, then light-eyed children would indeed always have at least one light-eyed gene from each parent (who may be carriers). If it were dominant, light-eyed parents could also have light-eyed children. The pattern described (light-eyed children having light-eyed parents) is consistent with both possibilities. However, since dark-eyed parents can also have light-eyed children (if both are carriers), the overall pattern in the population suggests that light eye colour is recessive in most cases — but the given information alone is insufficient for a definitive answer.
How are the areas of study — evolution and classification — interlinked?
Answer
Evolution and classification are deeply connected:
(i) Classification groups organisms based on similarities and differences in their body structures, functions, and genetic makeup.
(ii) Organisms with more similarities are placed in the same group — this reflects their shared ancestry (common evolutionary origin).
(iii) The fewer differences between two species, the more recently they diverged from a common ancestor.
(iv) Classification is essentially an attempt to map evolutionary relationships — a classification chart mirrors the evolutionary tree of life.
In short, evolution explains why organisms share similarities (common descent), and classification organises organisms based on those shared evolutionary features.
Explain the terms analogous and homologous organs with examples.
Answer
Homologous organs: Organs that have the same structural origin and basic plan but may have been modified to perform different functions in different organisms. They indicate common ancestry (divergent evolution).
Example: The forelimb of a human (grasping), whale (swimming flipper), bat (flying wing), and horse (running) all have the same bone pattern (humerus, radius, ulna, carpals, metacarpals, phalanges) despite different functions.
Analogous organs: Organs that have different structural origins but perform the same function. They evolved independently in unrelated organisms (convergent evolution).
Example: Wings of a butterfly (extension of exoskeleton) and wings of a bird (modified forelimb with feathers) — both used for flying but structurally very different.
Outline a project which aims to find the dominant coat colour in dogs.
Answer
Project outline:
(i) Survey: Observe and record the coat colours of dogs in a local area. Note which colours are most common.
(ii) Select parents: Choose pure-breeding dogs of two different coat colours (e.g., pure black × pure white). Ensure they are pure by checking that their parents and grandparents had the same colour.
(iii) Cross (F₁): Breed the selected parents and record the coat colour of all F₁ puppies. The colour that appears in all F₁ offspring is dominant.
(iv) Cross F₁ (F₂): Allow F₁ dogs to breed among themselves. Observe the F₂ generation for the reappearance of the hidden colour. A 3:1 ratio (dominant:recessive) would confirm the dominance pattern.
(v) Test cross: Cross F₁ dogs with pure recessive-coloured dogs. A 1:1 ratio in offspring confirms F₁ are heterozygous.
(vi) Conclude: The trait appearing in all F₁ dogs is dominant.
Explain the importance of fossils in deciding evolutionary relationships.
Answer
Fossils are crucial for understanding evolution because:
(i) They provide physical evidence of organisms that lived in the past but no longer exist.
(ii) Stratified rock layers: Fossils found in deeper (older) rock layers represent earlier life forms; those in upper (newer) layers represent more recent organisms. This reveals the chronological sequence of evolution.
(iii) Transitional fossils (like Archaeopteryx) bridge the gap between two groups, showing evolutionary links (e.g., between dinosaurs and modern birds).
(iv) Comparing the complexity of organisms across geological time shows a clear trend from simple to complex life forms.
(v) Fossils of the same species found on different continents (e.g., Mesosaurus fossils in both Africa and South America) support the theory of continental drift and common ancestry.
What evidence do we have for the origin of life from inanimate matter?
Answer
The strongest evidence comes from the Miller-Urey experiment (1953):
• They simulated early Earth conditions: a closed system with methane, ammonia, hydrogen gas, and water vapour, subjected to electric sparks (simulating lightning) for a week.
• Result: Amino acids and other simple organic molecules were formed from purely inorganic starting materials.
• This demonstrated that the building blocks of life could form spontaneously under primordial Earth conditions without any biological intervention.
Additionally, organic molecules have been found in meteorites and interstellar dust clouds, supporting the idea that basic chemistry of life is a natural process.
Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?
Answer
Sexual reproduction generates more variations through:
(i) Meiosis: During gamete formation, chromosome number is halved and crossing over exchanges genetic material between homologous chromosomes, creating new gene combinations.
(ii) Random segregation: Each gamete receives a random mix of maternal and paternal chromosomes.
(iii) Random fertilisation: Any sperm can fertilise any egg, multiplying the possible genetic combinations enormously.
(iv) Two-parent contribution: Offspring receive genes from two genetically different parents, increasing diversity.
In asexual reproduction, the only source of variation is mutation — a much rarer and less diverse source.
Impact on evolution: Sexually reproducing organisms evolve faster because they have a larger pool of genetic variations for natural selection to act upon. This allows them to adapt more quickly to changing environments, making them more successful in the long run.
How is the equal genetic contribution of male and female parents ensured in the offspring?
Answer
Equal contribution is ensured through the mechanism of meiosis and fertilisation:
• Normal body cells are diploid (2n) — they contain two sets of chromosomes (one from each parent).
• During meiosis, the chromosome number is halved, producing haploid (n) gametes — sperm cells in males and egg cells in females, each carrying only one set of chromosomes.
• At fertilisation, a haploid sperm (n) fuses with a haploid egg (n) to restore the diploid number (2n) in the zygote.
Since each parent contributes exactly one set of chromosomes (half the total), the offspring receives 50% of its genetic material from the father and 50% from the mother, ensuring equal contribution.
Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Answer
Not entirely. While advantageous variations do increase in frequency through natural selection (because individuals with beneficial traits survive and reproduce more), this is not the only mechanism:
(i) Many variations are neutral — they neither help nor harm the individual. These can persist in the population simply through genetic drift (random chance), especially in small populations.
(ii) Some mildly disadvantageous traits may also survive if they are linked to other advantageous genes or if their negative effects are not severe enough to affect survival significantly.
(iii) Only extremely harmful or lethal variations are consistently eliminated by natural selection.
So, it's more accurate to say that disadvantageous variations tend to be eliminated, while both advantageous and neutral variations can survive in a population.