Q.Mendel crossed a homozygous pea plant having yellow and round seeds with another pea plant bearing green and wrinkled seeds. He found that in some of the F2 population new combination of parental characters were observed. How will you explain the appearance of a new combination of parental characters in F2-offsprings ? Support your answer with the help of Punnett square.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Mendel's Law of Independent Assortment
Imagine you are packing two different suitcases for a trip. One suitcase is for clothes, the other for books. How you arrange the shirts inside the clothes suitcase has nothing to do with how you stack the books in the book suitcase. The two packing jobs happen independently.
That is the core intuition behind Mendel's Law of Independent Assortment. It says that when an organism passes on its genes to its offspring, the inheritance of one trait (like seed colour) does not influence the inheritance of another, separate trait (like seed shape). Each trait gets its own "lottery ticket" during reproduction, and the tickets are drawn separately.
The precise meaning
Gregor Mendel, working with pea plants in the 19th century, noticed that certain pairs of characteristics — for example, seed colour (yellow or green) and seed shape (round or wrinkled) — were inherited completely independently of each other. A plant could pass on a yellow colour gene and a wrinkled shape gene together, or a green colour and a round shape, or any combination. The chance of getting a particular colour was always 50-50 (in a certain cross), and the chance of getting a particular shape was also 50-50, and these chances did not affect each other.
The Law of Independent Assortment applies only to genes that are located on different chromosomes (or very far apart on the same chromosome). If two genes are on the same chromosome, they tend to be inherited together — that is called linkage, and it is an exception to this law. NCERT textbooks clearly state this limitation.
Why it matters
This law explains the enormous variety we see in living things. Because traits are shuffled independently, a single pair of parents can produce offspring with many different combinations of characteristics. For a human student, think of it this way: your eye colour and your hair colour are not tied together by fate. You could have your mother's eyes and your father's hair, or vice versa, or a mix. Independent assortment is one of the main reasons siblings (except identical twins) look different from each other.
In the NCERT Class 12 Biology textbook, Mendel's Law of Independent Assortment is stated as:
"When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters."
This means that during the formation of eggs and sperm (gametes), the alleles (versions of a gene) for one trait separate into gametes without being influenced by the alleles for another trait.
A few key points to remember
- It is the second law Mendel discovered (the first is the Law of Segregation, which says each organism has two copies of each gene and passes one copy randomly to offspring).
- It explains why a dihybrid cross (a cross involving two traits) yields a 9:3:3:1 ratio of offspring types — but you do not need to memorise that ratio for a prose subject. …
Part (b)Concept understanding — Darwinian Natural Selection
Imagine you walk into a crowded room. Some people are naturally louder, some are quieter. Some are taller, some shorter. Now imagine that, for some reason, the room is suddenly plunged into darkness, and everyone has to find the exit by touch alone. Who do you think will get out first? Probably not the loudest talker, but the person who happens to have the most sensitive fingertips or the best memory of where the door was.
That simple scenario captures the core of Darwinian natural selection. It is not about being "better" in some moral or absolute sense. It is about being a better fit for the specific situation you are in.
The Everyday Intuition: "Survival of the Fittest" — But What Does "Fittest" Mean?
The phrase "survival of the fittest" is often misunderstood. It does not mean the strongest, fastest, or most aggressive individual wins. In biology, "fitness" has a very specific meaning: the ability to survive long enough to reproduce and leave offspring.
Think of it this way: in a game of musical chairs, the "fittest" player isn't the one who dances the best. It's the one who, when the music stops, is standing on a chair. The chair is the environment. The player's ability to grab that chair is their fitness.
Darwin never used the phrase "survival of the fittest" in his first edition of On the Origin of Species. It was coined by Herbert Spencer, a philosopher, and Darwin adopted it in later editions. Darwin's own term was "natural selection," which is a more accurate description of the process.
The Precise Meaning: How Natural Selection Works
Natural selection is not a conscious force. It is a blind, automatic process that follows from three simple facts that are always true in any population of living things:
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Variation: Individuals in a population are not identical. Even within a species, there is a range of traits — different beak sizes in birds, different fur colours in rabbits, different heights in humans. This variation is the raw material.
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Inheritance: Many of these variations are passed from parents to offspring. A tall parent is more likely to have tall children. A fast-running cheetah is more likely to have fast-running cubs.
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Differential Survival and Reproduction: More individuals are born than can possibly survive. Resources (food, water, shelter, mates) are limited. This creates a "struggle for existence." Individuals with traits that give them even a slight advantage in this struggle are more likely to survive and, crucially, to reproduce.
The result: Over many generations, the traits that helped those individuals survive and reproduce become more common in the population. The traits that were less helpful become rarer. The population gradually changes to become better suited to its environment.
Natural selection does not create new traits. It only acts on the variation that already exists. It is like a sieve, not a sculptor. The sieve lets through the grains of sand that are the right size for the hole, and holds back the rest. The environment is the sieve.
Why It Matters: The Engine of Adaptation
Natural selection is the mechanism that explains how life becomes adapted to its surroundings. It is why:
- A cactus has spines instead of leaves (to reduce water loss in a desert).
- A polar bear has white fur (to blend in with snow and ice).
- A human has an opposable thumb (to grip tools).
It is not a plan or a goal. It is a consequence of simple, observable facts playing out over vast stretches of time. The NCERT textbook for Class 12 Biology (Chapter 6, "Evolution") states this clearly: "Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny."
A Few Key Points to Remember
- It acts on individuals, but it changes populations. An individual does not evolve. The population as a whole changes over generations. …
Part (a)
The cross is a dihybrid cross for seed colour (yellow Y dominant over green y) and seed shape (round R dominant over wrinkled r).
- P: yellow-round YYRR × green-wrinkled yyrr; F1: all YyRr (yellow, round).
- By Mendel's Law of Independent Assortment, the colour gene and shape gene segregate independently, so the F1 makes four gamete types in equal numbers: YR, Yr, yR, yr.
Punnett square (F1 YyRr × YyRr):
| YR | Yr | yR | yr | |
|---|---|---|---|---|
| YR | YYRR | YYRr | YyRR | YyRr |
| Yr | YYRr | YYrr | YyRr | Yyrr |
| yR | YyRR | YyRr | yyRR | yyRr |
| yr | YyRr | Yyrr | yyRr | yyrr |
Part (a): the new combinations (yellow-wrinkled, green-round) arise by Mendel's law of independent assortment, which makes the YyRr F1 form four equally frequent gametes and yields a 9:3:3:1 F2 ratio. Part (b): Miller sparked CH₄, NH₃, H₂ and water vapour to mimic the early reducing atmosphere and lightning, obtained amino acids, and gave experimental support to the Oparin–Haldane theory of chemical evolution.
Part (a)
Mendel here followed two characters together — seed colour (yellow/green) and seed shape (round/wrinkled) — a dihybrid cross.
- Parents: homozygous yellow-round (YYRR) × homozygous green-wrinkled (yyrr).
- F1: all YyRr, expressing the dominant phenotypes yellow and round.
The decisive step is gamete formation in the F1. Because the colour gene and the shape gene lie on different chromosomes, they assort independently: how Y/y separates has no bearing on how R/r separates. So each F1 plant produces four gamete types in equal proportion — YR, Yr, yR, yr. If the two genes had stayed linked together, only the parental gametes YR and yr would form and no new combinations could appear.
Random fusion of these gametes is shown in the Punnett square:
| YR | Yr | yR | yr | |
|---|---|---|---|---|
| YR | YYRR | YYRr | YyRR | YyRr |
| Yr | YYRr | YYrr | YyRr | Yyrr |
| yR | YyRR | YyRr | yyRR | yyRr |
| yr | YyRr | Yyrr | yyRr | yyrr |
Counting phenotypes gives the classic 9:3:3:1 ratio:
- 9 yellow-round (parental type)
- 3 yellow-wrinkled (new combination)
- 3 green-round (new combination)
- 1 green-wrinkled (parental type) …
Showing the 12 most recent of 36 on this concept.
- CBSE 2026Set 57/1/11 markMCQQ.Assertion (A) : The population of melanized moths increased in industrial areas after Industrial Revolution. Reason (R) : In Industrial environment lichen covered trees were replaced by soot-covered trees offering better camouflage to dark coloured moths. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
The assertion and reason are both true, and the reason correctly explains why melanized moths became more common in industrial areas after the Industrial Revolution.
This question draws on one of the most famous and well-documented examples of natural selection in action — the peppered moth (Biston betularia) in England. The story is a classic illustration of how environmental change can drive evolutionary change, and it is directly covered in the NCERT Class 12 Biology textbook under the chapter on Evolution.
Before the Industrial Revolution, the typical peppered moth had a light, speckled appearance that blended beautifully with the lichen-covered bark of trees. This camouflage protected them from predatory birds. A dark, melanic form of the same species existed but was very rare, because it stood out starkly against the pale lichen and was easily spotted and eaten.
The Industrial Revolution changed the landscape dramatically. Soot and smoke from coal-burning factories coated trees and killed the lichen, turning the bark dark. Now the situation reversed: the light-coloured moths became highly visible against the soot-blackened trees, while the dark (melanized) moths were well-camouflaged. Birds now ate more of the light moths, and the dark moths survived and reproduced in greater numbers. Over several decades, the population shifted from mostly light to mostly dark in industrial areas.
NoteThis phenomenon is called industrial melanism. It is a textbook case of natural selection acting on a heritable trait, not a change acquired during the moth's lifetime.
Now look at the Assertion (A): "The population of melanized moths increased in industrial areas after Industrial Revolution." This is exactly what happened — the dark form became dominant in polluted regions. …
- CBSE 2026Set 57/3/11 markMCQQ.Appearance of antibiotic-resistant bacteria is an example of evolution due to : (A) Adaptive radiation (B) Divergent evolution (C) Artificial selection (D) Anthropogenic action
›Reveal solutionSolution
Antibiotic-resistant bacteria evolve through anthropogenic action — human use of antibiotics creates the selective pressure that favours resistant strains, a direct example of evolution driven by human activity.
Darwin's theory of natural selection rests on a simple but powerful idea: organisms with traits better suited to their environment survive and reproduce more successfully, passing those advantageous traits to the next generation. Over time, populations change. The environment acts as the selective agent, and the result is evolution.
Now consider what happens when we introduce antibiotics into bacterial populations. Before antibiotics, a bacterial colony contains natural genetic variation — most individuals are susceptible to the drug, but a tiny fraction carry random mutations that confer resistance. These resistant bacteria have no particular advantage in the absence of antibiotics; they're just different.
The moment we administer an antibiotic, the landscape shifts entirely. The drug kills or inhibits the susceptible bacteria, but the resistant ones survive. Suddenly, resistance is no longer a neutral quirk — it's a life-or-death advantage. The resistant bacteria reproduce freely in the now-emptied niche, and within generations the population is dominated by resistant strains. The antibiotic didn't create the resistance; it simply selected for bacteria that already possessed it.
This is evolution in fast-forward, and the selective pressure is unmistakably human in origin. We manufacture the antibiotics, we prescribe them, we use them in agriculture, and we often misuse them by stopping treatment early or deploying them unnecessarily. Each use is an evolutionary experiment, and bacteria — with their rapid generation times — respond with breathtaking speed.
Let's see why the other options don't fit:
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Adaptive radiation describes the rapid diversification of a single ancestral species into multiple forms, each adapted to different ecological niches — think Darwin's finches spreading across the Galápagos. Antibiotic resistance doesn't involve branching into diverse new species; it's a single trait spreading through existing populations.
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Divergent evolution occurs when related species accumulate differences over time as they adapt to different environments, eventually becoming distinct. Again, we're not watching bacterial species diverge into separate lineages here; we're watching one trait sweep through a population. …
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- CBSE 2026Set 57/3/11 markMCQQ.Identify the statements that correctly describe Darwin's Theory of Evolution.(i) Overproduction of organisms leads to competition.(ii) Variation is inherited and causes evolution.(iii) Acquired characters are inherited.(iv) Survival depends on favourable traits.(v) New species arise due to accumulation of favourable variations. Choose the correct option. (A) (i), (ii),(iv) and(v) are correct (B) (ii),(iii) and(iv) are correct (C) All are correct (D) Only(i) and(iv) are correct
›Reveal solutionSolution
Darwin’s theory rests on overproduction, inherited variation, survival of the fittest, and the gradual accumulation of favourable variations — but it explicitly rejects the inheritance of acquired characters.
Darwin’s theory of evolution by natural selection is one of the most elegant and powerful ideas in biology. It explains how life changes over time without needing any guiding hand or purpose. The core logic is simple: organisms produce more offspring than can possibly survive, those offspring vary in their traits, and the environment “selects” the variants best suited to it. Over many generations, this process can give rise to entirely new species.
Let’s examine each statement in the question against what Darwin actually proposed.
Statement (i): Overproduction of organisms leads to competition.
This is absolutely correct. Darwin observed that every species has the potential to produce far more young than the environment can support. A single pair of elephants, for example, could in theory produce millions of descendants in a few centuries — yet elephant numbers remain roughly stable. The inevitable result is a struggle for existence: competition for food, shelter, mates, and safety. Overproduction sets the stage for natural selection.
Statement (ii): Variation is inherited and causes evolution.
Also correct. Darwin knew that individuals within a species differ from one another, and that many of these differences are passed from parents to offspring. He called this “inherited variation.” Without heritable variation, natural selection would have nothing to act upon — all individuals would be identical, and no change could occur. Evolution happens precisely because favourable variations are inherited and spread through a population.
Statement (iii): Acquired characters are inherited.
This is the one statement that does not belong to Darwin’s theory. The idea that traits acquired during an organism’s lifetime (like a blacksmith’s strong arm or a giraffe’s stretched neck) can be passed to offspring was proposed by Jean-Baptiste Lamarck, not Darwin. Darwin himself was unsure about the mechanism of inheritance, but he never made the inheritance of acquired characters a pillar of his theory. Modern genetics has shown that such acquired traits are not inherited — changes in body cells do not affect the DNA in eggs or sperm.
Watch outA common exam mistake is to confuse Lamarck’s idea with Darwin’s. Remember: Darwin said variation arises by chance and is then selected; Lamarck said organisms change in response to need and pass those changes on. They are fundamentally different.
Statement (iv): Survival depends on favourable traits. …
- CBSE 2026Set A1 markMCQQ.How many types of gametes would be formed if the genotype of plant is AaBbCc?(a) Two(b) Four(c) Eight(d) Sixteen
›Reveal solutionSolution
Number of gamete types = 2^n where n = number of heterozygous gene pairs; here n = 3, so 2^3 = 8.
Each heterozygous gene pair segregates independently during meiosis, giving 2 kinds of gamete for that gene. For AaBbCc there are 3 heterozygous pairs, so the total numb …
- CBSE 2026Set ANNUAL1 markQ.Which theory was given by Charles Darwin?
›Reveal solutionSolution
Darwin's theory of natural selection explains that organisms with heritable traits better suited to their environment survive and reproduce more successfully, gradually driving evolutionary change.
Charles Darwin, along with Alfred Russel Wallace, proposed the theory of Natural Selection, formally published by Darwin in 1859 in his book On the Origin of Species.
The key ideas of this theory are:
- Individuals within a population show heritable variations.
- Populations tend to produce more offspring than the environment can support, leading to a struggle for existence.
- Individuals with variations better suited ('fitter') to their environment are more likely to survive and reproduce — 'survival of the fittest'. …
- CBSE 2026Set ANNUAL1 markMCQQ.For a long time, it was believed that the organisms were fixed and unchanging. Which theory was proposed by Charles Darwin to challenge this belief?(a) Theory of Pangenesis(b) Theory of Use and Disuse(c) Theory of Natural Selection(d) Theory of Inheritance of Acquired Characters
›Reveal solutionSolution
Darwin's Theory of Natural Selection (published in 'On the Origin of Species', 1859) proposed that organisms change over generations through differential survival and reproduction of the fittest variants.
Darwin observed that organisms produce more offspring than the environment can support, that individuals show heritable variations, and that individuals better suited to the environment survive and reproduce more successfully ('survival of the fittest'). Over generations, favourable variations accumulate, driving evolutionary change. This directly challenged the older belief that species were fixed and unchanging.
…
- CBSE 2026Set ANNUAL1 markMCQQ.If a plant has genotype AaBb, what will be the possible number of gametes it may produce?(a) 1(b) 2(c) 3(d) 4
›Reveal solutionSolution
Number of gamete types = 2ⁿ, where n = number of heterozygous gene pairs. For AaBb, n = 2, so 2² = 4 gamete types.
A plant with genotype AaBb is heterozygous at two independently assorting gene loci (A/a and B/b). During meiosis, each gene pair segregates independently (Law of Independent Assortment), and the alleles combine randomly into gametes. The possible combinations are:
AB, Ab, aB, ab
…
- CBSE 2026Set ANNUAL1 markMCQQ.Identify the effect brought about by single step large mutation in evolution from the options given below :(a) Saltational speciation(b) Founder effect(c) Gene migration(d) Evolution by special creation
›Reveal solutionSolution
Speciation brought about by a single-step large mutation is called saltation (saltational speciation).
While Darwinian evolution emphasises gradual accumulation of small heritable variations, Hugo de Vries (working on Oenothera, the evening primrose) argued that mutation caused evolution and that large, sudden, single-step mutations — which he called saltation — could produce a new species in one step, rather than by slow, minor variations. Thus the effect of a single-step large mutation is saltational speciation.
…
- CBSE 2025Set A1 markQ.Match the correct pair and write the match for 'Darwin'. Column I:(i) Apomixis(ii) Darwin(iii) Toddy(iv) Agarose(v) Detritivorous. Column II:(a) Galapagos Islands(b) Southern India(c) Sea weeds(d) Earthworm(e) Grass family.
›Reveal solutionSolution
Darwin matches with (a) Galapagos Islands, the site of his famous finch observations during the voyage of the Beagle.
Charles Darwin, during his voyage aboard HMS Beagle, observed several closely related but distinct species of finches on the different islands of the Galapagos archipelago, each with beak shapes adapted to the specific food sources available on its island. These observations of adaptive radiation from a common anc …
- CBSE 2025Set ANNUAL1 markMCQQ.The essence of Darwinian theory about evolution is -(a) Mutation(b) Spontaneous generation(c) Natural selection(d) Chemical change
›Reveal solutionSolution
Darwin proposed that heritable variations giving a survival/reproductive advantage are naturally selected and become more common over generations.
Darwin's theory rests on the observations that populations show heritable variation, more offspring are produced than can survive, and individuals best suited to the environment survive and reproduce more successfully ('survival of the fittest'). This differential, non-random survival and reproduction of variants — natural selection — is the essence of Darwinism, driving gradual evolutionary change. Mutation is a source of variation (emphasised later by neo- …
- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A): Evolution is not occurring at present. Reason (R): Evolution takes a very long time to occur.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).(c) Assertion (A) is true but Reason (R) is false.(d) Both Assertion (A) and Reason (R) are false.
›Reveal solutionSolution
Evolution is a continuous, ongoing process happening in populations right now, so the Assertion is false; and while large-scale (macro)evolutionary change is typically slow, evolutionary/genetic change at the population level (microevolution) can be observed within a few generations, so the blanket Reason is also not correct.
The Assertion claims evolution is not occurring at present — this is factually false. Evolution is a continuous process driven by ongoing mutation, genetic recombination, gene flow, genetic drift, and natural selection acting on populations every generation. Directly observable modern examples include the rapid rise of antibiotic-resistant bacteria, pesticide-resistant insects, and the classic industrial melanism seen in the peppered moth (Biston betularia) population within a human lifetime — all clear evidence that evolutionary change is happening today, not just in the deep geological past.
…
- CBSE 2025Set ZOOLOGY1 markMCQQ.Which of the following is the modern concept of origin of life?(i) Special creation(ii) Spontaneous generation(iii) Biogenesis(iv) Chemical evolution
›Reveal solutionSolution
The modern concept of the origin of life is chemical evolution — life arose abiotically from simple molecules under early-Earth conditions.
The modern concept, the theory of chemical evolution proposed by Oparin and Haldane, holds that the first forms of life arose from non-living inorganic and organic molecules (chemogeny). Under the conditions of the primitive Earth — reducing atmosphere, energy from UV rays, lightning and heat — simple molecules combined to form complex organic molecules, then aggregates (coacervates/protobionts), and finally the first cells. The Miller–Urey experiment provided experimental support …
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