Q.(a)
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Self Incompatibility Genetics
Imagine you are a plant. You cannot walk over to a neighbour and exchange pollen. Instead, you rely on wind, bees, or butterflies to carry your pollen to another flower. But here is the problem: if that pollen lands on your own stigma (the female part of the same flower, or a flower on the same plant), you might end up fertilising yourself. That would be like marrying your sibling — it leads to weak, inbred offspring. Plants have evolved a clever way to prevent this: self-incompatibility.
At its core, self-incompatibility is a plant's built-in "no-self-marriage" rule. It is a genetic mechanism that allows the pistil (the female organ) to recognise and reject pollen from the same plant (or a genetically identical plant). The pollen grain may land on the stigma, but the plant actively blocks it — the pollen tube fails to grow, or the ovule is not fertilised. The result is that only pollen from a different, genetically distinct plant can successfully fertilise the ovules.
This is not a random failure. It is a precise, genetically controlled rejection system. The NCERT textbook (Class 12 Biology, Chapter 2) describes it as "the inability of a plant with a functional male and female reproductive system to produce seeds when self-pollinated." In other words, the plant is perfectly fertile — it just refuses to mate with itself.
Self-incompatibility is not the same as male sterility. In male sterility, the plant produces no functional pollen. In self-incompatibility, the pollen is perfectly viable — it just gets rejected by the plant's own stigma.
Why does this matter? Because it forces cross-pollination — pollen from one plant to another. This ensures genetic diversity, which is the raw material for evolution. A diverse population is more resilient to diseases, pests, and changing climates. For farmers and plant breeders, self-incompatibility is both a challenge and a tool. It makes it harder to produce pure inbred lines (since the plant refuses to self-pollinate), but it also makes hybrid seed production easier — you can plant two varieties together and let nature enforce cross-pollination.
The genetics behind it can be simple or complex, but the core idea is this:
- There is a single gene (called the S-locus) with many different versions (alleles) in the population.
- If the pollen carries an S-allele that matches either of the two S-alleles in the pistil, the pollen is rejected.
- If the pollen carries a different S-allele (one not present in the pistil), it is accepted and fertilisation proceeds.
So, a plant with S1 and S2 alleles will reject pollen carrying S1 or S2, but accept pollen carrying S3, S4, S5, etc. This is called gametophytic self-incompatibility (common in many fruits like apples, pears, and tomatoes). There is also sporophytic self-incompatibility (seen in cabbage, mustard, and sunflowers), where the rejection is determined by the pollen parent's genetics, not the pollen grain's own S-allele. …
Part (b)Concept understanding — Human Reproduction
Human Reproduction: A First Look
Think about what makes you you. You have your mother's eyes, your father's laugh, a mix of traits from two people you may never have met — your grandparents. That chain of life, stretching back thousands of years, runs through one process: human reproduction. It is not a topic for biology students alone. It is the story of how every single person on this planet came to be here.
The Big Picture: Why Does Reproduction Exist?
Every living thing has one fundamental drive: to continue its kind. A mango tree grows fruit so its seeds can sprout elsewhere. A bacterium splits in two. For humans, reproduction is more complex — and more personal. It is the reason families exist, why you have siblings or cousins, and why populations grow or shrink.
At its simplest, human reproduction is the biological process by which a new human individual is created. It requires two parents — a male and a female — each contributing a special cell that fuses to form a single, new cell. That single cell then grows, divides, and develops into a baby over about nine months.
Reproduction is not the same as sex. Sex is an act; reproduction is the entire sequence from cell fusion to birth. Not every act of sex leads to reproduction, and reproduction can sometimes happen with medical help (like IVF) without sex.
The Two Key Players: Male and Female Reproductive Systems
Your body, whether male or female, has a set of organs designed specifically for reproduction. These are not the same as the organs that keep you alive — they are the "spare set" dedicated to creating the next generation.
The Male System: Production and Delivery
The male reproductive system has one main job: to produce male sex cells (sperm) and deliver them to the female body. The key parts are:
- Testes (two oval organs in a pouch called the scrotum): These are the factories. They produce sperm continuously from puberty onwards — millions every day. They also produce testosterone, the hormone that drives male physical changes at puberty.
- Duct system (a series of tubes): Sperm travel from the testes through a long coiled tube (epididymis) where they mature, then through a muscular tube (vas deferens) that carries them toward the urethra.
- Accessory glands (seminal vesicles, prostate gland, bulbourethral glands): These add fluids to the sperm to nourish them and help them swim. The mixture of sperm and these fluids is called semen.
- Penis: The organ that delivers semen into the female reproductive tract.
The Female System: Production, Reception, and Nurturing
The female system has a bigger job: it produces female sex cells (eggs), receives sperm, and if fertilization occurs, houses and nourishes the developing baby for nine months. The key parts are:
- Ovaries (two almond-sized organs): These produce eggs — but unlike sperm, eggs are not made continuously. A female is born with all the eggs she will ever have (about 1–2 million at birth, reducing to around 300,000–400,000 by puberty). Each month, one egg matures and is released.
- Fallopian tubes (also called oviducts): These are not connected directly to the ovaries. When an egg is released, the finger-like ends of the tube sweep it inside. Fertilization — the meeting of sperm and egg — happens here, in the tube.
- Uterus (womb): A hollow, pear-shaped muscular organ. If fertilization happens, the embryo implants in the lining of the uterus and grows here. If not, the lining is shed — that is menstruation.
- Cervix: The lower, narrow part of the uterus that opens into the vagina. It produces mucus that changes consistency during the month to help or hinder sperm.
- Vagina: A muscular canal that receives the penis during intercourse and serves as the birth canal during delivery.
The female reproductive system has a monthly cycle (the menstrual cycle) that prepares the body for a possible pregnancy. This cycle is controlled by hormones and typically lasts about 28 days. The release of the egg (ovulation) happens around day 14. If no pregnancy occurs, the uterine lining is shed as menstrual blood — this is a normal, healthy process, not a "curse" or something to be ashamed of.
The Moment of Creation: Fertilization
When sperm from the male meet the egg from the female inside a fallopian tube, one sperm may penetrate the egg's outer layer. That single moment — fertilization — creates a new cell called a zygote. This zygote contains 23 chromosomes from the mother and 23 from the father, making 46 in total. That is the complete genetic blueprint for a new human being.
The zygote immediately begins to divide as it travels down the fallopian tube toward the uterus. By the time it reaches the uterus (about 5–7 days later), it has become a ball of about 100 cells called a blastocyst. This blastocyst burrows into the soft, thick lining of the uterus — a process called implantation. That is when pregnancy truly begins.
The Nine-Month Journey: Development
Once implanted, the developing human is called an embryo for the first eight weeks, and then a fetus from the ninth week until birth. During these months:
- A special organ called the placenta develops, connecting the mother's blood supply to the fetus. It delivers oxygen and nutrients and removes waste — without the mother's and baby's blood ever mixing directly.
- The umbilical cord links the baby to the placenta.
- The baby is protected by a fluid-filled sac (amniotic sac) that cushions it.
- All major organs form in the first three months (first trimester). The next three months (second trimester) are about growth and refinement. The final three months (third trimester) are about rapid weight gain and preparation for life outside the womb. …
Part (a)
(i) Four devices that encourage cross-pollination:
- Dichogamy — anther and stigma mature at different times (protandry/protogyny).
- Self-incompatibility — genetic inhibition of the pollen tube when self-pollen lands on the stigma.
- Herkogamy — spatial separation of anther and stigma (e.g. heterostyly) so self-pollen cannot reach the stigma.
- Unisexuality — male and female flowers on the same (monoecious) or different (dioecious) plants. …
Part (a): Flowering plants use dichogamy, self-incompatibility, herkogamy and unisexual flowers to encourage cross-pollination, and discourage self-pollination because it causes inbreeding depression.
Part (b): The menstrual cycle is driven by pituitary (FSH, LH) and ovarian (oestrogen, progesterone) hormones through the follicular/proliferative, luteal/secretory and menstrual phases.
Part (a)
(i) Four devices that encourage cross-pollination:
- Dichogamy — the anther and stigma of a bisexual flower mature at different times (protandry = anthers first; protogyny = stigma first), preventing self-pollen from reaching a receptive stigma.
- Self-incompatibility — a genetic mechanism that inhibits the growth of the pollen tube when pollen lands on the stigma of the same flower, preventing self-fertilisation.
- Herkogamy — a spatial separation of anther and stigma within the flower (e.g. heterostyly in Primula) so that self-pollen cannot easily fall on the stigma.
- Unisexuality (unisexual flowers) — production of separate male and female flowers, either on the same plant (monoecious, e.g. maize) or on different plants (dioecious, e.g. papaya), which prevents self-pollination.
(ii) Why discourage self-pollination? …
Showing the 12 most recent of 19 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.A genetic mechanism which prevents inbreeding depression in majority of angiospermic plants is ________.(a) Parthenogenesis(b) Parthenocarpy(c) Mutation(d) Self-Incompatibility
›Reveal solutionSolution
Self-incompatibility is a genetic mechanism by which the stigma recognises and rejects self-pollen (or pollen from a genetically similar plant), so that flowers are forced to be cross-pollinated — this maintains genetic variability and prevents inbreeding depression.
In many flowering plants, especially those that are bisexual and where the anther and stigma mature at the same time, there is a strong chance of self-pollination (autogamy). To avoid the inbreeding depression that would result from continued self-fertilisation, angiosperms have evolved several devices to encourage cross-pollination, of which self-incompatibility is genetically the most important.
- Self-incompatibility (SI) is a genetic mechanism that prevents both autogamy and geitonogamy: it operates through a pollen-pistil interaction in which self-pollen grains (or their pollen tubes) are recognised as 'self' and are inhibited from germinating on the stigma, or their pollen tubes are arrested in the style, so fertilisation of that flower's own ovules by its own pollen does not take place. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which one of the following involves the transfer of genetically different pollen grains to the stigma?(a) Geitonogamy(b) Cleistogamy(c) Chasmogamy(d) Xenogamy
›Reveal solutionSolution
Transfer of genetically different pollen (from another plant) to a stigma is xenogamy — true cross-pollination.
The three kinds of pollination based on pollen source are:
- Autogamy — pollen transferred to the stigma of the same flower (genetically identical).
- Geitonogamy — pollen transferred to another flower of the same plant; genetically it is still self-pollination. …
- CBSE 2026Set ANNUAL1 markMCQQ.Geitonogamy involves transfer of pollen grains :(a) from anther to stigma of the same flower.(b) from anther to stigma of another flower on the same plant.(c) from anther to stigma of a flower on a different plant of the same species.(d) between flowers of different species.
›Reveal solutionSolution
Geitonogamy transfers pollen from the anther of one flower to the stigma of a different flower on the same plant, so (b) is correct.
The CBSE/NCERT Sexual Reproduction in Flowering Plants chapter names three pollination types:
- Autogamy — anther to stigma of the same flower (option a).
- Geitonogamy — anther to stigma of another flower on the same plant (option b). Genetically it is like self-pollination (same plant), but it functionally needs a pollinator. …
- CBSE 2024Set 57/2/11 markMCQQ.In humans, the secondary oocyte completes meiotic division when : (A) it gets implanted in the uterine endometrium. (B) it is released from the matured Graafian follicle. (C) it is penetrated by the sperm cell. (D) acrosomal enzymes break down the zona pellucida.
›Reveal solutionSolution
The secondary oocyte arrests in metaphase II and completes meiosis II only upon fertilization—specifically when a sperm penetrates it. The answer is (C).
Why the secondary oocyte waits
Human oogenesis is a carefully timed process. Unlike spermatogenesis, which runs to completion once started, oogenesis has two built-in arrest points. The first is in prophase I (during fetal development), and the second is in metaphase II—right after the first meiotic division produces the secondary oocyte.
This second arrest exists because completing meiosis is metabolically expensive and produces a second polar body that will be discarded. Evolution has optimized this: why finish the division if no sperm arrives? The secondary oocyte therefore remains frozen at metaphase II, chromosomes aligned on the spindle, waiting for a signal that fertilization has begun.
That signal is sperm entry.
Walking through the oocyte's journey
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Ovulation releases a metaphase II cell
When the Graafian follicle ruptures during ovulation, what is released is a secondary oocyte, not a mature ovum. This cell has already completed meiosis I (producing the first polar body) but is arrested in metaphase II. So option (B) describes the release, but meiosis II is not yet complete at this moment.
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Sperm penetration triggers completion
When a sperm successfully penetrates the secondary oocyte—after the acrosome reaction has allowed it to breach the zona pellucida and the sperm head fuses with the oocyte membrane—the oocyte receives a calcium signal. This calcium wave is the trigger that releases the metaphase II arrest. The cell now completes meiosis II, extruding the second polar body and forming the mature ovum with a haploid nucleus.
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What about the other options? …
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- CBSE 2024Set D1 markQ.Write True or False: Humans are oviparous.
›Reveal solutionSolution
The statement is False — humans give birth to live young (viviparous), they are not egg-laying (oviparous) organisms.
Animals can be classified by their mode of reproduction/birth as oviparous (egg-laying, where the embryo develops outside the mother's body inside an egg, e.g., birds, reptiles, most fish) or viviparous (where the embryo develops inside the mother's body and she gives birth to live young, e.g., most mammals).
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- CBSE 2024Set ANNUAL1 markMCQQ.Choose the correct sequence of processes in human reproduction:(a) Gametogenesis → Gamete fusion → Zygote → Embryonic development(b) Zygote → Gametogenesis → Gamete fusion → Embryonic development(c) Embryonic development → Gametogenesis → Zygote → Gamete fusion(d) Gamete fusion → Embryonic development → Gametogenesis → Zygote
›Reveal solutionSolution
Human (and most sexual) reproduction proceeds through gamete formation, their fusion, zygote formation, and then development of the embryo.
Sexual reproduction follows a fixed logical order: first, the parents produce haploid gametes through gametogenesis (spermatogenesis in the male, oogenesis in the female). Next, during fertilisation, a male and a female gamete undergo gamete fusion (syngamy) to restore the diploid number. This fusion produces a diploid zygote. T …
- CBSE 2023Set ANNUAL1 markMCQQ.The animals in which embryonic development completes inside the female body are called(a) oviparous(b) viviparous(c) both(1) and(2)(d) none of these
›Reveal solutionSolution
Viviparity means the embryo develops completely inside the mother's body, nourished directly by her, and is born alive — as opposed to oviparity, where development happens outside, inside a laid egg.
Animals are classified by where embryonic development occurs: oviparous animals (e.g., birds, reptiles, most fish) lay fertilised eggs, and the embryo develops OUTSIDE the mother's body, drawing on the egg's stored nutrients, hatching later; viviparous animals (most mammals, including humans) retain the developing embryo inside the mother's body (uterus), whe …
- CBSE 2023Set ANNUAL1 markMCQQ.Haemochorial placenta is found in which of the following?(a) cat(b) dogs(c) human(d) horse
›Reveal solutionSolution
Humans have a haemochorial placenta.
Placentae are classified by how many maternal tissue layers separate maternal blood from the foetal chorion. In a haemochorial placenta the foetal chorionic tissue is directly bathed by maternal blood (maternal endothelium and connective tissue are eroded), allowing efficient exchange. This intimate type is found in hum …
- CBSE 2022Set HE2201 markMCQQ.Choose the correct answer: Which of the following the continuity of life is maintained?(a) Adaptation(b) Respiration(c) Reproduction(d) Photosynthesis
›Reveal solutionSolution
Reproduction is the biological process by which organisms produce offspring, ensuring the species' continuity across generations — the other options are life processes but do not by themselves perpetuate the species.
- Adaptation allows a species to survive better in its environment but does not by itself create new generations.
- Respiration releases energy for cellular activities but does not produce offspring. …
- CBSE 2022Set ANNUAL1 markMCQQ.The female animal in which the menstrual cycle occurs is(a) monkey(b) human(c) both (A) and (B)(d) cow
›Reveal solutionSolution
The menstrual cycle occurs in primates such as monkeys and humans, so the answer is (C).
The cyclical reproductive changes with periodic shedding of the uterine lining (menstruation) is called the menstrual cycle and is characteristic of primates — humans, apes and Old World monkeys. Non-primate mammals such as the cow instead show an oestrous cycle, in which the endometrium is reabsorbed rather than shed. Si …
- CBSE 2022Set ANNUAL1 markMCQQ.In humans, the placenta is formed by(a) the amnion(b) the allantois(c) the chorion(d) the chorion and allantois
›Reveal solutionSolution
The placenta's foetal contribution comes from the chorion (chorionic villi), so the answer is (C).
After implantation, finger-like chorionic villi grow out from the trophoblast/chorion of the embryo and interdigitate with the uterine endometrial tissue. This intimate contact between the chorionic villi (foetal side) and the maternal uterine tissue forms the placenta, the structural and functional link for exchange of nutrients, gases and wastes between mother and foetus.
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- CBSE 2022Set ANNUAL1 markMCQQ.Capacitation is a natural process which occurs(a) in the epididymis(b) in the female reproductive tract(c) in the vas deferens(d) in the rete testis
›Reveal solutionSolution
Capacitation of sperm occurs in the female reproductive tract, so the answer is (B).
Sperm leaving the male are motile but not yet able to fertilise an egg. Capacitation is the final physiological maturation of sperm that gives them the ability to fertilise; it takes place in the female reproductive tract (over some hours after ejaculation), where secretions remove coating factors from the sperm surface and prepare them for the acrosomal reaction. …
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