Reproduction: How Life Continues explains how living beings produce new individuals of their own kind so that life carries on. It starts with asexual reproduction by mitosis (vegetative propagation, budding, spores), moves to meiosis and the variation it creates, then covers flowers, pollination, fertilisation and seed formation, fertilisation strategies in animals, and the human reproductive system, menstrual cycle, pregnancy and reproductive health. School exams test labelled parts, sequences, comparisons and data from the textbook tables, and the chapter prepares the ground for reproduction and heredity in Class 10.
Key Concepts
1. Reproduction and Its Two Modes
Every organism has a definite life span: it is born, grows, matures, reproduces and eventually dies. Reproduction is the biological process by which living beings produce new individuals of their own kind; it is how life on the Earth continues. A mango tree may die, but its seeds grow into new mango plants.
Living beings reproduce in two main ways.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Number of parents | One | Two |
| Gametes | Not formed | Male and female gametes are formed and fuse |
| Type of cell division involved | Mitosis | Meiosis (to form gametes), then mitosis as the zygote grows |
| Offspring | Almost exact copies of the parent (clones) | A mix of characteristics from both parents |
| Variation | Very little | Creates variation |
| Speed | Fast; population rises quickly in favourable conditions | Generally slower, since gametes must form and meet |
The mixing in sexual reproduction causes small differences between parents and young. Accumulated over many generations, these help living beings adapt and sometimes give rise to new species.
2. Asexual Reproduction: Vegetative Propagation, Budding and Spores
Asexual reproduction is seen in many unicellular organisms (bacteria, amoeba, yeast), in simple multicellular organisms (hydra, sponge) and in many plants. It involves only one parent, and hence produces genetically identical individuals.
Vegetative propagation in plants
In vegetative propagation, new plants arise from the vegetative parts, that is, from the growing parts of a plant, without producing seeds.
- Plants with fleshy underground stems, such as potato and ginger, sprout new plants.
- Stem cuttings of money plant and sugarcane grow into new plants.
- Bryophyllum leaves sprout tiny plantlets, which eventually grow into new plants.
Scientists and horticulturists have adapted this process into cutting, grafting, layering and tissue culture, which let farmers grow desirable crops on a large scale.
| Method | How it is done (as in the NCERT activity) |
|---|---|
| Cutting | Remove leaves from the lower half of a shoot cutting and insert it up to about half its length in soil mixed with compost, at about 45 to 60° to the soil surface. Water regularly. |
| Grafting | Make a slit on a twig of a healthy rooted plant (Plant A, for example a wild rose), fit a stem piece of another variety (Plant B, for example a yellow rose) into it, and wrap the joint with cotton cloth or film until it heals. Cut the other branches of Plant A. Plant B grows along with Plant A. |
| Layering | Bury the middle part of a flexible, thin twig (for example lemon) under the soil. Roots develop from the buried part after 10 to 15 days; then cut the twig from the parent. |
| Tissue culture | Healthy plantlets are mass-produced from the shoot tip (apical meristem), eliminating virus-infected plants and ensuring high yields, as in banana farming. |
Budding in yeast and hydra
Yeast kept in sugar solution in a warm place for 1 to 2 hours (Activity 11.2) shows small, round buds on the parent cells.
In hydra, repeated cell division at a specific site on the parent body produces a small outgrowth, the bud, which enlarges and separates to live independently. This is budding; one hydra often carries many buds at once.
Spore formation in moulds
Louis Pasteur proved that new life always comes from pre-existing life.
In NCERT Activity 11.3, moist bread or roti kept in a closed moist chamber in a warm, dark place grows mould in about three days. Stained with cotton blue, it shows thread-like hyphae with a round sac at the tip containing tiny round spores. In some fungi (Rhizopus type), spores form inside a sac-like structure; in others (Aspergillus type), they form on a swollen vesicle at the end of a long hypha.
- The mould came from spores already present in the air, which settled on the moist bread and began reproducing rapidly.
- Spores are produced in huge numbers (millions from one mould colony), are lightweight and usually single-celled, and float easily on air currents.
- They germinate when they find moisture and nutrients; the chamber is kept warm (25 to 35 °C) because spores need warmth.
- Lower temperatures slow or stop their reproduction, which is why perishable food is refrigerated.
Fungi also degrade wastes and pollutants and give antibiotics such as penicillin.
Mitosis: the common process behind all of these
In every example above, the central process is mitosis, a type of cell division that produces two daughter cells, each having the same number of chromosomes, identical to the parent cell. So the offspring are genetically identical to the parent and are called clones. This method is fast and helps organisms increase their population quickly, especially when conditions are favourable.
3. Meiosis, Gametes and Variation
In sexual reproduction both parents contribute genetic material. If each generation received the full set of chromosomes from both parents, the chromosome number would double in every generation. This problem is solved by a special type of cell division called meiosis.
Chromosomes are thread-like structures in the nucleus that carry genetic information. Each species has a fixed number. Humans have 23 pairs, a total of 46, one chromosome of each pair coming from each parent.
In meiosis, the chromosome number of a parent cell (diploid) is reduced to half (haploid) in the daughter cells. These haploid cells are used only for reproduction and are called gametes. The chromosomes of each pair separate, so each gamete receives only one chromosome from each pair. Every human gamete therefore has 23 chromosomes.
- In animals, male gametes are sperm and female gametes are eggs.
- In plants, the pollen grain contains the male gametes and delivers them to an ovule, which contains the female gamete (egg).
The bead model (NCERT Activity 11.4)
Three pairs of beads stand for three pairs of contrasting characters on three different chromosomes: light green or dark green (blonde or black hair, chromosome 1), light blue or dark blue (straight or curly hair, chromosome 2), light red or dark red (brown or black eyes, chromosome 3). Picking one bead at random from each pair makes one “gamete”. With just three pairs, eight combinations are possible, because each pair offers two choices: 2 × 2 × 2 = 8.
With 23 pairs the number is enormous, so children differ from their parents and siblings; each of us has a unique combination.
Why variation matters: it is important for the survival of a species. Variation helps some individuals adapt better to changing environments, and over time this contributes to evolution. Examples from the textbook: some people can tolerate low oxygen levels at high altitudes, and some can digest milk in adulthood.
4. The Flower and Pollination
Flowering plants (angiosperms) are the most diverse group of plants on the Earth, and flowers are their reproductive organs.
Parts of a flower
A complete flower has four parts, from the outside in: sepals, petals, stamens and pistil. In some flowers, sepals and petals are fused.
| Part | Description | Role |
|---|---|---|
| Sepal | Thin, flat, green outermost whorl; covers the bud | Protects the flower in the bud stage |
| Petal | Coloured and often fragrant projections | Attract pollinators |
| Stamen (male part) | Filament (stalk) and anther (tip) | Anther produces pollen grains containing male gametes |
| Pistil (female part) | Stigma, style and ovary | Receives pollen; ovary holds ovules |
In the pistil, the stigma is at the tip and may be flat and/or sticky. The style is a thin long tube connecting the stigma to the ovary. The ovary is the swollen base of the pistil; it contains ovules, and each ovule has an egg cell (female gamete).
Proving that pollination is needed (NCERT Activity 11.6)
On a pea plant, buds and open flowers are bagged, some with stamens removed. Pods form in all treatments except the bud whose stamens were removed (Solved Example 5), so pollen transfer from anther to stigma is necessary for fruit formation.
Pollination is the transfer of pollen grains from the anther to the stigma of a flower.
- Self-pollination: pollen reaches the stigma of the same flower, or of another flower on the same plant.
- Cross-pollination: pollen goes from the anther of a flower on one plant to the stigma of a flower on another plant of the same type.
Pollination strategies
Pollination depends on external agents called pollinators: wind, water, insects or birds.
| Agent | Examples | Features of the flower |
|---|---|---|
| Wind | Wheat, maize, rice | Pollen light, small and produced in large numbers; stigma long and feathery to trap pollen |
| Water | Vallisneria, Hydrilla (aquatic) | Water currents carry pollen from one flower to another |
| Insects (bees, butterflies) | Sunflower, hibiscus, marigold | Brightly coloured, produce nectar, fragrant; pollen large, sticky or spiny; stigma sticky |
| Birds (Indian white-eye, sunbirds) | Coral tree, hibiscus | Flowers visited by birds that carry pollen |
Pollen and seed data (NCERT Table 11.3)
| Pollination strategy | Approximate pollen grains released per flower | Estimated average seeds formed |
|---|---|---|
| Wind-pollinated grasses (maize, wheat) | 5,00,000 to 10,00,000 | 50 to 200 |
| Insect-pollinated plants (sunflower) | 20,000 to 40,000 | 800 to 1,000 |
Wind scatters pollen at random, so most grains are wasted; insects carry it from flower to flower, so fewer grains give more seeds (Solved Example 1).
5. Fertilisation, Seed and Fruit Formation
Once pollen reaches a compatible stigma, the following sequence takes place:
- The pollen grain germinates and produces a pollen tube, which grows down through the style into the ovary.
- The male gamete moves through this tube and arrives at the ovule.
- In the ovule, it fuses with the egg cell. This fusion of gametes is fertilisation, and it marks the beginning of a new life.
- The fertilised egg is the zygote, which develops into an embryo.
- Meanwhile, the ovary enlarges into the fruit and the ovules develop into seeds inside it.
- Seeds are dispersed by wind, water and animals. When water, air and temperature are favourable, a seed germinates and grows into a new plant.
The full order to remember: pollination → pollen germination on stigma → pollen tube growth → fertilisation → zygote → embryo; ovule → seed; ovary → fruit.
P. Maheshwari, the “Father of Indian Embryology”, achieved in-vitro fertilisation in flowering plants by fusing an egg and a male gamete in a test tube.
Plant breeding
- Selective breeding: farmers select plants with desirable characters for reproduction.
- Artificial hybridisation: stamens are removed from the flowers, the flower is covered with a bag to prevent self-pollination, and pollen with desired characters is transferred by hand.
- Genetic engineering: genetic material for desired characters is inserted into the DNA of selected varieties.
These methods have produced high-yielding and disease-resistant varieties.
6. Fertilisation Strategies in Animals
All animals must make sure the male and female gametes meet and the young survive long enough to reproduce.
- External fertilisation: in many aquatic animals, such as frogs and most fish, the female releases eggs into water and the male releases sperm over them. Many eggs are destroyed by water currents or eaten by other animals.
- Internal fertilisation: in reptiles, birds and mammals, fertilisation takes place inside the body of the female. The fertilised egg or embryo is better protected, so survival of the young is generally higher.
NCERT Table 11.4: Variation in reproductive strategies
| Animal | Habitat | Mode of fertilisation | Number of eggs produced | Estimated survival of young |
|---|---|---|---|---|
| Fish | Water | External | 100s to 1000s at a time | Low |
| Frog | Water/land | External | 5,000 to 50,000 at a time | Low |
| Lizard | Land | Internal | 2 to 20 at a time | Moderate |
| Bird | Water/land | Internal | 1 to 15 at a time | Moderate to high |
Yolk, larva and care of young
Fish, amphibians and insects lay several hundred to several thousand eggs at a time. The mother cannot provide much yolk (nutritive substance) for so many eggs, so each egg has just enough to produce a larva. The larva eats organic wastes such as rotten food and manure; it is essentially a feeding stage. Once enough nutrition is stored, a transformation forms the adult body, as in butterflies (egg, larva, pupa, adult) and frogs.
Reptiles and birds lay eggs with enough yolk to nourish the embryo until it hatches into a young one. In mammals, the zygote grows and develops inside the female body. Mammals typically feed their young with breast milk after birth.
7. Human Reproductive System and the Menstrual Cycle
As a child grows into an adult, the reproductive organs mature and begin producing gametes: sperm in males and eggs in females.
Male reproductive system
| Organ | Function |
|---|---|
| Testes (two, oval) | Produce sperm; also produce a hormone that controls sperm production and causes the physical changes in boys at puberty |
| Scrotum | Pouch of skin holding the testes; keeps them slightly cooler than normal body temperature, which sperm formation needs |
| Vas deferens | Long tube carrying sperm from the testes; opens into the urethra |
| Urethra | Common passage for urine and sperm |
| Seminal vesicles and prostate gland | Add fluids that nourish the sperm and help them stay active and move |
| Penis | Organ through which the urethra opens outside |
Each sperm has a head containing the genetic material and a long tail that helps it swim towards the egg.
Female reproductive system
| Organ | Function |
|---|---|
| Ovaries (a pair) | Produce eggs and release hormones that bring about changes at puberty |
| Oviducts (fallopian tubes) | Connect each ovary to the uterus; fertilisation takes place here |
| Uterus | Bag-like structure where the foetus develops |
| Cervix | Narrow passage through which the uterus opens into the vagina |
| Vagina | Receives sperm; the birth canal |
Gametogenesis and the two gametes
Gametogenesis is the formation of gametes. It takes place in the testes and the ovaries by meiosis. Human body cells have 46 chromosomes; sperm and eggs have only 23, so that the zygote has the same number (46) as the parents. In males, gametogenesis produces numerous tiny, motile (moving) sperm. In females, it produces a single large egg.
| Feature | Sperm | Egg |
|---|---|---|
| Size | Very small | Large |
| Number produced | Millions | Few |
| Stored nutrients | Absent | Present |
| Motility | Actively motile | Non-motile |
Ovulation, fertilisation and implantation
- A girl’s ovaries have millions of immature eggs at birth. From puberty, usually one mature egg is released every month from one ovary: ovulation. Before it, the uterine lining starts to thicken.
- The egg travels from the ovary into the oviduct.
- During sexual intercourse, millions of sperm enter through the vagina and swim through the reproductive tract. They may reach the egg in the oviduct.
- If a sperm fuses with the egg, a zygote forms. The uterine lining becomes thicker and richer in blood vessels.
- The zygote undergoes a series of mitotic divisions while travelling to the uterus.
- It then implants into the inner lining of the uterus to receive nourishment. Implantation marks the beginning of pregnancy.
When the egg is not fertilised: menstruation
An unfertilised egg stays viable for about a day and then degenerates. The thick, blood-rich lining is no longer needed, so it sheds. The lining leaves the body through the vagina along with some blood. This is menstruation (a period), and it usually lasts 3 to 7 days.
The cycle of ovulation, preparation of the uterus and menstruation repeats typically every 21 to 35 days (often around 28 days). It usually begins at puberty, between the ages of 10 and 14 in girls, and continues until menopause, around the age of 50.
| Days (typical 28-day cycle) | Event |
|---|---|
| Day 1 to 5 | Menstruation: the uterine lining is shed |
| Day 6 to 14 | Uterine lining gradually rebuilds; an egg starts maturing in the ovary |
| Day 14 | Ovulation: the ovary releases a mature egg |
| Day 15 to 28 | Lining becomes thicker and rich in blood vessels; if there is no fertilisation, it starts breaking down around day 28 and the cycle repeats |
Sex determination
Every person has two sex chromosomes. Females have XX and males have XY. The mother always gives an X. The father gives either an X (baby is female, XX) or a Y (baby is male, XY). So the biological sex of the baby is decided by the chromosome in the father’s sperm.
Menstrual hygiene: use sanitary pads (preferably biodegradable), tampons or menstrual cups; change pads every 4 to 6 hours, or more often if the flow is heavy; wash hands before and after; clean the genital area with water; wrap used pads before putting them in a bin and never flush them. Menstruation is a sign of a healthy reproductive system.
8. Pregnancy, Childbirth and Reproductive Health
Pregnancy and childbirth
Human pregnancy lasts about nine months and is divided into three trimesters.
| Stage | What happens |
|---|---|
| First trimester | The fertilised egg develops into an embryo during the first two months and major organs start forming. From about the ninth week, the developing embryo is called a foetus. |
| Second trimester | The foetus grows bigger and stronger; the mother can usually feel its movements. |
| Third trimester | The baby grows rapidly and gets ready for life outside the womb. |
During childbirth, strong contractions of the uterine muscles push the foetus out through the birth canal; doctors may use medical or surgical procedures if a normal birth is not safe. After birth, breastfeeding is essential because mother’s milk gives complete nutrition and protects the infant from many diseases.
Mother’s health: a balanced diet rich in proteins, vitamins and minerals, regular check-ups, rest and family support; no smoking, alcohol or medicines without medical advice. Anxiety and fatigue after delivery (post-partum depression) is a treatable condition; mothers should talk to a doctor, nurse or ASHA worker.
Sexual maturity and emotional maturity
Sexual maturity (sperm production in boys, menstrual cycles in girls) comes gradually in adolescence; emotional maturity, meaning handling feelings, communicating clearly and making thoughtful decisions, takes longer. Responsible choices help prevent unplanned pregnancies and STIs.
Sexually transmitted infections (STIs)
STIs pass from an infected to an uninfected person through sexual contact: gonorrhoea, herpes, syphilis, genital warts and HIV (which can lead to AIDS). Some are not curable yet. Condoms prevent their transmission and also help prevent pregnancy.
Contraceptive methods
| Type | Examples | How it works | Note |
|---|---|---|---|
| Barrier | Condoms, vaginal covers | Stop sperm from reaching the egg | Condoms also protect against STIs |
| Medicines | Oral pills | Change the release of eggs by altering hormones | May have some side effects |
| Intra-Uterine Device (IUD) | Copper-T | Placed in the uterus to avoid pregnancy | May sometimes irritate the uterus |
| Surgical | Blocking the vas deferens (males) or fallopian tubes (females) | Sperm and egg cannot meet | - |
The Central Drug Research Institute, Lucknow, developed the world’s first non-steroidal, non-hormonal oral contraceptive pill, taken once a week.
Removing an unwanted pregnancy by surgery, generally only within the first trimester, is abortion. Sex-selective abortion upsets the sex ratio, so prenatal sex determination is strictly prohibited by law in India.
IVF (in-vitro fertilisation): egg and sperm are combined outside the body in a laboratory dish and the fertilised egg is implanted in the uterus. In 1978, Subhash Mukhopadhyay of Kolkata pioneered India’s first test tube baby.
Key Terms Sheet
This chapter has no formulas. The numbers and relations below are the ones questions are built on.
| Item | Value or relation |
|---|---|
| Chromosomes in a human body cell | 46 (23 pairs), diploid |
| Chromosomes in a human sperm or egg | 23, haploid |
| Fertilisation in humans | Sperm (23) + egg (23) → zygote (46) |
| Combinations from n chromosome pairs | 2 × 2 × … (n times) = 2n; 3 pairs give 8 |
| Sex determination | Mother X + father X → XX (female); mother X + father Y → XY (male) |
| Plant sequence | Pollination → pollen tube → fertilisation → zygote → embryo; ovule → seed; ovary → fruit |
| Menstrual cycle | 21 to 35 days, often about 28; ovulation around day 14 of a 28-day cycle |
Read the rest of the chapter →Hide the rest ↑
Important Definitions
- Asexual reproduction: reproduction in which a single parent produces offspring that are almost exact copies of the parent.
- Sexual reproduction: reproduction involving two parents, both of whom contribute genetic material to the offspring.
- Vegetative propagation: formation of new plants from the vegetative (growing) parts of a plant.
- Budding: formation of a small outgrowth (bud) by repeated cell division at a specific site on the parent, which enlarges and separates to live independently.
- Spore: a lightweight, usually single-celled reproductive body of a fungus that germinates into a new individual when it gets moisture and nutrients.
- Mitosis: cell division that produces two daughter cells, each with the same number of chromosomes, identical to the parent cell.
- Meiosis: a special cell division that forms gametes, in which the chromosome number of the parent cell (diploid) is reduced to half (haploid).
- Gametes: haploid cells used only for reproduction; sperm and eggs in animals.
- Pollination: the transfer of pollen grains from the anther to the stigma of a flower.
- Fertilisation: the fusion of male and female gametes, which marks the beginning of a new life.
- Gametogenesis: the process of formation of gametes, which takes place in the testes and ovaries.
- Ovulation: release of a mature egg from one of the ovaries, usually once a month from puberty.
- Implantation: attachment of the developing zygote to the inner lining of the uterus; it marks the beginning of pregnancy.
- Menstruation: shedding of the thick uterine lining, with some blood, through the vagina when the egg is not fertilised.
Solved Examples (NCERT-Based)
Example 1: Pollen-to-seed ratio from Table 11.3
Using NCERT Table 11.3, find the range of pollen grains used per seed formed in (a) wind-pollinated grasses and (b) insect-pollinated plants. Which strategy is more efficient, and why can the other still succeed?
Solution: Pollen per seed = pollen released / seeds formed.
(a) Wind: best case = 5,00,000 / 200 = 2,500 pollen per seed. Worst case = 10,00,000 / 50 = 20,000 pollen per seed.
(b) Insect: best case = 20,000 / 1,000 = 20 pollen per seed. Worst case = 40,000 / 800 = 50 pollen per seed.
Insect pollination is far more efficient: it needs only 20 to 50 pollen grains per seed, compared with 2,500 to 20,000 for wind. Wind pollination still works because the plant releases an enormous number of light grains, so even with most of them wasted, some reach the long, feathery stigmas.
Example 2: Counting gamete combinations (Activity 11.4)
Three pairs of beads represent three chromosome pairs. List all the combinations a gamete can carry and state the number for 23 pairs.
Solution: Let L = light and D = dark bead for green (G), blue (B) and red (R). Each pair gives 2 choices, so 2 × 2 × 2 = 8 combinations:
- LG, LB, LR
- LG, LB, DR
- LG, DB, LR
- LG, DB, DR
- DG, LB, LR
- DG, LB, DR
- DG, DB, LR
- DG, DB, DR
Combination 1 is blonde, straight hair with brown eyes. For 23 pairs, the same rule gives 223 = 83,88,608 (about 8.4 million) possible combinations in one person’s gametes, which is why siblings differ.
Example 3: Why meiosis is needed
Human body cells have 46 chromosomes. (a) How many chromosomes are in a sperm, an egg and a zygote? (b) If gametes were made by mitosis instead of meiosis, how many chromosomes would a child and a grandchild have?
Solution: (a) Meiosis halves the number, so sperm = 23 and egg = 23. Zygote = 23 + 23 = 46, the same as the parents.
(b) With mitosis, each gamete would have 46. Child = 46 + 46 = 92; grandchild = 92 + 92 = 184. The number would double every generation, which meiosis prevents.
Example 4: Predicting the next period (Pause and Ponder 7)
Rina’s period occurs every 28 days. Her last period was on the 5th of March. On which day is she most likely to get her next period? Around which date is ovulation likely?
Solution: Day 1 is 5 March; the next period starts 28 days later. From 5 March, 26 days reach 31 March, and the remaining 2 days reach 2 April.
Ovulation in a typical 28-day cycle is around day 14. Day 14 = 5 March + 13 days = 18 March (approximately).
Example 5: Reading the pea plant experiment (Activity 11.6)
Five pea flowers or buds are treated as follows: (A) bud wrapped, (B) bud with stamens removed, wrapped, (C) open flower with stamens removed, wrapped, (D) open flower wrapped, (E) open flower uncovered. Predict fruit formation and explain.
Solution:
| Treatment | Fruit formed? | Reason |
|---|---|---|
| A | Yes | Its own stamens are present, so pollen reaches its stigma inside the bag |
| B | No | No stamens of its own, and the bag keeps outside pollen away, so no pollination |
| C | Yes | Most likely, pollen had already reached the stigma from its own stamens before they were removed from the open flower |
| D | Yes | Stamens present |
| E | Yes | Stamens present and open to pollinators |
NCERT reports fruit in every treatment except B, and concludes that pollen transfer from anther to stigma is necessary for fruit formation. The reasons for A and C are inferences: a pea flower’s own pollen reaches its stigma, but a juvenile bud has not yet reached that stage, so removing its stamens stops pollination.
Example 6: Who decides the sex of the baby?
A mother has XX and a father has XY sex chromosomes. Work out the possible children and state who determines the sex.
Solution: In meiosis the pair separates, so every egg carries one X. The father’s pair also separates, so half his sperm carry X and half carry Y.
| Egg | Sperm | Zygote | Sex |
|---|---|---|---|
| X | X | XX | Female |
| X | Y | XY | Male |
A boy and a girl are equally likely. The father’s sperm decides the sex, because the mother always contributes an X.
Example 7: Reading Table 11.4 on animal strategies
From NCERT Table 11.4, explain why a frog lays 5,000 to 50,000 eggs at a time while a bird lays only 1 to 15.
Solution: The frog uses external fertilisation in water. Many eggs are destroyed by water currents or eaten, so survival is low, and a very large number of eggs ensures a few survive. Each egg has only enough yolk to produce a larva, which then feeds itself.
The bird uses internal fertilisation. Its egg has enough yolk to nourish the embryo until hatching, and survival is moderate to high, so a few eggs are enough.
Example 8: Choosing a propagation method (Revise, Reflect, Refine Q9)
A farmer wants to produce a large number of genetically identical plants quickly. Suggest suitable reproduction methods and explain why they are effective.
Solution: Vegetative propagation: cutting, grafting, layering and, for the largest numbers, tissue culture.
- They use only one parent and work by mitosis, so every new plant is genetically identical to the chosen parent and keeps its desirable characters.
- No flowers, pollination or seeds are needed, so the process is faster.
- Tissue culture mass-produces virus-free plantlets from the shoot tip.
Competency-Based Questions (with answers)
1. Case-based: The bread mould experiment
Aman keeps moist roti in a closed moist chamber in a warm, dark cupboard; his friend keeps an identical set-up in a refrigerator. After three days, only Aman’s roti has fuzzy patches.
(a) Where did the mould on Aman’s roti come from? From fungal spores already present in the air, which settled on the moist roti and began reproducing.
(b) What will Aman see under the microscope after cotton blue staining? Thread-like hyphae with a round sac at the tip containing tiny round spores.
(c) Explain the friend’s result. Lower temperatures slow or stop the reproduction of spores, which is why perishable food is refrigerated.
2. Case-based: Banana plantlets
A farmer’s banana crop is hit by a virus. He is offered plantlets raised in a laboratory from the shoot tips of healthy plants.
(a) Name the technique used. Plant tissue culture.
(b) Is this sexual or asexual reproduction? Give a reason. Asexual. Only one parent is involved and the plantlets arise by mitosis, so they are genetically identical to the source plant.
(c) State one advantage and one risk of planting only these identical plants. Advantage: virus-free, high-yielding plants in large numbers. Risk: all are genetically identical, so a disease that attacks one can attack all.
3. Assertion-Reason
Assertion (A): Offspring produced by budding in hydra are genetically identical to the parent. Reason (R): Budding involves mitosis, which gives daughter cells with the same chromosomes as the parent cell.
Answer: (a) Both A and R are true, and R is the correct explanation of A.
4. Assertion-Reason
Assertion (A): Frogs lay thousands of eggs at a time. Reason (R): In external fertilisation, many eggs are destroyed by water currents or eaten by other animals.
Answer: (a) Both A and R are true, and R is the correct explanation of A. Large numbers make up for the low survival of each egg.
5. Assertion-Reason
Assertion (A): Wind-pollinated flowers have long, feathery stigmas. Reason (R): Wind-pollinated flowers produce nectar and fragrance to attract pollinators.
Answer: (c) A is true, but R is false. Feathery stigmas trap light, windborne pollen. Nectar and fragrance belong to insect-pollinated flowers such as sunflower and marigold.
6. Assertion-Reason
Assertion (A): Human gametes contain 23 chromosomes. Reason (R): Gametes are formed by mitosis.
Answer: (c) A is true, but R is false. Gametes are formed by meiosis, which halves the chromosome number from 46 to 23.
7. Source-based: Tomato, wheat and papaya (Revise, Reflect, Refine Q11)
Read the prompts and find out which type(s) of pollination might have been followed: Tomato: stamens cover the stigma. Wheat: flowers open after pollination. Papaya: male and female flowers are often borne on different papaya trees.
Answer: Tomato: self-pollination, since the stamens surround the stigma and pollen falls directly on it. Wheat: self-pollination, since pollination is complete before the flower opens. Any cross-pollination in wheat is by wind, which is why the chapter lists it among wind-pollinated plants. Papaya: cross-pollination only, because a flower on a female tree can receive pollen only from a male flower on another tree, carried by a pollinator.
8. Experiment design: Pollen in sugar solutions (Revise, Reflect, Refine Q10)
Suresh prepares slides with pollen grains in different sugar concentrations (0%, 2.5%, 5%, 7.5%, 10%) to study the germination of pollen. (i) What are the different hypotheses which can be tested using this set-up? (ii) What parameters should be kept the same in this set-up?
Answer: (i) Possible hypotheses: sugar concentration affects pollen germination; pollen germinates better in some sugar than in plain water (0%); there is a best (optimum) concentration for germination; very high sugar concentration reduces germination or pollen tube growth. (ii) Keep the same: the plant and flower age the pollen comes from, the amount of pollen and solution per slide, temperature, time before observation and the counting method. Only sugar concentration changes; the 0% slide is the control.
9. Data-based: Apple orchards and bees (Revise, Reflect, Refine Q12)
In apple orchards at Place A, natural pollinators pollinated the flowers. At Place B, farmers added beekeeping (mixed farming) and got honey along with apples. Fruit setting (number of fruits / total number of corresponding fruit-bearing branches) and fruit drop (premature falling of developing fruits) were compared in a figure.
(i) What hypothesis did the group test? That keeping bee colonies in the orchard increases pollination, which raises fruit setting and lowers fruit drop, so the apple yield rises.
(ii) What are the parameters? Changed: presence or absence of managed bees. Measured: fruit setting and fruit drop (per cent). Kept similar: apple variety, age of trees, climate and orchard care.
(iii) How should the data be compared? Higher fruit setting and lower fruit drop mean higher yield.
(iv) Inference: If Place B shows higher fruit setting and lower fruit drop than Place A, the inference is that the decline in natural pollinators is limiting apple yield, and that beekeeping restores pollination while giving farmers honey as a second income.
10. Critical thinking: The day-14 claim (Revise, Reflect, Refine Q13)
A student claims, “In humans, ovulation always happens on day 14 of the menstrual cycle”. Critically examine this claim and state whether the claim is correct or not. Give at least two reasons for your answer.
Answer: The claim is not correct. (1) Cycle length varies from 21 to 35 days, so the day of ovulation cannot be the same for everyone. (2) Day 14 describes only a typical 28-day cycle, and the textbook itself says ovulation happens around day 14.
Important Questions for Exams
1-Mark Questions
- Name the type of cell division involved in asexual reproduction. Mitosis.
- How many chromosomes are present in a human zygote? 46.
- Which part of the flower develops into the fruit? The ovary.
- Where does fertilisation take place in the human female? In the oviduct (fallopian tube).
- Name one plant pollinated by water. Vallisneria (or Hydrilla).
2-Mark Questions
- Why does asexual reproduction produce offsprings that are genetically identical to the parent? Asexual reproduction involves only one parent, and the process behind it is mitosis. Mitosis gives daughter cells with the same number of chromosomes, identical to the parent cell, so the offspring (clones) carry the same genetic information as the parent.
- Distinguish between self-pollination and cross-pollination. In self-pollination, pollen reaches the stigma of the same flower or another flower on the same plant. In cross-pollination, pollen goes from a flower on one plant to the stigma of a flower on another plant of the same type.
- What is the role of the scrotum in sperm formation? The scrotum is a pouch of skin that holds the testes and keeps the testes slightly cooler than normal body temperature, which is necessary for sperm formation.
- If a couple uses oral contraceptive pills but not condoms, which risks remain and why? The risk of sexually transmitted infections remains. Pills only alter hormones to change the release of eggs; condoms are the barrier that prevents STI transmission.
3-Mark Questions
- Arrange the following stages of sexual reproduction in plants in the correct order: (i) Pollen germination on stigma (ii) Fertilisation (iii) Pollination (iv) Formation of zygote. (iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote.
- Explain why the menstrual cycle stops during pregnancy. Menstruation is the shedding of the thick uterine lining when the egg is not fertilised and the lining is no longer needed. In pregnancy, the zygote has implanted in this lining and is nourished through it for about nine months. The lining is needed throughout, so it is not shed.
- Why do vegetatively propagated plants tend to be more vulnerable to diseases than sexually reproduced plants? Vegetatively propagated plants are clones with no variation. A disease that can attack one plant can attack all of them, and there is no individual with a different set of characters that might resist it. Sexually reproduced plants vary, so some individuals may survive a disease.
- If all flowers in a type of plant were only capable of self-pollination, how would it affect the genetic diversity over several generations? Explain. Genetic diversity would decrease. With only self-pollination, gametes from the same plant fuse, so no new characters come in from other plants. Over generations the plants become more and more alike, and the species loses the variation that helps some individuals survive changes in the environment.
5-Mark Questions
- Describe the structure of a complete flower. Explain what happens from the arrival of pollen on the stigma to the formation of fruit and seed. A complete flower has four parts: sepals (green, protect the bud), petals (coloured, attract pollinators), stamens (male part: filament and anther, which produces pollen with male gametes) and pistil (female part: stigma, style and ovary with ovules, each with an egg cell). After pollination, the pollen grain on a compatible stigma produces a pollen tube that grows down through the style into the ovary. The male gamete moves through the tube to the ovule and fuses with the egg cell; this is fertilisation. The zygote formed develops into an embryo. The ovules develop into seeds and the ovary enlarges into the fruit.
- Explain how meiosis creates variation. Why is variation important for a species? Humans have 23 pairs of chromosomes. In meiosis the chromosomes of each pair separate, so each gamete gets one chromosome from every pair, chosen at random. With 3 pairs there are 2 × 2 × 2 = 8 combinations; with 23 pairs the number is enormous. Fertilisation then combines one such gamete from each parent. So children differ from their parents and from each other, and each person has a unique combination. Variation is important because it helps some individuals adapt better to changing environments; over time this contributes to evolution.
- Describe the events of the menstrual cycle in a typical 28-day cycle. What happens if the egg is fertilised? Day 1 to 5: menstruation, the uterine lining is shed with some blood. Day 6 to 14: the lining gradually rebuilds and an egg matures in the ovary. Around day 14: ovulation, a mature egg is released and travels into the oviduct. Day 15 to 28: the lining becomes thicker and rich in blood vessels; if there is no fertilisation, the egg degenerates after about a day and the lining starts breaking down around day 28, and the cycle repeats. If the egg is fertilised in the oviduct, a zygote forms, undergoes mitotic divisions while moving to the uterus, and implants in the thick lining. Implantation marks the beginning of pregnancy, and menstruation does not occur.
Common Mistakes and Examiner Tips
- Writing “meiosis” for asexual reproduction. Asexual reproduction works by mitosis; meiosis only forms gametes.
- Mixing up pollination and fertilisation. Pollination is transfer of pollen to the stigma. Fertilisation is fusion of the male gamete with the egg inside the ovule. Pollination comes first.
- Swapping ovary and ovule outcomes. Ovary → fruit; ovule → seed. Tie it to a pea pod: the pod developed from the ovary and each pea from an ovule.
- Defining cross-pollination as “between two flowers”. Two flowers on the same plant is still self-pollination. Cross-pollination needs two different plants of the same type.
- Saying fertilisation happens in the uterus. In humans, the sperm meets the egg in the oviduct. The uterus is where the zygote implants and the foetus develops.
- Saying the zygote attaches to the uterus immediately. The zygote first undergoes a series of mitotic divisions while travelling to the uterus, and only then implants.
- Saying ovulation “always” happens on day 14. Day 14 applies to a typical 28-day cycle. Cycles range from 21 to 35 days, so write “around day 14 of a 28-day cycle”.
- Getting chromosome numbers wrong. Body cell 46 (23 pairs), gamete 23 single chromosomes, zygote 46.
- Saying the mother decides the sex of the baby. The mother always gives an X. The father’s sperm carries X or Y, so the father’s chromosome decides.
- Listing oral pills as protection against STIs. Condoms prevent the transmission of STIs; pills and Copper-T only prevent pregnancy.
- Assuming more eggs means more young survive. Animals with external fertilisation lay more eggs because survival per egg is low.
- Confusing the grafting plants. Plant A is the rooted plant with the slit; Plant B is the inserted stem piece.
Quick Revision Points
- Asexual: one parent, mitosis, clones, fast. Sexual: two parents, meiosis, variation.
- Vegetative propagation: potato, ginger (underground stems), money plant and sugarcane (stem cuttings), Bryophyllum (leaf plantlets).
- Cutting, grafting, layering (roots in 10 to 15 days) and tissue culture (shoot tip, virus-free).
- Spores: millions per colony, light, airborne; cold slows them.
- Humans: 23 pairs = 46 chromosomes; gametes 23; zygote 46.
- Meiosis: diploid to haploid; one chromosome from each pair goes to a gamete; 3 pairs give 8 combinations.
- Flower: sepal, petal, stamen (filament + anther), pistil (stigma + style + ovary).
- Pollination: anther to stigma. Self: same flower or same plant. Cross: another plant of the same type.
- Wind (wheat, maize, rice), water (Vallisneria, Hydrilla), insects (sunflower, marigold), birds (coral tree, hibiscus).
- Insect pollination gives far more seeds per pollen grain than wind pollination.
- Pollen tube → male gamete fuses with egg in ovule → zygote → embryo; ovule → seed; ovary → fruit.
- External fertilisation (frog, most fish): many eggs, low survival. Internal (reptiles, birds, mammals): fewer eggs, higher survival.
- Larva is a feeding stage (butterfly, frog); reptile and bird eggs carry enough yolk to hatch a young one.
- Male: testes (sperm + hormone), scrotum (cooler), vas deferens, urethra, seminal vesicles and prostate (fluids).
- Female: ovaries, oviducts, uterus, cervix, vagina.
- Sperm: tiny, millions, no stored food, motile. Egg: large, few, stored food, non-motile.
- Fertilisation in oviduct; zygote divides by mitosis on the way; implantation begins pregnancy.
- Cycle 21 to 35 days (often 28); menstruation 3 to 7 days; puberty 10 to 14; menopause around 50.
- Sex: XX female, XY male; father’s sperm decides.
- Pregnancy about nine months, three trimesters; foetus from about week 9.
- Condoms prevent STIs and pregnancy; pills, Copper-T and surgery prevent pregnancy only; prenatal sex determination is banned in India.
Weightage and Exam Pattern
Class 9 has no board exam. This chapter is assessed in school exams (periodic tests, half-yearly and annual papers), and the NCERT source gives no fixed mark allocation. Questions come from definitions, labelled structures, sequences, comparisons and the textbook activities.
| Question type | What this chapter typically supplies |
|---|---|
| MCQ and one-word | Chromosome numbers, which part becomes the fruit, examples of each pollinator, site of fertilisation |
| Assertion-Reason | Mitosis and clones, meiosis and gamete number, egg numbers and survival, zygote and implantation |
| Short answer (2 to 3 marks) | Asexual versus sexual, self versus cross-pollination, ordering stages, functions of reproductive organs |
| Long answer (5 marks) | Flower structure with fertilisation, meiosis and variation, the menstrual cycle and pregnancy |
| Case-based and data | Pollen-to-seed data, egg-survival table, pea pollination experiment, mould and tissue culture scenarios, cycle date calculations |
This chapter is the foundation for reproduction and heredity in Class 10.
Class 9 Science · Chapter 11 – swipe through all 10 cards to understand the whole chapter.
Vegetative Propagation
New plants grow from vegetative parts of one parent, so they are genetically identical.
Tissue culture from the shoot tip gives virus-free plantlets, as in banana farming.
- Natural: potato, ginger, sugarcane and money plant cuttings, Bryophyllum leaf plantlets
- Cutting: leaves off the lower half, half buried at 45 to 60°
- Layering: buried twig roots in 10 to 15 days, then cut from the parent
Yeast, Hydra and Moulds
Budding and spore formation are asexual methods in simple organisms.
Spores need warmth and moisture; refrigeration slows or stops mould growth.
- Budding: an outgrowth forms by repeated cell division and separates (yeast, hydra)
- Bread mould: hyphae with a round sac of spores at the tip
- Spores are light, usually single-celled and produced in millions
Halving the Chromosomes
Meiosis reduces the chromosome number from diploid to haploid to form gametes.
Without meiosis the chromosome number would double every generation.
- Humans have 23 pairs, 46 chromosomes in body cells
- Each gamete gets one chromosome from every pair
- 3 pairs give 2 × 2 × 2 = 8 combinations; 23 pairs give an enormous number
The Complete Flower
A complete flower has sepals, petals, stamens and a pistil.
The ovary is the swollen base of the pistil and holds ovules with egg cells.
- Sepals protect the bud; petals attract pollinators
- Anther produces pollen grains containing male gametes
- Stigma may be flat or sticky; style is a thin tube to the ovary
Pollen from Anther to Stigma
Pollination is the transfer of pollen from anther to stigma, by self or cross-pollination.
Insect-pollinated plants need about 20 to 50 pollen per seed; wind needs 2,500 to 20,000.
- Wind (wheat, maize, rice): light pollen, feathery stigma
- Water: Vallisneria, Hydrilla; birds: coral tree, hibiscus
- Insects (sunflower, marigold): colour, nectar, fragrance, sticky pollen
From Pollen Tube to Fruit
The pollen tube carries the male gamete to the ovule, where it fuses with the egg.
Order: pollination, pollen germination on stigma, fertilisation, zygote.
- Zygote develops into an embryo inside the seed
- Seeds are dispersed by wind, water and animals
- Seeds germinate when water, air and temperature are favourable
External vs Internal Fertilisation
The lower the survival of each egg, the more eggs an animal lays.
A larva is a feeding stage; bird and reptile eggs carry enough yolk to hatch a young one.
- Frog: external, 5,000 to 50,000 eggs, low survival
- Lizard: internal, 2 to 20 eggs, moderate survival
- Bird: internal, 1 to 15 eggs, moderate to high survival
Male and Female Reproductive Organs
Testes make sperm; ovaries make eggs; fertilisation happens in the oviduct.
The father’s sperm (X or Y) decides the sex of the baby.
- Scrotum keeps testes slightly cooler, needed for sperm formation
- Vas deferens to urethra; seminal vesicles and prostate add fluids
- Ovaries, oviducts, uterus, cervix and vagina form the female system
Ovulation and Menstruation
The cycle of ovulation, uterine preparation and menstruation repeats every 21 to 35 days.
An unfertilised egg stays viable for about a day, then the lining is shed.
- Menstruation usually lasts 3 to 7 days
- Begins at puberty (10 to 14 years), continues till menopause around 50
- Change pads every 4 to 6 hours and dispose of them wrapped
Pregnancy and Reproductive Health
Implantation starts a pregnancy of about nine months in three trimesters.
Condoms prevent both pregnancy and STIs; pills and Copper-T prevent pregnancy only.
- Mother needs a balanced diet, check-ups and rest; no smoking or alcohol
- STIs include gonorrhoea, herpes, syphilis, genital warts and HIV
- Prenatal sex determination is prohibited by law in India
📝 Practice Reproduction: How Life Continues - 10 questions
Concept-check questions · with answers & explanations
Start →Close ✕
Why C: Tissue culture mass-produces plantlets from the shoot tip (apical meristem) of a chosen plant. Being asexual, it gives genetically identical copies, and the chapter notes it helps eliminate virus-infected plants and ensures high yields in banana farming.
Why not A: Seeds come from sexual reproduction and carry a mix of characters, so the plants would not be identical to the parent.
Why not B: Cross-pollination deliberately mixes genetic material from two plants, producing variation, the opposite of what the farmer wants.
Why not D: Natural seed dispersal is slow, uncontrolled and still produces variable seed-grown plants.
Remember: Identical + many + fast + virus-free = tissue culture.
Why C: Mitosis keeps the full chromosome number, so each gamete would carry the complete set. Fertilisation would then add two full sets, doubling the number every generation. The chapter says exactly this: if each generation received the full set from both parents, the number would double, and meiosis solves this problem.
Why not A: Fertilisation adds the two gametes’ sets; it can only restore the normal number if each gamete brought half.
Why not B: Halving is what meiosis does to gametes; nothing in mitosis or fertilisation reduces the number.
Why not D: Mitosis does not halve the chromosome number; it produces daughter cells with the same number as the parent.
Remember: Two full sets at fertilisation means doubling; only halved gametes keep the number fixed.
Why D: The muslin bag keeps out pollen from outside, so no cross-pollination was possible. The bud kept its own stamens, so pollen from its anthers could reach its own stigma. A pod formed, so self-pollination inside the bag must have occurred. The chapter notes that the only treatment without fruit was the bud whose stamens were removed and bagged.
Why not A: The bag is used precisely to stop insects and outside pollen reaching the flower.
Why not B: The cloth blocks wind-borne pollen just as it blocks insects.
Why not C: The bagged bud with stamens removed formed no fruit, proving that pollen transfer is necessary for fruit formation.
Remember: Bag without stamens = no fruit; bag with stamens = fruit by self-pollination.
Why C: A is true: birds hatch as young ones. R is false: the chapter says reptile and bird eggs each contain enough yolk to nourish the developing embryo until it hatches into a young one. Eggs with only enough yolk for a larva are laid by insects, fish and amphibians, which produce hundreds to thousands of eggs at a time.
Why not A: R is false, so it cannot be the correct explanation of A; a bird egg with only larva-level yolk would give a larva, the opposite of A.
Why not B: Option B needs both statements to be true, but R wrongly gives the bird egg the insect, fish and amphibian strategy.
Why not D: A is true; birds hatch as young ones, and larvae are seen in insects, fish and amphibians such as frogs.
Remember: Lots of yolk per egg means a young one hatches; little yolk per egg means a larva hatches.
Why A: Both are true and R explains A. The chapter says the uterine lining becomes thick and rich in blood vessels to receive and nourish the developing zygote, and that it sheds only when an egg is not fertilised and the lining is no longer needed. When the zygote implants, the lining is needed for nourishment throughout pregnancy, so no shedding, and therefore no menstruation, occurs.
Why not B: The need to keep the lining is the direct cause of menstruation stopping, so R is the correct explanation.
Why not C: R is true; implantation into the lining is how the embryo receives nourishment.
Why not D: A is true; menstruation is the shedding of an unneeded lining, which does not happen while an embryo is implanted in it.
Remember: Menstruation is the sign of no pregnancy; pregnancy keeps the lining, so periods stop.
Why B: Oral pills are medicines that alter hormones to change the release of eggs, so they prevent pregnancy. STIs pass through close physical contact, and the chapter says condoms can prevent their transmission. Pills do nothing to stop contact, so the infection risk stays.
Why not A: Only barrier methods like condoms protect against STIs; pills give no such protection.
Why not C: Pills do prevent pregnancy when used as prescribed, so this understates their effect.
Why not D: This reverses the facts: pills act on pregnancy, not on infection.
Remember: Pills stop eggs, condoms stop contact; only condoms cover both pregnancy and STIs.
Why A: Both statements are true and R explains A. Vegetatively propagated plants are clones produced by mitosis, so if one plant lacks resistance to a disease, all of them do, and the disease spreads through the whole crop. Seed-grown plants come from sexual reproduction and vary, so some may resist.
Why not B: R is not a separate fact; the shared weakness is precisely why the whole clonal crop is vulnerable, so R does explain A.
Why not C: R is true; the chapter states that asexual reproduction produces genetically identical individuals.
Why not D: A is true; the chapter’s own review question asks why vegetatively propagated plants are more vulnerable to disease.
Remember: Clones share strengths and weaknesses alike; variation is a species’ insurance.
Why A: Both are true and R explains A. The random separation of chromosome pairs in meiosis gives each gamete a different combination (8 from just three pairs, vastly more from 23). Each child is formed from a different sperm and a different egg, so each receives a unique combination of chromosomes and differs from siblings.
Why not B: R is exactly the mechanism that produces the sibling differences described in A, so it is the correct explanation.
Why not C: R is true; the chapter states that during meiosis the chromosomes of each pair separate so each gamete receives only one from each pair.
Why not D: A is true; the chapter says this random mixing makes children different from their parents and from their siblings.
Remember: Same parents, different gametes, different children.
Why A: The chapter defines self-pollination as transfer of pollen to the stigma of the same flower or another flower of the same plant. Cross-pollination needs the pollen to reach a flower on a different plant of the same type. Both flowers here are on one plant, so it is self-pollination.
Why not B: Two flowers do not make it cross-pollination; what matters is whether they are on different plants.
Why not C: The pollinating agent (insect, wind, water) does not decide self versus cross; the source and destination plants do.
Why not D: Pollination is any transfer of pollen from anther to stigma; it happens within one plant too.
Remember: Same plant = self, different plant = cross, whatever the number of flowers.
Why B: The mother cannot pack much yolk into thousands of eggs, so the yolk is just enough to produce a larva. The larva then gets nutrition by eating organic wastes like rotten food and manure, and grows. The chapter calls this intermediate stage essentially a feeding stage; once enough nutrition is accumulated, the adult body forms.
Why not A: The yolk was used up producing the larva; that is the whole reason a feeding stage is needed.
Why not C: Feeding young with milk is a mammal strategy; insects, fish and amphibians do not produce milk.
Why not D: The larva must feed and accumulate nutrition before transformation into the adult can happen.
Remember: Little yolk per egg means a hungry larva that eats its way to adulthood.
Chapter Navigation
Previous: Sound Waves: Characteristics and Applications Class 9 Notes
Next: Patterns in Life: Diversity and Classification Class 9 Notes
Related Chapters in Class 9 Science
- Exploration Class 9 Notes
- Cell: The Building Block of Life Class 9 Notes
- Tissues in Action Class 9 Notes
- Describing Motion Around Us Class 9 Notes
- Exploring Mixtures and their Separation Class 9 Notes
- How Forces Affect Motion Class 9 Notes
Explore More
Frequently Asked Questions
Asexual reproduction involves a single parent, and the process behind it is mitosis. Mitosis produces two daughter cells, each with the same number of chromosomes, identical to the parent cell. So the offspring carry exactly the parent’s genetic information and are called clones. This makes the method fast and useful for increasing population quickly when conditions are favourable.
Both parents contribute chromosomes to the offspring. If gametes carried the full set, the chromosome number would double every generation. Meiosis halves the number from diploid to haploid, so a human sperm and egg each have 23 chromosomes and the zygote has 46, the same as the parents. Meiosis also creates variation by sending one chromosome of each pair into a gamete at random.
Pollination is the transfer of pollen grains from the anther to the stigma. Fertilisation comes after it: the pollen grain grows a pollen tube through the style into the ovary, and the male gamete fuses with the egg cell inside the ovule. The fused cell is the zygote, which develops into an embryo. The ovule then becomes the seed and the ovary becomes the fruit.
Frogs and most fish use external fertilisation in water. Many of their eggs are destroyed by water currents or eaten by other animals, so survival of young is low, and a large number of eggs makes sure a few survive. Birds use internal fertilisation, their eggs carry enough yolk to nourish the embryo until hatching, and survival is moderate to high, so 1 to 15 eggs are enough.
Fertilisation takes place in the oviduct (fallopian tube), where a sperm fuses with the egg to form a zygote. The zygote undergoes a series of mitotic divisions while travelling to the uterus, then implants into the thick, blood-rich inner lining of the uterus to receive nourishment. Implantation marks the beginning of pregnancy, which lasts about nine months in three trimesters.
No. Day 14 applies to a typical 28-day cycle. The cycle usually repeats every 21 to 35 days, so the day of ovulation varies from person to person. The correct way to write it is that ovulation happens around day 14 of a 28-day cycle. If the egg is not fertilised it degenerates in about a day and the lining is shed.
The father. Females have XX and males have XY sex chromosomes. The mother always contributes an X chromosome. The father’s sperm carries either an X, giving a girl (XX), or a Y, giving a boy (XY). Prenatal sex determination is strictly prohibited by law in India to maintain a healthy sex ratio.
Condoms. They act as a barrier that stops sperm from reaching the egg and also prevents the transmission of STIs such as gonorrhoea, herpes, syphilis, genital warts and HIV. Oral pills work by altering hormones and changing the release of eggs, and a Copper-T is placed in the uterus; both prevent pregnancy but give no protection against STIs.
