This MCQ module is based on: Sexual Reproduction in Animals and Humans
TOPIC 40 OF 50
Sexual Reproduction in Animals and Humans
🎓 Class 9
Science
CBSE
Theory
Ch 11 — Reproduction: How Life Continues
⏱ ~16 min
🌐 Language: [gtranslate]
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This assessment will be based on: Sexual Reproduction in Animals and Humans
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Introduction: Two Parents, Mixed Inheritance
In Part 1 we saw how a single parent can produce offspring all by itself. But in most plants, animals and even some fungi, reproduction is a two-parent affair. The new individual carries features of both parents — its mother's eyes, perhaps, and its father's height; the seed of a hybrid mango bearing characters of both varieties used in cross-pollination. This is the world of sexual reproduction.
Sexual reproduction looks more elaborate than asexual reproduction — it needs two parents, special reproductive organs, gametes that must meet, and often a long developmental period. So why do most complex organisms still take this route? The answer is one word: variation.
11.5 Why Sexual Reproduction? The Power of Variation
Asexual reproduction makes near-identical copies of the parent. If the environment never changed, this would be perfect. But environments do change — temperatures rise and fall, new pathogens emerge, food supplies fluctuate. A population of clones is dangerously uniform: a single new disease could wipe out everyone.
Sexual reproduction continually shuffles the genes of two parents into new combinations every time an offspring is produced. The chance that some individuals carry just the right combination of features to survive a new challenge is far higher in a varied population. Over many generations, this is the engine that drives evolution.
Key Idea: Sexual reproduction is "expensive" in time and energy, but it produces the genetic variety that allows species to keep up with a changing environment.
11.6 Gametes — Half a Genetic Set Each
The cells used in sexual reproduction are called gametes. They are produced by the reproductive organs of the parents — testes in the male animal and ovary in the female animal; anther in the male part of a flower and ovary inside the pistil in the female part.
Body cells (somatic cells) of an organism contain two sets of chromosomes — one set inherited from each parent. Such cells are called diploid (2n). If two diploid cells fused, the offspring would have four sets of chromosomes — and this number would double again every generation. To prevent this, gametes are produced with only one set of chromosomes — they are haploid (n). When two haploid gametes fuse, the diploid number is restored and remains constant from generation to generation.
Body cell (parent): diploid \(2n\) → Gamete: haploid \(n\)
Male gamete \(n\) + Female gamete \(n\) → Zygote \(2n\)
Male gamete \(n\) + Female gamete \(n\) → Zygote \(2n\)
11.7 Fertilization — The Meeting of Gametes
The fusion of a male gamete with a female gamete to form a zygote is called fertilization. Depending on where this fusion happens, fertilization is of two kinds:
External fertilization
The female releases eggs into the surrounding water and the male releases sperm over them. The two gametes meet outside the body. This is common in fish, frogs and most aquatic animals. Because many gametes are wasted (eaten, dispersed, dried out), these animals release eggs and sperm in huge numbers — a single frog may lay thousands of eggs at a time.
Internal fertilization
The male transfers sperm directly into the female's body, where the egg is fertilized inside. This happens in reptiles, birds and mammals (including humans). Far fewer gametes are produced because the chance of fusion is much higher when fertilization is internal. The developing embryo is also better protected.
11.8 The Zygote Becomes an Embryo
Once fertilization is complete, the zygote does not stay a single cell for long. It begins to divide — first into 2, then 4, 8, 16 cells. As the cell number rises, groups of cells start to specialise: some will form the nervous system, some the muscles, some the skin. This developing ball of cells is now called an embryo.
The embryo continues to grow and differentiate. In flowering plants the embryo develops inside the seed and waits for favourable conditions to germinate. In mammals the embryo develops inside the mother's uterus, drawing nutrition through the placenta until birth. In birds and reptiles the embryo develops inside an egg laid outside the body, using the food stored in the yolk.
👶 Zygote to Newborn — Step through the four stages L3 Apply
A single fertilised cell becomes a complete new individual through four major stages. Click each stage in order to walk through the process — what is happening at the cellular level, and what comes next?
Click each stage from left to right to step through the development from zygote to a new individual.
11.9 Variation — The Hidden Engine of Evolution
Sexual reproduction creates variation in three powerful ways:
- Mixing of two genomes. The offspring inherits a combination of genes from two different individuals. The new combination has never existed before.
- Random distribution of chromosomes. When gametes are formed, the chromosomes inherited from the grandfather and grandmother are shuffled into different combinations.
- DNA copying errors. As in asexual reproduction, occasional small copying errors add new variants.
These variations are tested by the environment. Individuals with useful variations are more likely to survive and reproduce, and those features become more common in the next generation. Over thousands of generations, this slow filter — known as natural selection — produces new species. So the link is direct: reproduction → variation → selection → evolution.
Definition. The genetic differences seen between offspring of the same parents and between members of the same species are collectively called variations. They are essential raw material for evolution.
11.10 Reproduction in Animals — A Quick Tour
Animal reproduction shows great diversity, but the underlying logic is the same — male gamete (sperm) meets female gamete (egg), forms a zygote, develops into an embryo and finally produces a new individual.
- Fish & amphibians (frog, toad): External fertilization in water. Eggs hatch into a larval stage (e.g. tadpole) before becoming an adult.
- Reptiles & birds: Internal fertilization. Eggs are laid with a tough shell that protects the embryo as it develops outside the mother's body.
- Mammals: Internal fertilization. The embryo develops inside the mother's uterus, nourished by the placenta. Young are born live and fed on mother's milk.
11.11 Activity — Tracing Variation in Your Family
Activity 11.2 — Family Trait SurveyL4 Analyse
Predict first: Which of your features look like your father's, which like your mother's, and which look unlike either parent? Will all your siblings (or cousins) show the same combination as you?
- Make a small table with rows for: hair colour, eye colour, ear lobe (free / attached), ability to roll the tongue, dimples on cheek.
- Fill in your own value for each row, then your mother's, your father's, and one or two siblings/cousins.
- For each trait, mark whether it matches the father, the mother, both, or neither.
- Compare the rows across siblings — are any two of you exactly identical?
Observations: Most students find that they share some traits with each parent, and a few traits differ from both. Siblings will have overlapping but not identical sets of traits.
Conclusion: Sexual reproduction has shuffled the genes of two parents into a new combination in each child. This is exactly the variation that, on a large scale and over many generations, drives evolution. No two children of the same parents are genetically identical (except identical twins).
Conclusion: Sexual reproduction has shuffled the genes of two parents into a new combination in each child. This is exactly the variation that, on a large scale and over many generations, drives evolution. No two children of the same parents are genetically identical (except identical twins).
Quick Recap
| Term | Meaning |
|---|---|
| Gamete | Haploid (n) reproductive cell — sperm or egg/ovum. |
| Fertilization | Fusion of male gamete and female gamete → zygote. |
| Zygote | Diploid (2n) first cell of the new individual. |
| Embryo | Multicellular stage formed by repeated division of the zygote. |
| External fertilization | Outside the body, in water — fish, frog. |
| Internal fertilization | Inside female's body — reptiles, birds, mammals. |
| Variation | Genetic differences among offspring; raw material of evolution. |
Competency-Based Questions
A pair of frogs lays nearly 4000 eggs in a pond at the start of the monsoon. Within a week, only about 2200 of these have visibly developed into tadpoles. Some unfertilized eggs decay; many fertilized embryos are eaten by fish or get washed away.
Q1. Why does the female frog lay such a large number of eggs at once? L3
Frogs use external fertilization. A large fraction of the gametes never fuse, and even fertilized eggs are at high risk in open water. Producing many eggs ensures that at least some develop successfully into the next generation.
Q2. If a frog used internal fertilization like mammals, how would the egg number likely change? L4
It would fall sharply. Internal fertilization keeps the gametes safely inside the female's body, sperm reach the eggs reliably, and the embryo is better protected — so very few eggs are needed. Most mammals produce only a handful of offspring at a time.
Q3. Match column A with column B. L1
(i) Sperm — (ii) Ovum — (iii) Zygote — (iv) Embryo
(p) 2n cell formed after fertilization (q) Multicellular developing stage (r) Female gamete (s) Male gamete
(i) Sperm — (ii) Ovum — (iii) Zygote — (iv) Embryo
(p) 2n cell formed after fertilization (q) Multicellular developing stage (r) Female gamete (s) Male gamete
(i)–(s); (ii)–(r); (iii)–(p); (iv)–(q).
Q4. State whether True or False: "Variation is harmful for a species and should be minimized." L2
False. Variation is essential — it allows a species to cope with new diseases or environmental changes. Populations without variation are far more vulnerable to extinction.
Q5. If gametes were diploid (2n) instead of haploid (n), what problem would arise after a few generations? L5
The chromosome number of the zygote would double every generation (2n + 2n = 4n, then 8n, 16n …). Such enormous chromosome sets would disrupt cell function and the species would not survive. Haploid gametes are the elegant solution that keeps the chromosome number constant after fertilization.
Assertion–Reason Questions
Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.
A: Sexual reproduction produces more variation among offspring than asexual reproduction.
R: Sexual reproduction combines genes from two different parents and shuffles chromosomes during gamete formation.
(A) Both statements are true and R is the exact reason — mixing of two genomes plus chromosomal shuffling is what generates the extra variation.
A: Mammals release very large numbers of eggs each cycle.
R: Internal fertilization in mammals reduces gamete wastage, so few eggs are required.
(D) Assertion is false — mammals release very few eggs at a time. Reason is true and is exactly why mammals do not need many eggs.
A: A zygote is diploid, while gametes are haploid.
R: The zygote is formed by the fusion of two haploid gametes.
(A) Both statements are true and R correctly explains A — the fusion of n + n gives 2n.
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