Ch 2 · Sexual Reproduction · O Level Biology 5090
Topic 2 of 5 in Reproduction — O Level Biology 5090 — 48 lessons.
Introduction
Brothers and sisters share the same two parents yet look and behave differently. That difference is the signature of sexual reproduction. Two specialised cells, the gametes, each carry half a set of genetic instructions and combine to make a new, unique individual. This short lesson fixes the Cambridge definitions of sexual reproduction and fertilisation, introduces haploid and diploid nuclei, and asks candidates to weigh the costs and benefits of variation for wild populations and for farmers.
Key ideas — The definitions
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other.
🧑🏫 Teacher's explanation
Think of sexual reproduction as a high-stakes card game. Each parent has a full deck of 46 cards (chromosomes), but they only deal you half the deck (23 cards) via their gametes. When the sperm and egg meet, they don't just mix; they shuffle their specific cards together. This ensures that while you look like your parents, you are a unique combination of their genetic material, unlike a photocopy (asexual reproduction).
📖 Definitions
🔍 Worked example — Easy
Define fertilisation and state the ploidy of the cells involved before and after the process in humans.
🔍 Worked example — Standard
Explain why offspring produced by sexual reproduction are genetically different from each other, whereas offspring from asexual reproduction are clones.
🔍 Worked example — Exam-style
Discuss one advantage and one disadvantage of sexual reproduction for a population of a species in the wild.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain why the offspring of sexual reproduction are genetically different from each other. (2 marks) (c) State the chromosome number in a human gamete and a human zygote. (2 marks) [6 marks]
Key ideas — Haploid gametes, diploid zygote
chromosome numbers. The nuclei of gametes are haploid: they contain a single set of chromosomes (23 in humans). The nucleus of a zygote is diploid: it contains two sets (46 in humans), one from each gamete. Gametes are produced by meiosis, which halves the chromosome number, so that fertilisation restores the diploid number rather than doubling it every generation. Human body cells formed from the zygote by mitosis are all diploid.
🧑🏫 Teacher's explanation
Imagine you have a recipe book with 23 pages. To make a new person, you need a complete recipe, but you don't want the book to double in size every time a baby is born. So, the parents each create a special 'half-recipe' card (a gamete) containing just one copy of each page. When these two half-recipes meet and fuse, they create a full 23-page recipe again (the zygote). This ensures the next generation has the correct number of instructions, not 46 or 92.
📖 Definitions
🔍 Worked example — Easy
A fruit fly has a diploid chromosome number of 2n = 8. How many chromosomes are in its sperm cell, and what is the chromosome number of the zygote after fertilisation?
🔍 Worked example — Standard
Explain why the chromosome number in a species remains constant from generation to generation, despite the fusion of two gametes during sexual reproduction.
🔍 Worked example — Exam-style
A student claims: 'If a human egg cell (23 chromosomes) fuses with a human sperm cell (23 chromosomes), the resulting zygote will have 46 chromosomes, which is double the number in the parents.' Explain why the student's conclusion is incorrect regarding the stability of the species' chromosome number.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify which cell is a gamete and which is a zygote. (b) Explain how the chromosome number in Cell C was restored to 46 after the fusion of the nuclei of two gametes. (c) State one advantage of sexual reproduction in terms of population survival. [6 marks]
Key ideas — Sources of genetic difference
Two events make each zygote unique:
🧑🏫 Teacher's explanation
Imagine you have two standard 52-card decks, one red and one blue. During meiosis, you shuffle each deck and deal a hand of 23 cards to a new player. Because the shuffle is random, no two hands are identical. Then, during fertilisation, you pick one red hand and one blue hand at random from a massive pile of millions of red hands and one blue hand. The resulting 46-card deck (the zygote) is a unique combination that has never existed before and will never exist again.
📖 Definitions
🔍 Worked example — Easy
A human sperm cell has 23 chromosomes. A human egg cell has 23 chromosomes. How many chromosomes are in the zygote formed by their fusion?
🔍 Worked example — Standard
Explain why two children from the same parents are genetically different, even if they are not twins.
🔍 Worked example — Exam-style
A farmer grows a crop using cuttings (asexual reproduction) versus seeds (sexual reproduction). Discuss one advantage and one disadvantage of using sexual reproduction for crop production in a wild environment.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Explain why sexual reproduction leads to genetic variation in a population. (3 marks) (c) A population of rabbits lives in a stable environment with no new predators. A new virus is introduced. Explain why sexual reproduction is advantageous for the survival of the rabbit population in this scenario. (2 marks) [7 marks]
Key ideas — Advantages and disadvantages
In the wild, advantages: offspring show genetic variation, so if the environment changes (new disease, new predator, climate change) some individuals are likely to have features that let them survive and reproduce; the population can adapt through natural selection and is less likely to become extinct. Harmful mutations are less likely to be passed to every offspring.
🧑🏫 Teacher's explanation
Think of sexual reproduction as a 'shuffle' rather than a 'copy'. When two parents contribute genetic material, the resulting offspring are like a new, unique deck of cards. This shuffling creates variation, which is the raw material for evolution. In the wild, this is a survival strategy: if a new disease hits, a uniform population (asexual) might die out, but a varied population (sexual) will likely have some survivors with resistance.
📖 Definitions
🔍 Worked example — Easy
Explain why sexual reproduction is advantageous for a population of animals living in a habitat where the climate is changing rapidly.
🔍 Worked example — Standard
State the ploidy of the nuclei in a human sperm cell, an egg cell, and the resulting zygote.
🔍 Worked example — Exam-style
A farmer grows wheat using sexual reproduction (seeds) rather than asexual reproduction (clones). Explain one advantage and one disadvantage of this choice for large-scale commercial production.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
(a) The fusion of the nuclei of (two) gametes [B1].
🔍 Worked example — Easy
A species of flowering plant has a diploid number of 2n = 20. How many chromosomes are found in its pollen grain (gamete)?
🔍 Worked example — Standard
Compare the genetic makeup of offspring produced by asexual reproduction versus sexual reproduction.
🔍 Worked example — Exam-style
A population of bacteria reproduces asexually. A new antibiotic is introduced. Explain why the entire population might be wiped out, whereas a sexually reproducing population might survive.
⚠️ Common errors
Common mistakes
Practice
Questions