Ch 3 · Meiosis · IGCSE Biology 0610

Topic 3 of 4 in Inheritance — IGCSE Biology 0610 — 48 lessons.

Introduction

If sperm and egg cells were made by mitosis, each would carry 46 chromosomes and every zygote would have 92. The chromosome number would double in every generation. Meiosis solves this by halving the number, and at the same time it shuffles the alleles so that every gamete is different. This short lesson states where meiosis happens, describes it (Supplement) as a reduction division, and sets out the comparison with mitosis that examiners ask for again and again, usually as a table.

Key ideas — Meiosis makes gametes

Meiosis is involved in the production of gametes. In humans it occurs in the testes (producing sperm) and the ovaries (producing egg cells); in flowering plants it occurs in the anthers (pollen) and ovules. Every other cell division in the body is mitosis.

🧑‍🏫 Teacher's explanation

Imagine you have a massive instruction manual for building a house, consisting of two identical sets of blueprints: one from your mother and one from your father. If you wanted to pass this manual on to your child, you couldn't give them the whole thing, or the manual would double in size every single generation! Instead, you must perform a special kind of 'splitting' process where you pick only one set of blueprints to pass on.

📖 Definitions

🔍 Worked example — Easy

A human cell contains 46 chromosomes. Calculate the number of chromosomes in a gamete produced by this cell via meiosis.

🔍 Worked example — Standard

Compare the products of mitosis and meiosis in terms of genetic identity and chromosome number.

🔍 Worked example — Exam-style

A plant species has a diploid number of 2n = 12. Describe the outcome of meiosis in this plant regarding the number of cells produced and their chromosome count.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Meiosis as a reduction division

🧑‍🏫 Teacher's explanation

Imagine you have a deck of cards consisting of two identical sets: one red deck and one blue deck. If you want to make a new, smaller deck that contains only one card of each rank (e.g., one Ace, one King, one Queen), you have to split the sets. Meiosis is like this process of splitting and shuffling. Instead of just making exact copies of the deck, you are splitting the pairs and mixing the cards up so that every new small deck is unique.

📖 Definitions

🔍 Worked example — Easy

A human skin cell contains 46 chromosomes. If this cell were to undergo mitosis, how many chromosomes would the daughter cells have? If it were to undergo meiosis, how many chromosomes would the gametes have?

🔍 Worked example — Standard

Compare mitosis and meiosis in terms of the number of daughter cells produced and their genetic composition.

🔍 Worked example — Exam-style

Explain why meiosis results in genetic variation. Mention the two specific processes involved.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the number of chromosomes in each of the resulting gametes. [1] (b) Describe two ways in which the resulting gametes are genetically different from each other. [2] [3 marks]

Key ideas — Comparing mitosis and meiosis

The syllabus specifies four points of comparison. Learn them as a table and reproduce it in that form when asked.

🧑‍🏫 Teacher's explanation

Think of mitosis as a high-quality photocopier. When you use a photocopier to make a copy of a document, the goal is to get an exact replica that is identical to the original. This is what your body does when you grow or heal a wound; it needs exact copies of cells to maintain the body's blueprint.

📖 Definitions

🔍 Worked example — Easy

A skin cell in a human has 46 chromosomes. If this cell undergoes mitosis, how many chromosomes will each daughter cell have?

🔍 Worked example — Standard

A plant cell has 20 chromosomes. Calculate the number of chromosomes in each of the gametes produced via meiosis.

🔍 Worked example — Exam-style

Compare the outcome of mitosis and meiosis in terms of the number of daughter cells produced and their genetic composition.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the number of chromosomes in each of the resulting daughter cells. [1] (b) Compare the daughter cells produced by meiosis with those produced by mitosis in terms of their genetic identity. [2] [3 marks]

Key ideas — Why the halving matters

Consider a diploid organism with 2n = 46. Meiosis produces gametes with n = 23. Fertilisation: 23 + 23 = 46, so the offspring have the same diploid number as the parents and the number is constant across generations. Without meiosis the number would double each generation. The halving also means that each parent passes on one allele of every gene to each offspring, which is the basis of the genetic diagrams in the next lesson.

🧑‍🏫 Teacher's explanation

Imagine you are building a massive Lego castle. Every generation, you want to build the exact same castle. If you just kept adding more bricks without ever organizing them, the castle would grow uncontrollably until it covered the whole room. To keep the castle the same size, you need a rule: every new generation must start with exactly half the bricks from the previous one.

Ch 3 · Meiosis · IGCSE Biology 0610

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