Ch 1 · 16.1 Passage of information from parents to offspring · AS & A Level Biology 9700
Topic 1 of 3 in Inheritance — AS & A Level Biology 9700 — 68 lessons.
Introduction
Sexual reproduction joins two gametes, so each gamete must carry half the normal number of chromosomes. Meiosis achieves this, and at the same time shuffles the genetic material so that every gamete, and every offspring, is different. (Syllabus section 16.1.)
Key ideas — Haploid, diploid and homologous pairs
🧑🏫 Teacher's explanation
Think of your DNA like a massive library of instruction manuals. In a diploid cell, you have two copies of every single manual—one set from your mother and one set from your father. These copies are like 'matching editions' of the same book; they have the same chapters (genes) in the same order, even if one edition uses different words (alleles) for a specific sentence.
📖 Definitions
🔍 Worked example — Easy
A species of plant has a diploid number of 2n = 24. Calculate the number of chromosomes in a pollen grain produced by this plant.
🔍 Worked example — Standard
A cell is observed under a light microscope during meiosis. It contains 20 chromosomes, but they are arranged in 10 pairs of homologous chromosomes. Determine if this cell is in a diploid or haploid state and explain why.
🔍 Worked example — Exam-style
Explain how the process of meiosis leads to genetic variation in offspring. Focus on the mechanisms occurring during meiosis.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the number of chromosomes in a human sperm cell. [1] (b) Explain why it is necessary for gametes to be haploid. [2] (c) Describe the significance of 'crossing over' during meiosis in terms of genetic variation. [2] [5 marks]
Key ideas — Why a reduction division is needed
At fertilisation two gametes fuse. If gametes were diploid, the zygote would have four sets of chromosomes, and the number would double every generation. Meiosis is a reduction division: it halves the chromosome number from diploid to haploid, so that when two haploid gametes fuse the diploid number is restored and stays constant from generation to generation.
🧑🏫 Teacher's explanation
Imagine you have a recipe book that contains 46 pages. If you and a partner each contributed a full 46-page book every time you had a child, the child's book would have 92 pages, and the grandchild's would have 184. This exponential growth would quickly make life impossible. To keep the 'book' the same size for every generation, we need to make a photocopy of only half the pages before passing it on.
📖 Definitions
🔍 Worked example — Easy
A species of flowering plant has a diploid number of 2n = 24. Calculate the number of chromosomes present in a pollen grain produced by this plant.
🔍 Worked example — Standard
A cell is observed under a light microscope during meiosis. It contains 20 chromosomes, but they are arranged in 10 pairs of homologous chromosomes. State whether the cell is in a haploid or diploid state and explain why.
🔍 Worked example — Exam-style
Explain why meiosis is described as a 'reduction division' and why this is essential for sexual reproduction.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the number of chromosomes in a gamete of this species. [1] (b) Explain why it is necessary for gametes to be haploid rather than diploid. [3] [4 marks]
Key ideas — The stages of meiosis I
Before meiosis, DNA replicates in interphase, so each chromosome has two sister chromatids.
🧑🏫 Teacher's explanation
Imagine you have two identical decks of playing cards, one red and one blue. Each deck represents a set of homologous chromosomes. In your body cells (diploid), you have both decks. To make a baby, you can't just give them both decks, or the baby would have double the cards every generation. Instead, you perform a 'reduction division' to shuffle the cards and hand out only half a deck (haploid) to each gamete.
📖 Definitions
🔍 Worked example — Easy
A cell in a species has a diploid number of 2n = 46. How many chromosomes will be present in each cell after the completion of Meiosis I?
🔍 Worked example — Standard
A student observes a cell under a light microscope during meiosis. The chromosomes are arranged in pairs along the equator of the cell. Identify the stage of meiosis and explain why the cell is not yet haploid.
🔍 Worked example — Exam-style
Compare the outcomes of mitosis and meiosis I in terms of genetic variation and chromosome number.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain how crossing over and independent assortment contribute to genetic variation in the resulting gametes. [3] (b) Describe the behavior of chromosomes during Anaphase I. [2] [5 marks]
Key ideas — The stages of meiosis II
There is no further DNA replication. Meiosis II is like mitosis, in each of the two haploid cells:
🧑🏫 Teacher's explanation
Think of meiosis II as the final sorting process in a factory. In meiosis I, you already separated the big 'families' (homologous chromosomes) into two different rooms. Now, in meiosis II, you are taking those specific bundles and splitting the individual 'instruction manuals' (sister chromatids) so that each final package contains only one copy of each instruction.
📖 Definitions
🔍 Worked example — Easy
A diploid organism has a chromosome number of 2n = 8. How many chromosomes will be present in each cell after the completion of meiosis II?
🔍 Worked example — Standard
Compare the chromosome count and DNA content of a cell in Metaphase I versus a cell in Metaphase II for a cell where 2n = 4.
🔍 Worked example — Exam-style
An examiner shows a diagram of a cell where centromeres are splitting and sister chromatids are moving to opposite poles. Identify the stage and explain why this stage results in haploid cells.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the behavior of the chromosomes during Anaphase II. [2] (b) Explain why the resulting four gametes are haploid. [2] [4 marks]
Key ideas — Identifying stages in photomicrographs
Count the chromosomes: in meiosis II each cell has only half the diploid number.
🧑🏫 Teacher's explanation
Imagine you are organizing a massive dance where everyone must find their exact partner. In a diploid cell, you have pairs of dancers (homologous chromosomes) that look similar but carry different versions of the same instructions. Meiosis is the process of taking these pairs and splitting them up so that each resulting dancer (gamete) only has one partner's instructions.
📖 Definitions
🔍 Worked example — Easy
A cell in a plant anther is observed under a light microscope. You see chromosomes arranged in homologous pairs (bivalents) at the equator of the cell. Identify the stage of meiosis.
🔍 Worked example — Standard
A cell has a diploid number of 2n = 46. If this cell undergoes meiosis, how many chromosomes will be present in each cell during Anaphase II?
🔍 Worked example — Exam-style
A student observes a photomicrograph of a cell where chromosomes are being pulled towards opposite poles, but they are single-stranded (no sister chromatids attached). The cell is part of a division resulting in four distinct nuclei. Identify the stage and explain the genetic significance of the movement observed.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the stage of meiosis shown. [1] (b) Explain how this stage contributes to the genetic variation in the resulting gametes. [2] [3 marks]
Key ideas — Sources of genetic variation
🧑🏫 Teacher's explanation
Imagine you have two decks of cards: one red (maternal) and one blue (blue). If you want to make a new deck, you don't just keep them separate. First, you might swap a few cards between a red and a blue card—this is like 'crossing over'. Then, when you deal the cards out, you don't always pick the red one first; the order is totally random—this is 'independent assortment'. Finally, you take one random hand from the red deck and mix it with one random hand from the blue deck—this is 'random fusion'.
📖 Definitions
🔍 Worked example — Easy
A species of plant has a diploid number of 2n = 10. How many different combinations of chromosomes can be produced in the gametes due to independent assortment alone?
🔍 Worked example — Standard
Describe the effect of crossing over on the genetic composition of gametes.
🔍 Worked example — Exam-style
Explain why meiosis is described as a 'reduction division' and why this is essential for sexual reproduction.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the stage of meiosis shown. [1] (b) Explain how this stage contributes to the genetic variation of the resulting gametes. [2] [3 marks]
Worked example
Question. A cell from an animal with a diploid number of 8 undergoes meiosis. (a) State the number of chromosomes in each cell at the end of meiosis I and at the end of meiosis II. [2] (b) Calculate the number of different combinations of chromosomes possible in the gametes, from independent assortment alone. [1] (c) Explain why meiosis is necessary for sexual reproduction. [2]
🔍 Worked example — Easy
A species of plant has a diploid number of 2n = 12. State the number of chromosomes present in a gamete produced by this plant.
🔍 Worked example — Standard
A cell with 2n = 6 undergoes meiosis. Calculate the number of possible chromosome combinations in the resulting gametes due to independent assortment.
🔍 Worked example — Exam-style
Explain how crossing over and independent assortment contribute to genetic variation in offspring.
⚠️ Common errors
Common mistakes
Practice
Answers