Passage of information
Cambridge A-Level Biology (9700) · Unit 16: Inheritance · 9 flashcards
Passage of information is topic 16.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 16 — Inheritance , alongside The roles of genes in determining phenotype and Gene control. In one line: Haploid refers to a cell or organism having a single set of unpaired chromosomes. In humans, gametes (sperm and egg cells) are haploid, containing 23 chromosomes each. This is represented as 'n'.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 9 flashcards — 3 definitions and 6 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'haploid' (n)
Haploid refers to a cell or organism having a single set of unpaired chromosomes. In humans, gametes (sperm and egg cells) are haploid, containing 23 chromosomes each. This is represented as 'n'.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain what the terms diploid (2n) and haploid (n) mean.
- Explain what homologous chromosome pairs are.
- Explain why gamete production by meiosis needs a reduction division.
- Describe what chromosomes do during meiosis in animal and plant cells, and what happens to the spindle, nuclear envelope and cell surface membrane at the same time. You need the names of the main stages (the four stages of meiosis I, namely prophase, metaphase, anaphase and telophase, and the same four in meiosis II) but not the sub-divisions of prophase I.
- Interpret diagrams and photomicrographs showing cells at various points in meiosis, and identify which main stage each cell is in.
- Explain that genetically different gametes result from crossing over and from the random orientation (independent assortment) of homologous chromosome pairs and sister chromatids in meiosis.
- Explain that genetically different individuals arise because gametes fuse randomly at fertilisation.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Passage of information
- › Specify that crossing over involves the exchange of 'alleles' between 'non-sister chromatids' of homologous chromosomes.
- › In anaphase I, spindle fibres shorten and pull homologous chromosomes (not chromatids) to opposite poles.
- › Use 'independent assortment' when discussing the random alignment of homologous pairs at the equator; use 'crossing over' for the exchange of alleles between non-sister chromatids.
- › Define haploid (n) as one set of chromosomes and diploid (2n) as two sets of chromosomes, applicable to all organisms.
Define the term 'haploid' (n).
Haploid refers to a cell or organism having a single set of unpaired chromosomes. In humans, gametes (sperm and egg cells) are haploid, containing 23 chromosomes each. This is represented as 'n'.
Define the term 'diploid' (2n).
Diploid refers to a cell or organism containing two complete sets of chromosomes, one from each parent. Somatic cells (body cells) in humans are diploid, containing 46 chromosomes (23 pairs). This is represented as '2n'.
What are homologous pairs of chromosomes?
Homologous chromosomes are pairs of chromosomes, one inherited from each parent, that have the same genes at the same loci. They are similar in size, shape, and banding pattern. They pair up during meiosis I.
Explain the necessity of reduction division during meiosis in gamete production.
Reduction division (meiosis) is necessary to halve the chromosome number in gametes. Without it, fertilization would result in offspring with double the normal chromosome number, leading to genetic abnormalities. Meiosis ensures that the diploid number is maintained across generations.
Outline the main events that occur during Anaphase I of meiosis.
During Anaphase I, homologous chromosomes are separated and pulled to opposite poles of the cell. Sister chromatids remain attached at the centromere. This segregation reduces the chromosome number from diploid to haploid.
Outline the main events that occur during Metaphase II of meiosis.
During Metaphase II, in each of the two haploid cells, the chromosomes (each still made of two sister chromatids joined at the centromere) line up singly on the equator of a new spindle. Spindle fibres from opposite poles attach to the centromere of each chromosome, and the chromatids are randomly oriented.
Explain how crossing over during Prophase I of meiosis increases genetic variation.
Crossing over involves the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I. This creates new combinations of alleles on each chromosome, increasing genetic variation in the resulting gametes.
Explain how independent assortment (random orientation) increases genetic variation.
Independent assortment refers to the random orientation of homologous chromosome pairs at the metaphase plate during Metaphase I of meiosis. Each pair aligns independently of the others, resulting in different combinations of maternal and paternal chromosomes in the daughter cells, which increases genetic variation.
Explain how the random fusion of gametes at fertilization produces genetically different individuals.
The fusion of any one sperm cell with any one egg cell during fertilization is a random process. Since gametes are genetically unique due to crossing over and independent assortment, and fertilization is random, an enormous number of genetically different zygotes are possible.
Review the material
Read full revision notes on Passage of information — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 16 — Inheritance
Passage of information sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Passage of information deck
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