16.1 Passage of Information from Parents to Offspring
- Syllabus
- 9700–2028–2029
- Topic
- 16.1
- Level
- A2
Learning objectives
16.1.1Haploid (n) and diploid (2n)• Haploid (n) and diploid (2n)16.1.2Homologous pairs of chromosomes• Homologous pairs of chromosomes16.1.3Reduction division• Need for reduction division during meiosis for gamete production16.1.4Chromosome behaviour during meiosis• Chromosome behaviour during meiosis: nuclear envelope, cell surface membrane, spindle16.1.5Meiosis I and II stages• Main stages: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II16.1.6Meiosis image interpretation• Interpret photomicrographs and diagrams of meiosis stages16.1.7Genetic variation in meiosis• Crossing over and random orientation (independent assortment) produce genetically different gametes16.1.8Random fertilisation and variation• Random fusion of gametes at fertilisation produces genetically different individuals
Haploid cells carry one chromosome set; diploid cells carry homologous pairs
A haploid cell has one complete set of chromosomes, written n. A diploid cell has two complete sets, written 2n, so its chromosomes occur as homologous pairs: one chromosome of each pair was inherited through each parent.
- Gametes, such as egg and sperm cells, are haploid and contain one chromosome from each homologous pair.
- Most body cells are diploid and contain both homologues of each pair.
- Haploid and diploid describe the number of chromosome sets, not the total number of chromosomes; n does not mean one chromosome in every species.
During fertilisation, two haploid gamete nuclei fuse: n + n → 2n. The zygote is therefore diploid, restoring the two chromosome sets needed for the next generation. Keeping the gametes haploid prevents chromosome-set number from doubling at each fertilisation.
Chromosome-set number is not the same as DNA amount or chromatid number. This card defines ploidy and the gamete/body-cell boundary; it does not explain the stages of meiosis or sources of genetic variation.
Homologous chromosomes carry the same gene loci but may carry different alleles
A homologous pair contains two chromosomes with the same genes in the same order at corresponding loci. They have the same characteristic shape, length and centromere position, but the allele at a locus may differ between the two homologues.
- In a diploid zygote, one chromosome of each homologous pair came from the female gamete and the other came from the male gamete.
- The homologues therefore carry corresponding genetic information, but they are not necessarily genetically identical because their alleles may differ.
- A locus is the position of a gene on a chromosome; the allele is the particular form found at that locus.
Matching genes in the same order allows homologous chromosomes to line up alongside one another during meiosis. This pairing relationship is different from sister chromatids: sister chromatids are replicated copies of one chromosome joined at a centromere, whereas homologues are the maternal and paternal members of a pair.
Homologous chromosomes are not the same as sister chromatids and do not have to carry the same alleles. This card stops at homologue identity and pairing; crossing over and independent assortment are covered separately.
Meiosis is a reduction division that makes haploid cells from a diploid cell
Meiosis is a form of nuclear division in plants and animals that produces haploid cells for sexual reproduction. It starts from a diploid cell and includes one round of DNA replication followed by two nuclear divisions: meiosis I and meiosis II.
- DNA is replicated once, so each chromosome consists of two sister chromatids joined at a centromere.
- In meiosis I, homologous chromosomes pair and whole homologues are separated, reducing the chromosome set number; the centromeres do not divide at this division.
- There is no further DNA replication before meiosis II. In meiosis II, centromeres divide and sister chromatids separate.
- Cytokinesis after the two divisions produces four haploid cells. In animals these develop as gametes; in plants the products contribute to sexual reproduction.
Meiosis is a reduction division because the chromosome number changes from diploid to haploid before fertilisation. This allows two haploid gamete nuclei to fuse and restore the diploid number. Unlike mitosis, meiosis has two successive nuclear divisions and reduces chromosome number.
DNA replication happens once, not before each division. Meiosis I separates homologues; meiosis II separates sister chromatids. The detailed PMAT image-reading method and the mechanisms that generate genetic variation are covered separately.
Meiosis coordinates chromosomes, spindle, nuclear envelope and cell division
| Meiosis I | Chromosomes and spindle | Nuclear envelope and cell surface membrane |
|---|---|---|
| Prophase I | replicated homologues pair as bivalents; non-sister chromatids may cross over; spindle forms | nuclear envelope breaks down |
| Metaphase I | bivalents align at the equator; spindle fibres attach to centromere regions from opposite poles | envelope remains absent |
| Anaphase I | homologous chromosomes move to opposite poles; centromeres do not divide, so sister chromatids remain joined | cell elongates |
| Telophase I | chromosomes reach poles | nuclear envelopes may reform; cell-surface membrane divides the cytoplasm into two haploid cells |
| Meiosis II | Chromosomes and spindle | Nuclear envelope and cell surface membrane |
|---|---|---|
| Prophase II | chromosomes condense if needed; a new spindle forms in each cell | nuclear envelopes break down if they reformed |
| Metaphase II | chromosomes align singly at each equator; spindle fibres attach from opposite poles | envelope absent |
| Anaphase II | centromeres divide and sister chromatids move to opposite poles | each cell elongates |
| Telophase II | chromosomes reach poles and decondense | nuclear envelopes reform; membranes divide cytoplasm to produce four haploid cells |
The reduction step is anaphase I: homologues separate while centromeres stay intact. Centromeres divide only in anaphase II when sister chromatids separate.
Eight named stages form two related meiotic divisions
| Stage | Fast recognition cue |
|---|---|
| Prophase I | homologues pair as bivalents; crossing over may be visible as chiasmata |
| Metaphase I | homologous pairs align together at the equator |
| Anaphase I | whole homologues separate; centromeres remain intact |
| Telophase I | two haploid chromosome groups/nuclei form |
| Prophase II | chromosomes recondense and new spindles form in the two cells |
| Metaphase II | chromosomes align singly at each equator |
| Anaphase II | centromeres divide and sister chromatids separate |
| Telophase II | four haploid chromosome groups/nuclei form |
Use the repeated PMAT order twice, but identify the separating unit. Meiosis I pairs and separates homologous chromosomes; meiosis II aligns and separates sister chromatids. No DNA replication occurs between the two divisions.
Do not label a stage from PMAT position alone. Metaphase I shows paired homologues, whereas metaphase II shows chromosomes aligned singly; anaphase I preserves centromeres, whereas anaphase II divides them.
A meiosis image is read by identifying pairing, alignment and separation
Interpret a meiosis photomicrograph or diagram by matching visible chromosome behaviour to the division and stage. Use pairing, alignment, centromere behaviour and cell number together rather than relying on one shape or position.
- Decide whether meiosis I or meiosis II is shown. Homologous chromosomes paired side by side indicate meiosis I; single chromosomes suggest meiosis II. Two forming cells support meiosis I, while four forming haploid cells support meiosis II.
- Identify the PMAT evidence: condensed chromosomes in prophase; chromosomes aligned at the spindle equator in metaphase; chromosomes or chromatids moving to opposite poles in anaphase; nuclei reforming and cytokinesis beginning in telophase.
- Check the separation event. In anaphase I, whole homologous chromosomes move while centromeres remain intact. In anaphase II, centromeres divide and sister chromatids move apart.
- Cross-check spindle direction, nuclear-envelope change and cell-surface division, then state the stage only when multiple visible clues agree.
A crowded, rotated or unevenly stained image may hide a clue. Record only visible evidence, explain how the evidence supports the stage label, and avoid inventing chromosome details that the image does not show.
A pair of dark shapes is not automatically a homologous pair, and anaphase I is not the same as anaphase II. This card is an image/diagram interpretation method; it does not add a separate mechanism for genetic variation.
Three meiotic chromosome events create different gametes
| Meiotic event | What is random or exchanged | How gametes differ |
|---|---|---|
| Crossing over in prophase I | corresponding DNA sections exchange between non-sister chromatids of homologues at chiasmata | recombinant chromatids carry new combinations of alleles on the same chromosome |
| Random orientation at metaphase I | each homologous pair faces either pole independently of other pairs | different mixtures of whole maternal and paternal homologues enter daughter cells |
| Random orientation at metaphase II | sister chromatids, which may no longer be identical after crossing over, can face either pole | different chromatids enter the final gametes |
These events operate together: crossing over reshuffles linked alleles within chromosomes, while independent orientation distributes whole homologues and then chromatids into different cells. The four haploid products can therefore carry different allele combinations.
Crossing over is between non-sister chromatids of homologous chromosomes, not sister chromatids. Random fertilisation is a separate source of variation taught next; it does not occur during meiosis.
Random fertilisation adds another independent source of genetic variation
Meiosis produces haploid gametes that already differ because of chromosome behaviour. Random fertilisation adds a further source of variation: any genetically different male gamete may fuse with any genetically different female gamete, so the zygote receives a new combination of alleles from the two parents.
- Each parent contributes one gamete, but the particular sperm and egg that fuse are not predetermined. Their random fusion samples from two varied gamete pools.
- The resulting zygote is diploid and combines one haploid chromosome set from each gamete; different pairings can therefore produce different allele combinations in offspring.
Meiosis creates variation within each parent through crossing over and independent assortment. Random fertilisation combines that parental variation, so offspring from the same parents can differ even when no new mutation is being considered.
Random fertilisation describes the genetic sampling principle, not a claim that every pairing has equal population frequency. This card completes the meiosis-to-offspring variation chain; it does not replace the neighbouring cards’ mechanisms for crossing over, independent assortment or image-stage identification.