16. Inheritance
- Syllabus
- 9700–2028–2029
- Section
- 16
- Level
- A2
Exam analysis
16.1 Passage of Information from Parents to Offspring
- Syllabus
- 9700–2028–2029
- Topic
- 16.1
- Level
- A2
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.
16.2 The Roles of Genes in Determining Phenotype
- Syllabus
- 9700–2028–2029
- Topic
- 16.2
- Level
- A2
Genetic terms locate alleles, describe expression and track generations
| Term | Precise meaning |
|---|---|
| gene | DNA base sequence that codes for a polypeptide or functional RNA |
| locus | position of a gene on a chromosome |
| allele | alternative form of a gene at the same locus |
| dominant | allele whose phenotype is expressed in a heterozygote |
| recessive | allele whose phenotype is masked by a dominant allele in a heterozygote |
| codominant | two different alleles both contribute to the heterozygote phenotype |
| genotype / phenotype | allele combination / observable characteristics produced by genotype and environment |
| homozygous / heterozygous | two identical / two different alleles at a locus |
| linkage | loci on the same chromosome tend to be inherited together |
| F1 / F2 | first filial generation / offspring produced from an F1 cross or selfing |
| test cross | cross with a homozygous recessive individual to reveal an unknown dominant-phenotype genotype |
Dominant does not mean common, beneficial or stronger. Codominance means both alleles are expressed in the heterozygote; it does not mean the alleles blend into a new allele.
Choose the inheritance model before building a genetic diagram
For every cross: define allele symbols and relationships → write parental phenotypes and genotypes → list every possible gamete → combine one gamete from each parent in a Punnett square → state offspring genotypes and phenotypes with probabilities or ratios → check probabilities sum to 1.
| Inheritance model | Required diagram decision |
|---|---|
| monohybrid complete dominance | one locus; heterozygote has dominant phenotype |
| codominance / multiple alleles | use distinct superscripts; both codominant alleles appear in a heterozygote; a population may have more than two alleles although one individual has at most two |
| sex linkage | write alleles as X-chromosome superscripts; track male and female offspring separately because the Y usually lacks the locus |
| unlinked dihybrid dominance | list four gamete types from a double heterozygote if loci assort independently |
| autosomal linkage | write linked alleles together on homologues, e.g. AB/ab; parental gametes usually exceed recombinant gametes when crossing over occurs |
| epistasis | determine how one locus masks or modifies another before converting genotypes to phenotypes; do not assume or memorise one universal ratio |
| test cross | cross the unknown dominant-phenotype individual with a homozygous recessive individual; offspring phenotypes reveal its gametes |
The Punnett square combines gametes; it does not decide which gametes are possible. A 3:1 or 9:3:3:1 ratio applies only when its dominance, segregation, viability and independent-assortment assumptions are satisfied.
Chi-squared tests whether count deviations exceed chance expectation
\chi^2=\sum\frac{(O-E)^2}{E}
- State the null hypothesis: observed and expected counts do not differ significantly; any difference is due to chance.
- Convert the genetic ratio to expected counts with the same total as the observations.
- Calculate (O−E)²/E for every category and sum the contributions.
- Use degrees of freedom = number of categories − 1.
- Compare χ² with the critical value at the chosen probability, commonly p=0.05. Above the critical value: reject the null; at or below it: fail to reject the null.
For observed 78 dominant and 22 recessive offspring under a 3:1 expectation (total 100), E=75 and 25. χ²=(3²/75)+(−3)²/25=0.48. With df=1, 0.48 is below 3.841 at p=0.05, so fail to reject the null: the deviation is compatible with chance.
Failing to reject does not prove the genetic model true. Use independent count categories—not percentages—and check that expected counts and the biological assumptions are suitable.
Four gene variants alter proteins and produce distinct phenotypes
| Gene and inheritance | Protein consequence | Phenotypic consequence |
|---|---|---|
| mutant TYR alleles; albinism is recessive | little or no functional tyrosinase, so the melanin pathway is blocked | little/no melanin in skin, hair and eyes; visual effects can occur |
| HBB allele HbS; HbA and HbS are codominantly expressed at protein level | altered beta-globin forms haemoglobin S, which is less soluble and can form fibres at low oxygen | red cells sickle, carry oxygen less effectively and may block capillaries; HbS/HbS causes sickle-cell anaemia |
| mutant F8 allele; X-linked recessive | little or no functional factor VIII | clotting cascade is impaired, fibrin formation is reduced and bleeding lasts longer |
| mutant dominant HTT allele with expanded CAG repeats | abnormal huntingtin protein is produced | progressive damage to nervous tissue causes movement, cognitive and behavioural changes |
For each example, explain the full direction: DNA allele changes the amount or structure of a named protein; altered protein function changes a cellular or physiological process; that process produces the phenotype.
Do not jump directly from gene name to symptom. The four proteins have different jobs—enzyme, oxygen-carrying protein, clotting factor and neuronal protein—so their mechanisms are not interchangeable.
Le controls a gibberellin-synthesis enzyme and stem height
| Genotype/allele | Enzyme and gibberellin | Stem phenotype |
|---|---|---|
| Le_ (LeLe or Lele) | dominant Le codes for a functional enzyme in the gibberellin-synthesis pathway, so enough active gibberellin is produced | gibberellin promotes internode cell elongation and the plant is tall |
| lele | recessive le codes for a non-functional enzyme, reducing/blocking gibberellin synthesis | internode elongation is reduced and the plant is dwarf |
Applying gibberellin can restore stem elongation in an lele dwarf if its receptors and downstream response pathway are functional. This shows that the mutation affects hormone synthesis upstream, rather than preventing cells from responding to gibberellin.
Le is an allele coding for an enzyme; it is not the hormone itself. Dominance means one functional Le allele supplies sufficient pathway activity for the tall phenotype.
16.3 Gene Control
- Syllabus
- 9700–2028–2029
- Topic
- 16.3
- Level
- A2
Structural genes make pathway products; regulatory genes make controllers
| Feature | Structural gene | Regulatory gene |
|---|---|---|
| Encoded product | polypeptide or functional RNA that performs a cellular role | RNA or regulatory protein that controls expression of other gene(s) |
| Main effect | directly supplies an enzyme, transporter or structural product | changes whether, when or how strongly target genes are expressed |
| lac example | lacZ and lacY encode proteins used in lactose uptake/metabolism | lacI encodes the repressor that controls the structural genes |
A promoter or operator is a regulatory DNA sequence, not a regulatory gene, because it does not encode the repressor. Both structural and regulatory genes are transcribed; their products differ in function.
Inducible and repressible enzymes have opposite default controls
| Feature | Inducible enzyme | Repressible enzyme |
|---|---|---|
| Default production | low/off when substrate is absent | on while an end product is scarce |
| Signal | substrate/inducer removes or prevents repression | accumulated end product/corepressor enables repression |
| Typical pathway logic | catabolic: make enzymes when the substrate is available | anabolic: stop making enzymes when enough product exists |
| Resource benefit | avoids producing substrate-use enzymes unnecessarily | avoids overproducing an end product |
Inducible does not mean permanently active, and repressible does not mean permanently absent. State the default expression and the metabolite that switches production.
Lactose removes lac repressor control so uptake and digestion genes are transcribed
The lac operon contains a promoter where RNA polymerase binds, an operator control site and structural genes including lacZ (β-galactosidase) and lacY (lactose permease). A separate regulatory gene, lacI, produces the repressor protein.
| Lactose absent | Lactose present |
|---|---|
| repressor has a shape complementary to the operator and binds it | lactose (inducer) binds the repressor and changes its shape |
| bound repressor blocks RNA polymerase from transcribing structural genes | repressor cannot bind the operator, so RNA polymerase transcribes lacZ and lacY |
| little/no permease or β-galactosidase is produced | permease increases lactose entry and β-galactosidase hydrolyses lactose |
Lactose does not bind the operator or directly switch on RNA polymerase. It binds the repressor. cAMP control is outside this syllabus objective.
Transcription factors bind DNA and change transcription rate
A transcription factor is a protein that binds to a specific DNA sequence and controls gene expression in eukaryotic cells by changing the rate of transcription.
- An activating transcription factor increases transcription, for example by helping RNA polymerase begin transcription.
- A repressing transcription factor decreases transcription, for example by preventing effective transcription initiation.
- Different cells can contain or activate different transcription factors, so the same genome can produce different patterns of gene expression.
A transcription factor is a DNA-binding protein, not RNA polymerase and not a DNA sequence. It can increase or decrease transcription; the name does not imply activation only.
Gibberellin activates genes by removing DELLA repression
- Gibberellin binds to its receptor in a target cell.
- The hormone–receptor interaction leads to breakdown of DELLA protein repressors.
- DELLA proteins normally inhibit factors that promote transcription; their removal releases these transcription-promoting factors.
- Transcription of target genes increases, so the cell produces proteins required for responses such as amylase production during germination or stem elongation.
This is activation by removing an inhibitor: gibberellin does not have to bind DNA itself. The amount of target mRNA and then protein rises because a repressive protein has been degraded.
Gibberellin is the signal, DELLA is the protein repressor, and transcription-promoting factors control gene expression. Gibberellin does not directly transcribe genes or act as amylase.