Inheritance

Define diploid (2n) as a cell with two complete sets of chromosomes (one from each parent) and haploid (n) as a cell with one set. n is the number of distinct chromosomes in a set, not 'half the diploid number'

Define a homologous pair as two chromosomes (one maternal, one paternal) that are the same size, have the centromere in the same position, and carry the same genes at the same loci (they may carry different alleles)

State that the autosomes are the 22 true homologous pairs in humans; the X and Y are not a true homologous pair (different size, different gene content, pair only across pseudoautosomal regions)

Explain why a reduction division is needed: gametes are haploid so that fertilisation restores the diploid number in the zygote

Describe the eight named stages of meiosis: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II — and what the chromosomes, nuclear envelope, cell-surface membrane and spindle are doing at each

State that meiosis I separates homologous chromosomes; meiosis II separates sister chromatids (like mitosis)

Identify meiotic stages from a photomicrograph: prophase I = condensed chromosomes inside an intact nuclear envelope; metaphase I = paired homologs (bivalents) on the metaphase plate; anaphase I = homologs moving to opposite poles; telophase I = two cells with one chromosome from each pair

State the difference between anaphase I and anaphase II: anaphase I = whole chromosomes (each still with two chromatids) move apart; anaphase II = sister chromatids separate (as in mitosis)

Explain crossing over in prophase I: non-sister chromatids of a bivalent exchange segments at a chiasma, producing recombinant chromatids

Explain random orientation (independent assortment) at metaphase I: each pair of homologs lines up at the metaphase plate independently of every other pair, so the gametes receive all 2ⁿ possible combinations of maternal and paternal chromosomes (n = haploid number)

State that a single diploid parent makes 2ⁿ genetically different gametes (n = haploid number); in humans, n = 23, so 2²³ ≈ 8.4 million different gametes

Explain that the random fusion of gametes at fertilisation is a third source of variation: a parent that makes n gamete types and another that makes m gamete types can produce n × m different zygotes

Define the 14 genetic terms: gene, locus, allele, dominant, recessive, codominant, linkage, sex linkage, test cross, F1, F2, phenotype, genotype, homozygous, heterozygous

State that dominant and recessive describe the phenotype of the heterozygote; they do not describe the frequency of the allele in the population

Construct a Punnett square for a monohybrid cross: 2 × 2 with one row per paternal gamete and one column per maternal gamete; each box is the union of the row and column gametes

State the F2 ratio for a monohybrid cross with simple dominance: 3 dominant phenotype : 1 recessive phenotype (3 : 1)

State the F2 ratio for a monohybrid cross with codominance: 1 : 2 : 1 (three distinguishable phenotypes including the heterozygote)

State the F2 ratio for a dihybrid cross with independent assortment: 9 : 3 : 3 : 1

Use the ABO blood-group system as the multi-allele example: $I^{A}$ and $I^{B}$ codominant; both dominant to $i$; one person has two of the three alleles, never more

State that an X-linked gene is on the X chromosome; a male (XY) has only one copy, so a single recessive allele is expressed (the male is hemizygous, not heterozygous)

Construct an X-linked Punnett square: father's gametes are $X^{H}$ and Y (no allele on Y); mother's gametes are $X^{H}$ and $X^{h}$; the sex split in the offspring is the giveaway of X-linkage

State that X-linked recessive conditions (haemophilia A from F8, haemophilia B from F9, red–green colour blindness, DMD) appear more often in males than in females

Define autosomal linkage as two or more genes on the same autosome, close enough that crossing over between them is rare; the F2 collapses from 9 : 3 : 3 : 1 toward 3 : 1 (in the limit of complete linkage)

State that a dihybrid test cross (AaBb × aabb) reveals linkage: independent assortment gives 1 : 1 : 1 : 1; complete linkage gives 1 : 1 (parentals only); the rare classes are the recombinants

Define epistasis as one gene masking the expression of another gene at a different locus; the commonest pattern is 9 : 3 : 4 (one allele of one gene is epistatic to the other gene)

Define a test cross as a cross of an unknown genotype with a homozygous recessive; the offspring phenotypes reveal the unknown parent's gametes

State the chi-squared test: χ² = Σ ( (O − E)² / E ) summed over every class; df = number of classes − 1; if χ² < critical value (at p = 0.05) accept H₀; if χ² ≥ critical value reject H₀

State the chi-squared decision rule in words: a small χ² means the data fit the expected ratio; a large χ² means they do not; accepting H₀ does not prove H₀ true, only that the data are not inconsistent with it

Apply the χ² test to a 3 : 1 monohybrid and a 9 : 3 : 3 : 1 dihybrid; identify which class is most-deviated from expected and use that to suggest linkage or another modifier

State the TYR → tyrosinase → melanin → pigmentation chain: TYR codes for tyrosinase, tyrosinase makes melanin from tyrosine, melanin gives skin/hair/eye colour; tyr/tyr is albino; albinism is autosomal recessive

State the HBB → β-globin → haemoglobin → sickle cell chain: HBB codes for β-globin, β-globin is part of HbA; the HbS variant (Glu→Val at position 6) polymerises at low O₂ and distorts the RBC; sickle cell is autosomal codominant (HbA/HbA, HbA/HbS, HbS/HbS all distinguishable)

State the F8 → factor VIII → clotting → haemophilia chain: F8 is on the X chromosome; loss-of-function mutation gives no factor VIII, uncontrolled bleeding; haemophilia A is X-linked recessive

State the HTT → huntingtin → neuronal toxicity → Huntington's chain: HTT CAG expansion gives a toxic gain-of-function huntingtin protein that damages striatal neurones; HD is autosomal dominant (one copy is enough)

State the general principle: loss-of-function mutations are usually recessive (TYR, HBB, F8); gain-of-function mutations are usually dominant (HTT). The biology of the protein sets the inheritance pattern

Use the Le/le allele: Le = functional gibberellin-synthesis enzyme; le = non-functional; le/le plants are dwarf because they cannot make gibberellin to break down DELLA

Define structural gene as a gene that codes for a functional polypeptide; define regulatory gene as a gene that codes for a product (often a protein) that controls the expression of other genes

Define repressible enzyme as one made continuously until something stops it; inducible enzyme as one made only when the substrate is present

Describe the lac operon: lacI (regulatory) codes for the LacI repressor; the operator sits between the promoter and the structural genes lacZ (β-galactosidase), lacY (permease), lacA (transacetylase)

State the two states of the lac operon: repressed (no lactose — repressor bound to operator — no transcription) and induced (lactose present — converted to allolactose — allolactose binds repressor — repressor released — RNA polymerase transcribes the structural genes)

Do NOT mention cAMP, CAP or catabolite activator protein — these are off-syllabus for 2025–2027

Define transcription factor as a protein that binds DNA and controls gene expression in eukaryotes, either by recruiting RNA polymerase (increase) or by blocking it (decrease)

Describe the gibberellin → DELLA chain: GA binds GID1 receptor → complex binds DELLA repressor → DELLA is ubiquitinated → DELLA is broken down by the 26S proteasome → TFs are released → TFs enter nucleus → α-amylase gene is switched ON → α-amylase hydrolyses starch in the endosperm

State the two-step wording: DELLA represses the TFs (not 'DELLA represses the gene directly') — the MS credits the two-step wording