Inheritance is the transmission of alleles between generations and the prediction of offspring phenotype ratios from the rules of dominance, sex-linkage, linkage, and epistasis. The same five-line cross discipline runs every kind of cross AQA tests; only the ratios change with the rules.
Read this topic as an analogy.
Picture each parent as a holder of a deck of paired cards. The deck has many slots, and in each slot the parent holds two cards: one inherited from their mother, one from their father. When the parent makes a gamete, they deal one card from each slot at random into the gamete. Fertilisation combines two gametes; the offspring now has a new pair of cards in every slot, one card from each parent.
The phenotype the offspring displays is decided by the cards in each slot and the dominance rules for those cards. A dominant card overrides its partner: present in either copy, it decides the slot's display. A recessive card only shows up if both cards in the slot are recessive. Codominant cards both contribute to the slot's display at the same time.
Two heterozygous parents (each holding one dominant and one recessive card in a slot) deal at random. The offspring slot pairs come out as 1 dominant-dominant : 2 dominant-recessive : 1 recessive-recessive, and three of those four display the dominant phenotype. That is the 3:1 ratio. Two slots dealing independently give the 9:3:3:1 ratio of a dihybrid cross.
Some slots are sex-linked. The X-slot has many cards; the Y has very few or none for the same gene. Males hold two different chromosomes, so they have only one card in the X-slot. Whatever single card a male holds there is what shows up; he is hemizygous. Females have two X-slots and follow normal dominance. A female holding one recessive sex-linked card and one dominant one does not display the recessive trait herself, but she is a carrier who can deal that card to her children.
Two slots can be glued together on the same chromosome. That is autosomal linkage: the two cards in a glued pair are dealt as a package, not independently. Occasionally, during the deal, the glue gives way and the cards swap with their counterparts on the homologous chromosome (crossing over in prophase I of meiosis). That is why recombinant offspring exist, but in smaller numbers than parental offspring.
Sometimes one slot's card controls whether another slot's cards matter at all. That is epistasis: the masking gene is at one locus, the masked gene is at another. The 9:3:3:1 ratio collapses into modified ratios (9:3:4 for recessive epistasis; 12:3:1 for dominant epistasis) because some classes merge under the masking rule.
Some card combinations are an "instant out": the offspring is never viable and never gets counted in the ratio. That is a lethal allele. Derive the standard ratio first, strike the lethal class, then simplify what remains.
Mapping back to formal vocabulary. The slot is the gene; the cards are alleles; the pair in a slot is the genotype; the displayed result is the phenotype. A slot with two identical cards is homozygous; with two different cards, heterozygous. A card whose presence alone decides the display is a dominant allele; one that only decides the display when both cards match is recessive. Two cards both contributing to the display is codominance. The random one-card-per-slot deal is gamete formation by meiosis. A slot on the X with no Y counterpart is sex-linked, and a male with one card there is hemizygous. Two glued slots on the same chromosome are autosomal linkage; the occasional unglueing is crossing over in prophase I of meiosis. One slot's card controlling the other's visibility is epistasis. A card combination producing an out is a lethal allele.
Inheritance vocabulary names what is passed, what is expressed, and how alleles relate to each other.
A genotype is the full set of alleles an organism carries at every relevant locus, whether or not those alleles are currently expressed. A phenotype is the observable characteristic the organism displays. Diploid organisms carry two alleles for every gene, one on each chromosome of the homologous pair.
Phenotype = genotype + environment. Both components must be named for the full two marks. "Genotype", "alleles", or "genetic constitution" each earns the genetic component mark; "environment" earns the second. Either component alone caps the answer at one mark out of two.
Write both components of the phenotype definition:
genotype(oralleles) ANDenvironment. Either component alone caps you at 1 of 2 marks. The two parts score independently.
A dominant allele decides the phenotype whenever at least one copy is present. A recessive allele only decides the phenotype when the organism is homozygous for it; a single dominant allele masks a recessive one. Codominance applies when neither allele dominates the other: both are expressed simultaneously, and the phenotype combines contributions from both.
Evidence for dominance needs a specific cross outcome, not a ratio. Two parents that both show a dominant phenotype producing an offspring with the recessive phenotype is diagnostic: only heterozygous parents can produce a recessive offspring. A 3:1 ratio is consistent with dominance but is not proof on its own.
For dominance evidence, cite named individuals and a specific cross outcome. Don't argue from a 3:1 ratio alone. AQA rejects ratio-based reasoning here. The diagnostic outcome is two dominant-phenotype parents producing a recessive-phenotype offspring.
Every genetic cross is executed as five labelled lines; 3:1 and 9:3:3:1 are the standard ratios.
A monohybrid cross involves a single gene with two alleles; two heterozygous parents produce a 3:1 phenotypic ratio. A dihybrid cross involves two genes on different chromosomes; two double-heterozygotes produce a 9:3:3:1 ratio. Independent assortment is the requirement for 9:3:3:1; the ratio breaks down under linkage or epistasis.
- Parental phenotypes.
- Parental genotypes.
- Gametes (circled or otherwise distinguished).
- Offspring genotypes (from a Punnett square or equivalent method).
- Offspring phenotypes with ratio.
Use the specific phenotype and allele names from the stem. Don't write generic phrasing like
some offspring. If the stem gives parental genotypes, use them. Inventing your own overrides the stem and disqualifies the principle mark.
To determine whether a dominant-phenotype individual is homozygous dominant or heterozygous, cross with a homozygous recessive partner. All offspring showing the dominant phenotype indicates a homozygous dominant parent. Roughly half the offspring showing the recessive phenotype indicates a heterozygous parent.
A test cross uses a
homozygous recessivepartner, and only that. Crossing two dominant-phenotype individuals, looking at the parents, or drawing a Punnett square from speculation are all explicitly rejected as test-cross methods.
The principle mark rewards faithful execution. If the parental genotypes are wrong but the cross is then run faithfully from those wrong genotypes, AQA awards a mark for correctly derived offspring from incorrect parents. Abandoning the diagram after a miscalculation forfeits this mark; continuing the cross from whatever was written preserves it.
Codominance and multiple alleles are illustrated by ABO blood groups, with three alleles producing four phenotypes.
Codominance is when both alleles at a locus are expressed simultaneously; the phenotype combines contributions from both. A multiple-allele system has more than two alleles for one gene across the population, even though any one individual can carry at most two. ABO blood groups are the standard AQA example, combining both phenomena.
The four ABO phenotypes and the six genotypes that produce them.
| Blood group phenotype | Genotype(s) |
|---|---|
| Group A | IAIA or IAIO |
| Group B | IBIB or IBIO |
| Group AB | IAIB (codominant expression) |
| Group O | IOIO (recessive) |
The dominance pattern is specific: IA and IB are codominant with each other, so a heterozygote with one of each produces both antigens. IO is recessive to both. ABO crosses follow the standard five-line discipline, but the three-allele system produces ratios that are not simple monohybrid 3:1.
A sex-linked gene sits on the X chromosome with no Y equivalent; males express it from a single allele.
Human sex chromosomes are structurally asymmetric. Females carry two X chromosomes (XX); males carry one X and one Y (XY). The X is substantially larger than the Y and encodes many genes for which no allele exists on the Y. A sex-linked (or X-linked) gene is one on the X with no Y counterpart.
Males carry only one allele at any X-linked locus; they are hemizygous. Whatever allele sits on the single X is expressed, regardless of whether it would be dominant or recessive in a female. Females have two X-alleles and follow normal dominance. A female heterozygous for a sex-linked recessive allele does not show the trait but is a carrier.
Write the mechanism in allele terms:
hemizygous,one allele(in males),two recessive copies(required for female expression). Don't write "gene on the X chromosome" or "the Y chromosome has no genes" as the mechanism. AQA ignores chromosome-only descriptions for the mechanism mark.
Haemophilia is the standard X-linked recessive example. A male carrying the haemophilia allele on his single X is haemophiliac. A heterozygous female is a carrier; only a homozygous female is haemophiliac. Males always inherit their Y from their father and their single X from their mother, so X-linked alleles in males always come from the maternal lineage.
Sex-linked dihybrid crosses must include the sex in every offspring phenotype description. Same-trait males and same-trait females are distinct phenotypic classes. Pedigree evidence for X-linkage cites a specific cross outcome that is impossible under the alternative hypothesis (for example, a father with the dominant phenotype passing the recessive phenotype to a daughter is impossible under X-linkage, because he passes only his dominant X to all daughters).
Pitfall — Sex-linked dihybrid: sex matters in every phenotype
In sex-linked dihybrid crosses, include the sex in every offspring phenotype.
Each sex-trait combination is a distinct phenotypic class. A standard sex-linked dihybrid produces a 1:1:1:1 ratio across the four sex-trait combinations. Collapsing same-trait males and same-trait females into one class produces 2:1:1, and AQA loses both the phenotype mark and the ratio mark.
Pedigree evidence for X-linkage uses sex-specific cross outcomes, not overall ratios. A father with the dominant phenotype passing the recessive phenotype to a daughter is impossible under X-linkage and is the diagnostic outcome that excludes it. Citing the overall dominant-to-recessive ratio across the pedigree is not credited.
Autosomal linkage means two genes share a chromosome; alleles travel together except where crossing over separates them.
Autosomal linkage is when two genes occupy loci on the same autosome. Linked genes do not assort independently: the alleles on a single chromosome tend to be inherited as a package. Parental allele combinations predominate in the offspring; recombinant combinations are rare. The 9:3:3:1 ratio breaks down.
Crossing over occurs in prophase I of meiosis, when homologous chromosomes pair and exchange segments at chiasmata. The exchange swaps alleles between the two members of the homologous pair, producing recombinant chromosomes. Crossing over does not occur in mitosis. Recombination frequency is always less than 50%, which is why parental combinations always predominate over recombinants.
Write
prophase I of meiosisfor crossing over. Don't writemitosis. AQA rejects mitosis here; crossing over is a meiotic event, specifically prophase I.
The full autosomal-linkage Explain answer has three named elements, each worth its own mark: the genes are linked (on the same chromosome); crossing over occurs (in prophase I); and recombinant offspring are fewer in number than parental offspring. In a results table, parental-combinations outnumbering recombinant-combinations is the linkage signal.
All three elements are needed for full marks:
linked genes,crossing over,fewer recombinant offspring. One element alone is one mark, not three. Each element is its own independent mark target.
Epistasis, lethal alleles, and small samples explain why observed ratios miss the expected ratio.
Epistasis is the interaction between genes at different loci, in which alleles at one locus alter or suppress the expression of alleles at another. The gene producing the masking effect is the epistatic gene; the gene whose expression is affected is the hypostatic gene. Epistasis is conceptually distinct from dominance (between alleles of the same gene) and from codominance (both alleles of one gene expressed). Confusing these is a high-frequency vocabulary error.
Write
epistasis(orepistatic) for the one-gene-masks-another phenomenon. Don't writecodominance,dihybrid, orepigenetics. All three are explicit rejects in this context; they name different phenomena.
How standard ratios get modified.
| Cause | Modified ratio | When |
|---|---|---|
| Recessive epistasis | 9 : 3 : 4 | Homozygous recessive at gene 1 masks gene 2 |
| Dominant epistasis | 12 : 3 : 1 | One dominant allele at gene 1 masks gene 2 |
| Homozygous dominant lethal | 2 : 1 (from 1:2:1) | Lethal class struck before simplifying |
Strike the lethal class first, then simplify. A 1:2:1 cross with the homozygous dominant lethal becomes 2:1, not 3:1 or 1:2:1. Both alternatives are explicit rejects.
Only five reasons earn marks on a "suggest why the observed ratio differs from expected" stem: small sample size; random fusion of gametes; linked genes; epistasis; lethal genotypes. Maximum two marks available. Mutation and environmental factors are explicit rejects on this list, regardless of how plausible they sound.
The list is closed. Don't write
mutationorenvironmental factors. Both are explicit rejects on ratio-deviation Suggest stems, even when the explanation sounds biologically reasonable; only the five listed reasons score.
The chi-squared test is the formal statistical method for assessing whether the deviation between observed and expected offspring frequencies is small enough to be explained by chance. The null hypothesis states no significant difference. The calculated χ² is compared with a critical value at the 5% significance level (p = 0.05) and at degrees of freedom equal to the number of phenotypic categories minus one. Below the critical value, accept the null hypothesis (data consistent with the genetic model); equal or above, reject the null (the difference is significant).
Pitfall — Lethal allele ratios: standard, strike, simplify
Lethal allele ratios are derived in three steps: standard ratio first, strike the lethal class, then simplify.
A monohybrid cross between two heterozygotes (Rr × Rr) normally produces 1 RR : 2 Rr : 1 rr. If RR is lethal, no RR offspring survive to be counted. The surviving classes are 2 Rr : 1 rr, which simplifies to the 2:1 ratio.
The same procedure applies to dihybrid lethal cases: derive the 9:3:3:1 or epistatically modified ratio first, then strike whichever class is non-viable, then simplify. Writing 1:2:1 or 3:1 in a lethal-allele context is an explicit reject because both treat the lethal class as viable.
Key terms
- genotype
- phenotype
- alleles
- heterozygous
- homozygous recessive
- recessive
- codominance
- sex-linked
- autosomal linkage
- epistasis
- test cross
- lethal allele