Monohybrid and Dihybrid Crosses Revision Notes | A Level AQA Biology

1. Why Genetic Crosses Matter

Every observable trait you carry — eye colour, blood group, the shape of your earlobes — is the product of alleles inherited from your parents. Genetic crosses let us predict, with mathematical precision, which combinations of alleles offspring are likely to receive.

At A Level, you need to go beyond drawing a Punnett square. Examiners expect you to interpret results, work backwards from phenotypic ratios to deduce parental genotypes, and handle codominance alongside standard dominant-recessive inheritance. This lesson builds those skills systematically.

2. The Language of Genetics — Key Terms Pinned Down

Before drawing a single cross, you need the vocabulary locked in. Genetics has precise terminology, and using the wrong word in an exam costs marks.

The genotype is what the organism has; the phenotype is what you can observe. A dominant allele masks the effect of a recessive allele in the heterozygote, so two different genotypes (AA and Aa) can produce the same phenotype.

• Dominant allele: expressed in both AA and Aa — written as a capital letter (e.g. A)
• Recessive allele: expressed only in aa — written as a lower-case letter (e.g. a)
• Codominant alleles: both expressed in the heterozygote — often written with a shared base letter and superscripts (e.g. $C^R$ and $C^W$ for red and white flower colour in snapdragons)

3. Monohybrid Crosses — One Gene at a Time

A monohybrid cross tracks the inheritance of a single gene with two alleles. Picture a pea plant: tall (T) is dominant over dwarf (t). If you cross two heterozygous tall plants (Tt × Tt), what offspring do you expect?

The Punnett square is the standard tool for working this out. Each parent produces gametes carrying one allele, and the square maps every possible combination. The key is setting it up correctly and labelling every component — examiners award marks for the parental genotypes, gametes, and offspring genotypes, not just the final ratio.

Worked example: Monohybrid Cross: Tall × Tall (Tt × Tt)

Two heterozygous tall pea plants (Tt) are crossed. Construct a fully labelled genetic diagram and state the expected phenotypic ratio of the offspring.

1. Parent 1: Tall, genotype Tt Parent 2: Tall, genotype Tt — Always start by writing out the parental phenotypes and genotypes — this earns the first mark on AQA mark schemes.
2. Parent 1 gametes: T or t Parent 2 gametes: T or t — Each gamete carries only one allele because gametes are haploid. Circle or underline the gametes in your diagram to make them clear.
3. T t T | TT | Tt | t | Tt | tt | — Place one parent's gametes along the top and the other's down the side. Fill each cell by combining the row and column alleles.
4. TT: Tt: tt = 1: 2: 1 — Count each genotype from the four cells: one TT, two Tt, one tt.
5. Tall (TT + Tt): Dwarf (tt) = 3: 1 — Both TT and Tt are tall because T is dominant. Only tt produces the dwarf phenotype. The 3:1 ratio is the hallmark of a monohybrid cross between two heterozygotes.

Answer: Expected phenotypic ratio: 3 tall: 1 dwarf

The 3:1 ratio is a probability, not a guarantee. In a small litter of four offspring, you might see four tall plants by chance. Examiners sometimes ask you to explain why observed ratios differ from expected ones — the answer is always random fertilisation and small sample size.

4. Codominance — When Neither Allele Hides

Codominance is where the standard dominant-recessive model breaks down — and where many students lose marks by applying the wrong rules. In codominance, both alleles are fully expressed in the heterozygote, producing a phenotype that is distinct from either homozygote.

A classic example is coat colour in Shorthorn cattle. The allele for red coat ($C^R$) and the allele for white coat ($C^W$) are codominant. A heterozygous animal ($C^R C^W$) has a roan coat — a mixture of red and white hairs — not a blend or an intermediate shade. Each allele produces its own pigment independently.

Notice the notation: both alleles share the same base letter C, with superscripts to distinguish them. This signals to the examiner that you understand neither allele is dominant over the other. Using upper and lower case (e.g. C and c) would incorrectly imply dominance.

Worked example: Codominant Cross: Roan × Roan ($C^R C^W$ × $C^R C^W$)

Two roan Shorthorn cattle are crossed. Construct a fully labelled genetic diagram and predict the phenotypic ratio of the offspring.

1. Parent 1: Roan, genotype $C^R C^W$ Parent 2: Roan, genotype $C^R C^W$ — Both parents are heterozygous for the two codominant alleles.
2. Parent 1 gametes: $C^R$ or $C^W$ Parent 2 gametes: $C^R$ or $C^W$ — Each gamete carries one allele. Both allele types are equally likely.
3. $C^R$ $C^W$ $C^R$ | $C^R C^R$ | $C^R C^W$ | $C^W$ | $C^R C^W$ | $C^W C^W$ | — Four cells: one $C^R C^R$, two $C^R C^W$, one $C^W C^W$.
4. $C^R C^R$ = Red $C^R C^W$ = Roan $C^W C^W$ = White — Because the alleles are codominant, the heterozygote has its own distinct phenotype — roan — not a blend of the two.
5. Red: Roan: White = 1: 2: 1 — The 1:2:1 phenotypic ratio is the hallmark of a codominant cross between two heterozygotes. Compare this with the 3:1 ratio from a dominant-recessive cross — the difference arises because the heterozygote here has a unique, identifiable phenotype.

Answer: Expected phenotypic ratio: 1 red: 2 roan: 1 white

The 1:2:1 phenotypic ratio is the key diagnostic for codominance between two heterozygotes. In a dominant-recessive cross, the heterozygote and dominant homozygote look the same, collapsing the ratio to 3:1. Codominance reveals the heterozygote as a separate class, splitting that 3 into 1 + 2.

5. Dihybrid Crosses — Tracking Two Genes Simultaneously

Now that monohybrid crosses are secure, you can extend the same logic to two genes at once. A dihybrid cross tracks two independently assorting genes — this means genes on different chromosomes, or genes that are sufficiently far apart on the same chromosome to behave as if they assort independently due to high recombination frequency (typically >50 cM) during meiosis. This high recombination frequency allows for independent assortment, which is essential for classic dihybrid ratios.

Mendel's law of independent assortment underpins this: the alleles of one gene segregate independently of the alleles of a second gene during gamete formation. This means a parent with genotype AaBb produces four gamete types in equal proportions: AB, Ab, aB, and ab.

The Punnett square for a dihybrid cross is a 4 × 4 grid — 16 cells in total. This looks daunting, but the method is identical to the monohybrid version; you are combining gametes from each parent systematically.

Worked example: Dihybrid Cross: AaBb × AaBb

In pea plants, round seed shape (R) is dominant over wrinkled (r), and yellow seed colour (Y) is dominant over green (y). Two plants with genotype RrYy are crossed. Predict the phenotypic ratio of the offspring.

1. Parent 1: Round, Yellow — genotype RrYy Parent 2: Round, Yellow — genotype RrYy — Both parents are heterozygous for both genes. This is the classic dihybrid F1 cross.
2. Each parent produces: RY, Ry, rY, ry (in equal proportions, each with probability 1/4) — With two heterozygous genes, independent assortment generates four gamete types. Use FOIL or a systematic listing: fix R with Y and y, then fix r with Y and y.
3. Columns (Parent 1 gametes): RY | Ry | rY | ry Rows (Parent 2 gametes): RY | Ry | rY | ry Fill all 16 cells by combining row and column gametes. — Each cell represents one possible offspring genotype. With 16 cells and four gamete types from each parent, every combination is covered.
4. R_Y_ (Round, Yellow): 9 cells R_yy (Round, Green): 3 cells rrY_ (Wrinkled, Yellow): 3 cells rryy (Wrinkled, Green): 1 cell — R_ means at least one R allele (RR or Rr) — the dominant phenotype. Count carefully: 9 + 3 + 3 + 1 = 16. This is your check that the square is complete.
5. Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green = 9: 3: 3: 1 — The 9:3:3:1 ratio is the hallmark of a dihybrid cross between two double heterozygotes where both genes show simple dominance. Memorise this ratio — examiners use it as a starting point for many interpretation questions.

Answer: Expected phenotypic ratio: 9 Round Yellow: 3 Round Green: 3 Wrinkled Yellow: 1 Wrinkled Green

Notice how the 9:3:3:1 ratio is built from two independent 3:1 ratios multiplied together: (3 Round: 1 Wrinkled) × (3 Yellow: 1 Green). This multiplicative relationship is a direct consequence of independent assortment and gives you a quick mental check on your working.

6. Working Backwards — Deducing Genotypes from Cross Results

Examiners frequently present you with cross results and ask you to deduce the parental genotypes. This is the analytical skill that separates a grade 7 response from a grade 9 one. The approach is systematic: use the offspring phenotypic ratio as your clue.

Here is the logic chain. If offspring appear in a 3:1 ratio, both parents must be heterozygous for a single gene. If offspring appear in a 1:1 ratio, one parent is heterozygous and the other is homozygous recessive — this is a test cross. If offspring appear in a 9:3:3:1 ratio, both parents are double heterozygotes for two independently assorting genes.

The test cross is particularly useful in genetics: crossing an organism of unknown genotype with a homozygous recessive individual reveals whether the unknown is homozygous dominant or heterozygous. If any recessive offspring appear, the unknown must carry a recessive allele.

7. Exam Technique — How to Secure Full Marks

Genetic diagrams are one of the most mark-dense question types in AQA Biology. A six-mark question on a dihybrid cross can be answered entirely through a correct, fully labelled diagram — but only if every component is present. Here is what AQA mark schemes consistently reward.

• Parental phenotypes stated explicitly (e.g. 'Tall, Yellow')
• Parental genotypes written correctly using the notation specified or implied in the question
• Gametes listed separately and clearly — circle them or write 'Gametes:' as a label
• A Punnett square (or equivalent grid) with all cells completed
• Offspring genotypes listed from the Punnett square
• Offspring phenotypes stated with their expected ratio

A frequent source of lost marks is omitting the gametes row. Students often jump straight from parental genotypes to the Punnett square, skipping the step that shows they understand meiosis produces haploid gametes. Always include it.

For extended-response questions asking you to evaluate genetic cross data, structure your answer as: state the observed ratio, identify what that ratio indicates about parental genotypes, explain the mechanism (meiosis and independent assortment), and then address any discrepancy between observed and expected ratios by referencing random fertilisation and sample size.

Monohybrid and dihybrid crosses, including codominant alleles, form the quantitative core of AQA inheritance questions. Mastery here — correct notation, complete diagrams, and accurate ratio interpretation — directly translates to marks across both the written papers.