Genetic Inheritance and Punnett Squares

Genetic inheritance is how characteristics pass from parents to offspring through their genes. To work with it you first need a precise vocabulary — the exam tests whether you can use these terms correctly.

Term	Meaning
Gamete	A sex cell (sperm or egg) carrying one copy of each chromosome
Chromosome	A long molecule of DNA found in the cell nucleus
Gene	A short section of DNA that codes for a particular protein and so controls a feature
Allele	A different version (form) of the same gene
Dominant	An allele that is expressed even when only one copy is present
Recessive	An allele that is only expressed when two copies are present
Homozygous	Both alleles for a gene are the same (e.g. BB or bb)
Heterozygous	The two alleles for a gene are different (e.g. Bb)
Genotype	The combination of alleles an organism has (e.g. Bb)
Phenotype	The physical characteristic that results (e.g. brown fur)

Body cells carry two copies of every gene — one inherited from each parent. Because gametes carry only one copy, fertilisation restores the pair.

Genotype vs phenotype: the genotype is the alleles you carry; the phenotype is the characteristic those alleles produce. Genotype Bb produces the phenotype "brown fur".

A single gene can exist as different alleles. For a gene controlling fur colour in mice, one allele might give brown fur and another give white fur. Since each mouse has two copies of the gene, the two alleles it carries decide its phenotype.

We write a dominant allele as a capital letter and the recessive allele as the same letter in lower case — for example B (brown, dominant) and b (white, recessive).

• A dominant allele is expressed whenever it is present, even as a single copy.
• A recessive allele is only expressed when two copies are present, because then no dominant allele is there to mask it.

Genotype	Description	Phenotype
BB	Homozygous dominant	Brown fur
Bb	Heterozygous	Brown fur
bb	Homozygous recessive	White fur

Notice that BB and Bb give the same phenotype (brown). The single dominant B in the heterozygote is enough to mask the recessive b.

A recessive phenotype (white fur, bb) can only appear when an organism inherits a recessive allele from both parents.

The spec names two characteristics controlled by a single gene:

• Fur colour in mice — controlled by one gene with brown/white alleles.
• Red-green colour blindness in humans — controlled by a single gene.

These are useful for teaching inheritance because their outcomes follow simple, predictable patterns.

In reality, most characteristics are controlled by many genes interacting, not one. Height, body mass, eye colour and skin colour in humans each depend on several genes working together (and often the environment too). This is why these features vary smoothly across a population rather than falling into a few clear-cut categories.

Single-gene examples are the exception, not the rule. The exam may ask you to recall that ordinary characteristics are usually the result of multiple genes interacting — do not claim a feature like height is controlled by one gene.

So when you draw a genetic cross, you are modelling the simple single-gene case — a deliberate simplification that lets you predict outcomes precisely.

(Higher Tier only — constructing a genetic cross by Punnett square is HT content. Foundation students must understand the key terms and the dominant/recessive idea, and may be asked to interpret a completed cross.)

A Punnett square is a grid that predicts the genotypes of offspring from two parents. To build one:

1. Write each parent's genotype.
2. Split each parent into its two possible gametes (one allele each).
3. Put one parent's gametes along the top, the other's down the side.
4. Fill each inner cell by combining the allele from its row and column.

Worked example — heterozygous cross (Bb × Bb):

Each Bb parent produces gametes B and b.

The four offspring genotypes are: BB, Bb, Bb, bb.

• Genotype ratio — 1 BB : 2 Bb : 1 bb
• Phenotype ratio — BB, Bb, Bb are all brown; bb is white, giving 3 brown : 1 white

This classic 3 : 1 phenotype ratio is the signature of crossing two heterozygotes.

When a heterozygous parent is crossed with a homozygous recessive parent, the ratio changes.

(Higher Tier only — constructing the cross is HT; the idea that bb can only pass on a b allele follows from the rules above.)

Worked example — Bb × bb:

The Bb parent produces gametes B and b. The bb parent can only produce b gametes.

The four offspring genotypes are: Bb, bb, Bb, bb.

• Genotype ratio — 2 Bb : 2 bb, which simplifies to 1 Bb : 1 bb
• Phenotype ratio — Bb is brown, bb is white, giving 1 brown : 1 white

A 1 : 1 phenotype ratio is the hallmark of crossing a heterozygote with a homozygous recessive. This kind of cross (called a test cross) can reveal a hidden allele.

Always check: each parent contributes exactly one allele to every offspring, and the genotypes in the grid must use only the alleles that parent actually carries.

The outcome of a cross can be expressed three equivalent ways: as a ratio, a percentage, or a probability. AQA may ask for any of them, so you must be able to convert between them.

(Higher Tier only — using the theory of probability to make predictions from a constructed cross is HT.)

Return to the Bb × Bb cross, which gave 3 brown : 1 white out of 4 offspring.

Worked example — expressing the brown outcome:

• As a fraction: 3 out of 4 offspring are brown → $\frac{3}{4}$
• As a percentage: $\frac{3}{4}\times 100=75\%$
• As a probability: $\frac{3}{4}=0.75$

So there is a 0.75 probability that any one offspring is brown, and a $\frac{1}{4}=25\%=0.25$ probability it is white.

Using direct proportion — if these mice had 20 offspring, multiply the fraction by the total:

$\frac{3}{4}\times 20=15\text{ brown},\frac{1}{4}\times 20=5\text{ white}$

These are predicted (expected) numbers. Real litters vary because fertilisation is random — a Bb × Bb litter of 4 will not always be exactly 3 brown and 1 white. The ratio is the long-run expectation, not a guarantee.

A family tree (pedigree) diagram tracks a characteristic across generations. Circles are usually females, squares males; shaded shapes show individuals expressing the trait. You read it by working out which genotypes are forced by the rules — for example, two unaffected parents who have an affected (bb) child must both be heterozygous carriers (Bb).

1. Confusing genotype with phenotype

Genotype is the alleles (Bb); phenotype is the visible feature (brown fur). A question asking for the genotype wants letters, not a description.

2. Treating Bb as a different phenotype from BB

Both BB and Bb show the dominant phenotype. The heterozygote looks identical to the homozygous dominant because one dominant allele is enough.

3. Putting both of a parent's alleles in one gamete

Each gamete carries one allele only. A Bb parent makes B gametes and b gametes — never a Bb gamete.

4. Forgetting that a recessive phenotype needs two recessive alleles

White fur (bb) requires a recessive allele from each parent. A single b alongside a B gives the dominant brown phenotype.

5. Reporting predicted ratios as certainties

A 3 : 1 ratio is the expected proportion over many offspring, not a promise about a single small litter. Phrase predictions as probabilities.

6. Claiming everyday features are single-gene

Height, eye colour and similar traits result from multiple genes interacting. Only specific cases such as mouse fur colour and red-green colour blindness are treated as single-gene in this spec.