Topic 17.4 · Core + Supplement
Punnett Squares and Genetic Crosses
A genetic diagram tracks alleles from parents, through gametes, into every possible offspring combination. Set it up the same way every time and the method barely changes between a straightforward 3:1 cross and a Supplement test cross — only the interpretation at the end gets more demanding.
Setting up a monohybrid cross
A reliable sequence, worth using even on questions that look simple enough to skip it:
- Define the allele symbols.
- Write the genotypes of the parents.
- Identify which allele each parent’s gametes can carry.
- Combine the gametes.
- Write the possible offspring genotypes.
- Convert those genotypes into phenotypes.
- Calculate the genotype or phenotype ratio the question asks for.
A Punnett square is a grid that displays the possible allele combinations in the offspring. It predicts probabilities — it does not predict the exact order or exact number of offspring that will actually occur in a small, real sample. That distinction matters enough to be worth a sentence in an answer whenever a question asks you to interpret a ratio.
A 3:1 cross
Let T be the dominant allele for tall, and t be the recessive allele for short. Cross two heterozygous plants, Tt × Tt. Each parent can form gametes carrying either T or t.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The genotypes produced are TT, Tt, Tt and tt — a genotype ratio of 1 TT : 2 Tt : 1 tt. Because T is dominant, TT and both Tt genotypes show the dominant phenotype, and only tt shows the recessive phenotype, giving a phenotype ratio of 3 tall : 1 short, or 3:1.
A 1:1 cross
Now cross a heterozygous individual with a homozygous recessive individual, Tt × tt. The Tt parent can form gametes carrying T or t; the tt parent can only form gametes carrying t.
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
This gives a genotype ratio of 1 Tt : 1 tt, and since Tt is dominant-phenotype and tt is recessive-phenotype, a phenotype ratio of 1 dominant : 1 recessive, or 1:1. Core candidates need to be able to produce both the 3:1 and the 1:1 ratio from a genetic diagram — those are the two phenotypic ratios Cambridge specifically requires at Core level.
Probability and real offspring
A Punnett square gives the probability for each individual fertilisation. In a Tt × Tt cross, the probability of a recessive-phenotype offspring is 1/4 for every single fertilisation — but that does not mean any actual group of four offspring must contain exactly one recessive individual. Probability describes the expected pattern across many events; a small number of real offspring can differ from the predicted ratio by chance, in the same way four flips of a fair coin don’t have to land two heads and two tails. If a question gives you an actual family that doesn’t match a predicted ratio exactly, that is not evidence the genetics is wrong — it’s exactly what probability predicts can happen in a small sample.
Supplement: test crosses
A dominant phenotype can come from either of two genotypes — AA or Aa — and the phenotype alone cannot tell you which one you’re looking at. A test cross resolves that by crossing the unknown individual with a homozygous recessive individual, aa.
If the unknown individual is AA, every offspring receives A from the unknown parent and a from the recessive parent, giving all Aa offspring — every offspring shows the dominant phenotype. If the unknown individual is Aa instead, the offspring are expected in a 1 Aa : 1 aa ratio, giving 1 dominant phenotype : 1 recessive phenotype. Producing any recessive-phenotype offspring at all is therefore the result that actually reveals the unknown parent’s genotype: it shows that parent carried the recessive allele and was heterozygous.
The tester has to be homozygous recessive for this to work, because a recessive tester contributes only recessive alleles to every offspring — that’s what makes whatever allele the unknown parent contributes visible in the results. In an idealised Cambridge-style genetic diagram, use the predicted outcomes to identify the genotype directly. In a real, finite family, observing only dominant offspring is strong evidence for homozygous dominance rather than absolute proof, for the same reason a small sample can depart from a predicted ratio by chance.
What this sets up next
This same seven-step method — genotypes, gametes, offspring, ratio — is what codominance and ABO blood groups and sex-linked inheritance both extend, with richer notation but the same underlying reasoning. Before that, pedigree diagrams ask you to run a version of this reasoning backwards, starting from a real family’s phenotypes rather than a hypothetical cross.