A monk counting pea plants in a monastery garden worked out the rules of inheritance without knowing that genes existed. The Punnett grid you are about to learn is just a tidy way of writing down what he noticed — and it is worth marks in almost every Paper 2.
📘 What you need to know
Gametes are haploid, so each carries only one allele of each gene. Fusion at fertilisation gives a diploid zygote with two alleles of each gene.
If the two alleles are the same the genotype is homozygous; if they are different it is heterozygous.
A monohybrid trait is controlled by a single gene, usually considered to have two alleles.
A monohybrid cross starts with pure-breeding (homozygous) parents showing different phenotypes — the P generation.
A Punnett grid predicts the probability of offspring showing a particular genotype or phenotype.
Crossing two homozygous parents gives an F1 generation that is all heterozygous. Crossing two F1 heterozygotes gives an F2 phenotype ratio of 3:1.
Mendel called genes “units of inheritance” and established that characteristics are passed on in a predictable pattern.
Why gametes carry only one allele
This is the foundation, so it is worth stating carefully. A body cell is diploid: two copies of every chromosome, and therefore two alleles of every gene. Meiosis halves that, so a gamete carries one chromosome from each homologous pair and therefore one allele of each gene.
When two gametes fuse, the zygote has two alleles again — one from each parent. Those two may be identical or different, and that single fact is what makes all the ratios in this topic work.
The whole of genetics in one line
two alleles in the parent → one allele per gamete → two alleles in the offspring
If a cross ever confuses you, go back to this line. Every Punnett grid is just “list what one parent can put in a gamete, list what the other can, then combine every possibility.” The grid is bookkeeping, not magic.
Mendel and the pea plants
Gregor Mendel was an Austrian monk who, in the mid-nineteenth century, carried out breeding experiments on very large numbers of pea plants. He is sometimes called the father of genetics, and the reason his work stood up is worth understanding: he controlled the pollination himself.
Pea flowers contain both male and female parts, so they will happily self-pollinate. Mendel transferred pollen from the anther of one chosen plant to the stigma of another chosen plant, which removed all uncertainty about which pollen had fertilised which ovule. He then grew the resulting seeds and recorded the characteristics — height, flower colour and the smoothness of the seed coat among them.
Why the numbers had to be big. A 3:1 ratio is a probability, not a guarantee. With four offspring you might easily get 4:0. Mendel counted thousands, which is the only reason the underlying pattern became visible at all. If an exam asks why he used large numbers, that is the answer.
What Mendel actually counted
Characteristic
Second generation counts
Ratio
Stem length (long or short)
787 long : 277 short
2.84 : 1
Seed shape (round or wrinkled)
5474 round : 1850 wrinkled
2.96 : 1
Seed colour (yellow or green)
6022 yellow : 2001 green
3.01 : 1
Flower colour (purple or white)
705 purple : 224 white
3.15 : 1
Notice that not one of those is exactly 3:1. They cluster around it, and the counts based on the largest samples sit closest. That is precisely what you expect from a probabilistic process, and it is a much more honest picture than a textbook that prints 3:1 four times.
Building a Punnett grid
🧩 The five steps
Write down the parental phenotypes and genotypes.
Write down all the possible gametes each parent can make. A useful convention is to circle them, to remind yourself they are haploid.
Put one parent’s gametes along the top of a 2 x 2 grid and the other’s down the side.
Combine the gametes in each box to give the possible offspring genotypes — this is the F1 generation.
State the genotype ratio and the phenotype ratio.
The single shaded box in the second grid is the whole point. A recessive phenotype only appears when an organism inherits the recessive allele twice.
Why the ratio is 3:1 and not something else
Look at the second grid again. There are four equally likely combinations: GG, Gg, Gg and gg. Three of those four contain at least one G, and because G is dominant, all three look green. Only the single gg box has no dominant allele to mask the recessive one, so only that plant has yellow pods.
This is also why the genotype ratio (1:2:1) and the phenotype ratio (3:1) are different numbers for the same cross. Examiners ask for one or the other quite deliberately, so read the question.
Probability, not prophecy. A 3:1 ratio means each offspring has a 3 in 4 chance of showing the dominant phenotype. It does not mean that four seeds will give exactly three green. Say “chance” or “probability” in your answers and you will never be marked down for overclaiming.
Controlling crosses on purpose
Plants can be arranged sexually in several ways: male and female parts in the same flower, separate male and female flowers on the same plant, or entirely separate male and female plants. Where both parts are present on one plant, self-pollination and self-fertilisation are possible.
Growers exploit this. If a trait is useful, they can self-pollinate the favoured plants to keep it in the next generation. If they want to combine two useful traits held by different plants, they cross-pollinate artificially. Genetic crosses on paper are how they predict the outcome before spending a season finding out.
Worked examples
WORKED EXAMPLE
In tomatoes, the allele for red fruit (R) is dominant to the allele for yellow fruit (r). A heterozygous red plant is crossed with a yellow plant. Determine the expected phenotype ratio of the offspring. [3]
Step 1: parental genotypesheterozygous red = Rr, yellow must be homozygous recessive = rrStep 2: possible gametesRr parent makes R or r; rr parent makes r onlyStep 3: combineR + r = Rr, R + r = Rr, r + r = rr, r + r = rr2 Rr : 2 rr, so 1 red : 1 yellowa 1:1 ratio is the signature of a cross with a homozygous recessive parent — worth recognising on sight
WORKED EXAMPLE
Two pea plants with purple flowers are crossed. Among 929 offspring, 224 have white flowers. Explain what this tells you about the parents. [3]
Work out the ratio first705 purple : 224 white, which is close to 3 : 1Read what a 3:1 ratio meansa 3:1 phenotype ratio comes from crossing two heterozygotesDeduce the genotypesboth parents must be heterozygous, Pp, and white must be the recessive phenotype3 marks: ratio calculated, 3:1 recognised, both parents identified as heterozygouswhite parents could not produce purple offspring, so purple has to be dominant
💡 Exam tips
Always show the gametes. A completed grid with no gamete working can lose marks even when the answer is right.
Choose letters whose upper and lower cases look obviously different. If forced to use one like P and p, exaggerate the size difference so the marker cannot mistake them.
State whether you are giving a genotype ratio or a phenotype ratio. They are different for the same cross.
Define your symbols at the start: “R = allele for red, r = allele for yellow”. It costs one line and often earns a mark.
If given raw counts, divide through to find the ratio before trying to interpret it.
⚠️ Common mix-ups
Putting a whole genotype in a gamete box. Gametes are haploid, so each box on the edge holds one letter, never two.
Treating 3:1 as a promise. It is a probability. Mendel’s own results ranged from 2.84:1 to 3.15:1.
Using the same letter for two different genes. Each gene needs its own letter.
Mixing capitals and lower case carelessly. Gg and gG mean the same thing, but writing GG when you meant Gg changes the answer entirely.
Saying the recessive allele “disappeared” in the F1. It was present in every F1 plant, just masked.
Confusing the P, F1 and F2 generations. P are the pure-breeding parents, F1 their offspring, F2 the result of crossing two F1 individuals.
Up next: Inheritance: Key Terms — gene, allele, locus, genotype, phenotype. Precise vocabulary is worth real marks here, and the words are easy to blur together.
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