Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 17% of your exam marks.
increasing
Very High
Increasing17%
Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.
A monohybrid cross is a cross between two individuals looking at one gene with two alleles. The standard tool for predicting the outcome of a monohybrid cross is a Punnett square.
How to set up a Punnett square
- Identify the parent genotypes. Use a clear letter for the gene, with the capital for the dominant allele and the lowercase for the recessive (e.g. T and t, or B and b; avoid letters like C/c and S/s where the case is hard to tell apart in handwriting).
- Split each parent's alleles into the two gametes they can produce.
- Draw a 2×2 grid. Put one parent's possible gametes along the top, the other parent's possible gametes down the side.
- Fill in the four boxes of the grid by combining the alleles at the top and side of each box. These are the possible offspring genotypes.
- Work out the phenotype for each genotype using the dominant/recessive rules.
- State the ratio of phenotypes.
Example 1: heterozygous × heterozygous (Tt × Tt)
A tall pea plant with the genotype Tt is crossed with another tall pea plant with the genotype Tt. Both are heterozygous. T is dominant for tall, t is recessive for short.
Each parent can produce gametes carrying either T or t (50:50).
Punnett square:
| T | t | |
|---|---|---|
| T | TT (tall) | Tt (tall) |
| t | Tt (tall) | tt (short) |
Reading the grid:
- 1 in 4 chance of TT (homozygous tall)
- 2 in 4 chance of Tt (heterozygous tall)
- 1 in 4 chance of tt (homozygous short)
Phenotype ratio: 3 tall : 1 short (because both TT and Tt look tall, but only tt looks short).
In percentage terms: 75% tall, 25% short.
Example 2: heterozygous × homozygous recessive (Tt × tt)
A heterozygous tall plant (Tt) is crossed with a short plant (which must be tt because short is recessive).
| t | t | |
|---|---|---|
| T | Tt (tall) | Tt (tall) |
| t | tt (short) | tt (short) |
Phenotype ratio: 1 tall : 1 short. In percentages: 50% tall, 50% short.
Example 3: homozygous dominant × homozygous recessive (TT × tt)
A pure-breeding tall plant (TT) is crossed with a pure-breeding short plant (tt). This is the classic first cross in a breeding experiment.
| t | t | |
|---|---|---|
| T | Tt | Tt |
| T | Tt | Tt |
All offspring are Tt and all look tall. The phenotype ratio is all tall. None of the offspring is homozygous, and the short allele is hidden in all of them.
If two of these offspring are then crossed (the F1 generation × F1 generation, giving the F2 generation), the result is the 3:1 ratio seen in Example 1. This is what Gregor Mendel found in his classic pea-plant experiments in the 1860s.
From ratio to probability
Each new offspring is an independent event. The ratio from a Punnett square tells you the probability of each outcome for any single offspring.
For example, with Tt × Tt:
- Probability of a tall offspring = 3/4 = 0.75 = 75%
- Probability of a short offspring = 1/4 = 0.25 = 25%
If the parents have four offspring, the ratio will not always come out exactly 3:1. You might get 4 tall and 0 short, or 2 tall and 2 short. The 3:1 ratio only emerges accurately over many offspring. This is why pea plants are useful for these experiments, because a single plant can produce hundreds of seeds.