Genetics & Inheritance

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

1. 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).
2. Split each parent's alleles into the two gametes they can produce.
3. Draw a 2×2 grid. Put one parent's possible gametes along the top, the other parent's possible gametes down the side.
4. 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.
5. Work out the phenotype for each genotype using the dominant/recessive rules.
6. State the ratio of phenotypes.

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.