Monohybrid Crosses Punnett Squares IGCSE Biology

Monohybrid Crosses and Punnett Squares: Predicting Offspring

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

A monohybrid cross involves one gene with two alleles. A Punnett square shows all possible combinations of parental gametes. Crossing two heterozygous parents (Bb x Bb) gives a 3:1 phenotype ratio in the offspring.

Genetic diagrams earn marks in a strict sequence: parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes and phenotypes, ratio. Skipping any step costs marks even if the final answer is correct.

Setting up a genetic diagram

Follow this standard layout for full marks:

1. State the parental phenotypes (e.g. Brown eyes x blue eyes).
2. Write the parental genotypes using a letter (e.g. Bb x bb). Use the same letter for both alleles, with uppercase for dominant and lowercase for recessive.
3. Show the gametes each parent can produce (e.g. B or b from the heterozygous parent; b from the homozygous recessive parent).
4. Draw the Punnett square with one parent's gametes across the top and the other's down the side.
5. Fill in the offspring genotypes and state the phenotypes.
6. Give the ratio of offspring phenotypes.

The classic 3:1 ratio

When two heterozygous parents are crossed (Bb x Bb), the Punnett square produces offspring genotypes BB, Bb, Bb and bb. Three out of four show the dominant phenotype and one shows the recessive phenotype, giving the classic 3:1 ratio.

The genotype ratio is 1 BB: 2 Bb: 1 bb. This 3:1 ratio is a predicted ratio based on probability. With small numbers of offspring, the actual ratio may differ from the prediction due to chance. With larger numbers of offspring, the actual ratio is more likely to approach the predicted ratio.

Test crosses and the 1:1 ratio

A test cross is used to determine whether an organism showing the dominant phenotype is homozygous dominant (BB) or heterozygous (Bb). The unknown organism is crossed with a homozygous recessive organism (bb).

If the unknown is BB, all offspring show the dominant phenotype (Bb). If the unknown is Bb, offspring are approximately half dominant (Bb) and half recessive (bb), giving a 1:1 ratio. The appearance of any recessive offspring confirms the tested parent is heterozygous.

Choosing the right letter

Always choose a letter where the uppercase and lowercase forms look clearly different. Avoid letters like S/s, C/c, or O/o where confusion is possible. Good choices include B/b, T/t, R/r, or N/n.

Use the same letter for both alleles of the same gene, because they are alternative forms of the same gene at the same locus. Using two different letters (e.g. B for brown and b for blue) is incorrect if those letters look like different genes.

Try a focused question (4 marks)

In mice, black fur (B) is dominant to white fur (b). Two heterozygous black mice are crossed. Use a genetic diagram to determine the expected ratio of phenotypes in the offspring.

Show the answer and marking guidance

Parental genotypes: Bb x Bb; gametes: B or b from each parent.
Punnett square produces offspring genotypes: BB, Bb, Bb, bb.
Phenotypes: 3 black fur (BB and Bb): 1 white fur (bb).
Expected phenotype ratio is 3:1.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Omitting gametes from the genetic diagram.
Using different letters for alleles of the same gene (e.g. B and w instead of B and b).
Forgetting that the 3:1 ratio is predicted and may not match actual results with small sample sizes.
Not clearly stating phenotypes alongside genotypes in the Punnett square.