Genetic Diagrams and Ratios: Interpreting Expected Outcomes
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Genetic diagrams show parental phenotypes, genotypes, gametes and a Punnett square to predict offspring ratios. The 3:1 ratio comes from crossing two heterozygotes; the 1:1 ratio from a test cross; and the 1:2:1 ratio from co-dominance crosses.
Genetic diagrams are the most structured type of IGCSE Biology answer. Every step earns a mark, and missing one step (especially gametes) can cost marks even when the final ratio is correct. This topic consolidates the skills from monohybrid crosses, co-dominance and sex linkage.
Standard layout for full marks
Examiners expect a consistent, clearly laid out genetic diagram. The recommended sequence is:
1. Parental phenotypes (state what each parent looks like). 2. Parental genotypes (use correct letter notation). 3. Gametes (circle them to show they are separate from genotypes). 4. Punnett square (one parent's gametes across the top, the other down the side). 5. Offspring genotypes and phenotypes. 6. Ratio (phenotype ratio, and genotype ratio if asked).
Missing the gametes line is the single most common reason for losing marks in genetics questions.
Understanding ratios and probability
The ratios from genetic diagrams are predicted or expected ratios based on probability. They do not guarantee the exact outcome.
Cross type
Expected phenotype ratio
Heterozygous x Heterozygous (Bb x Bb)
3 dominant: 1 recessive
Heterozygous x Homozygous recessive (Bb x bb)
1 dominant: 1 recessive
Co-dominance heterozygous x heterozygous
1: 2: 1 (three phenotypes)
Sex determination (XX x XY)
1 female: 1 male
With small numbers of offspring, actual results may differ from predicted ratios. The prediction becomes more accurate with larger sample sizes.
Why actual results differ from predicted ratios
Fertilisation involves random events. Each gamete combination is independent, like flipping a coin. Getting heads three times in a row does not change the probability of the next flip being heads. Similarly, having three offspring with the dominant phenotype does not make the next one more likely to show the recessive phenotype.
When exam questions ask why results differ from a 3:1 ratio, the answer is that fertilisation is random and small sample sizes produce results that may deviate from the predicted ratio by chance. Never say the cross "went wrong" or the ratio is "incorrect".
Try a focused question (4 marks)
A tall pea plant (Tt) is crossed with a short pea plant (tt). Predict the ratio of tall to short offspring. Explain why the actual results from this cross might not match the predicted ratio.
Show the answer and marking guidance
Gametes: T or t from the tall parent; t from the short parent.
Offspring genotypes: Tt and tt in equal proportions.
Predicted phenotype ratio: 1 tall: 1 short.
Actual results may differ because fertilisation is a random event and small numbers of offspring may not match the predicted ratio.
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 diagram.
Giving a 3:1 ratio for every cross regardless of parental genotypes.
Saying the ratio is "wrong" rather than explaining that chance causes deviation in small samples.
Forgetting to state both genotypes and phenotypes of offspring.