Learning focus
Use precise biological vocabulary, interpret diagrams and data, explain mechanisms as linked sequences, and evaluate biological applications and environmental decisions.
Writing a genetic cross
Start by defining the allele symbols. Write the parental genotypes, list the gametes produced and combine them in a Punnett square. Each gamete contains one allele because it is haploid. Finish by stating genotypic and phenotypic ratios.
Heterozygote cross
For Tt × Tt, each parent produces T and t gametes. The predicted genotypes are TT, Tt, Tt and tt, giving a 1:2:1 genotypic ratio. If T is fully dominant, the phenotype ratio is 3 dominant to 1 recessive.
Probability, not certainty
Each fertilisation event is independent. A 3:1 ratio is an expected probability across many offspring, not a fixed sequence in every family. Four offspring do not have to include exactly three dominant and one recessive phenotype.
Presentation
Clearly separate parental genotype, gametes and offspring. Use lines or a Punnett square so that allele combinations can be checked. State whether the final ratio is genotypic or phenotypic and use the phenotype names rather than only letters.
Practical or data skill
Use counters representing alleles to simulate 50 fertilisations. Compare observed and expected ratios.
Examination tip
Always include allele definitions and parental genotypes before the Punnett square.
Review questions and suggested answers
Question 1
What phenotypic ratio is expected from Tt × Tt?
Suggested answer
3 dominant : 1 recessive.
Question 2
What genotype ratio is expected?
Suggested answer
1 TT : 2 Tt : 1 tt.
Question 3
Why is each gamete given one allele?
Suggested answer
Gametes are haploid.
