SACE Biology — Stage 2
Inheritance Patterns — Flashcards & Quiz
Inheritance patterns describe how traits pass from parents to offspring, and SACE Biology Stage 2 asks you to apply Mendelian and non-Mendelian rules to solve genetic crosses. You need to draw Punnett squares, interpret pedigree charts, and recognise dominant, recessive, co-dominant, incomplete dominant and sex-linked patterns. Exam questions often give you a family pedigree and ask you to identify the mode of inheritance.
Key Points
- Dominant allele masks the recessive allele; homozygous dominant (AA) and heterozygous (Aa) both show the dominant phenotype.
- Recessive trait only appears in homozygous recessive (aa) individuals — both parents must carry at least one recessive allele.
- Co-dominance: both alleles are fully expressed in the heterozygote (e.g. AB blood type shows both A and B antigens).
- Incomplete dominance: heterozygote shows a blend (e.g. red + white flowers → pink).
- Sex-linked (X-linked) traits: males have only one X chromosome, so they express recessive X-linked alleles when present (e.g. red-green colour blindness).
- Punnett squares predict offspring genotype and phenotype ratios; 1:2:1 genotype ratio gives 3:1 phenotype ratio for simple dominance.
Common Mistakes to Avoid
- Confusing genotype (allele combination) with phenotype (observable trait).
- Treating co-dominance and incomplete dominance as the same — they produce different phenotypes.
- Forgetting that males only need one recessive X-linked allele to express the trait.
- Assigning "dominant" to whichever trait is more common — dominance is about which allele masks the other, not frequency.
- Misreading pedigree charts — filled symbols are affected, circles are female, squares are male.
Exam Strategy
SACE Stage 2 genetics questions give you a cross or pedigree and ask you to predict offspring or identify the inheritance pattern. Method for crosses: (1) identify parental genotypes, (2) draw the Punnett square, (3) list genotype and phenotype ratios. Method for pedigrees: (1) check if trait appears in every generation (suggests dominant), (2) check sex ratio (equal = autosomal, mostly males = X-linked recessive), (3) identify unaffected parents with affected children (suggests recessive).
Revision Tip
Punnett squares are drillable — build a Revizi deck with 10+ crosses covering dominant, recessive, co-dominant and sex-linked scenarios for fluency.
Related Concepts
Last updated: March 2026