Knowledge Organiser
This topic summary covers Knowledge Organiser within Genetic Inheritance for GCSE Biology. Genetic inheritance patterns, alleles, and inheritance diagrams It is section 8 of 9 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 8 of 9
Practice
25 questions
Recall
25 flashcards
Knowledge Organiser
Key Terms
- Dominant allele — an allele that is expressed in the phenotype even when only one copy is present
- Recessive allele — an allele that is only expressed in the phenotype when two copies are present (homozygous recessive)
- Homozygous — having two identical alleles for a gene (e.g., BB or bb)
- Heterozygous — having two different alleles for a gene (e.g., Bb); the dominant allele is expressed
- Genotype — the combination of alleles an organism carries (e.g., Bb)
- Phenotype — the observable characteristic resulting from the genotype (e.g., brown eyes)
- Carrier — a heterozygous individual who does not show a recessive condition but can pass the recessive allele to offspring
Must-Know Ratios
- Bb x Bb: 3 dominant : 1 recessive phenotype (3:1); genotype ratio 1 BB : 2 Bb : 1 bb
- Bb x bb: 1 dominant : 1 recessive phenotype (1:1) — test cross result
- BB x bb: all Bb, all dominant phenotype (100%)
- BB x Bb: all dominant phenotype; 50% BB, 50% Bb
- 1 in 4 = 25% | 1 in 2 = 50% | 3 in 4 = 75%
Common Marks Lost
- Not showing gametes: gametes must be written above/beside the Punnett square before filling it in — these carry method marks even if the grid is wrong
- Saying "dominant = more common in population": dominant means expressed when one copy is present — it says nothing about how frequent the allele is (polydactyly is dominant but rare)
- Giving only the genotype ratio, not the phenotype ratio: mark schemes want "3 brown : 1 blue" not just "3:1" — always state what the phenotypes are
- Treating Punnett square results as certainties: results show probability for each pregnancy independently — do not say "exactly 1 in 4 children will be affected"
- Using same-looking letter pairs (C/c, O/o): choose letters whose upper and lower case look different (B/b, T/t, F/f) to avoid ambiguity in written answers
Grade 7–9: Pedigree Charts
- Circles = female; squares = male; shaded = affected; horizontal line = mating; vertical line = offspring
- Two unaffected parents → affected child: condition must be recessive (both parents are carriers)
- Every affected individual has an affected parent: condition is likely dominant
- Test cross (Bb x bb): a 1:1 offspring ratio proves the dominant-phenotype parent was heterozygous (Bb), not homozygous (BB)
- Codominance: both alleles expressed equally in the phenotype (e.g., AB blood group, roan coat in cattle) — neither allele is dominant over the other; written as I^A I^B or C^R C^W
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Practice Questions for Genetic Inheritance
What is the term for an allele that is always expressed when present?
What is the purpose of a Punnett square in genetic inheritance?
Quick Recall Flashcards
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