Key Terms
- Dominant allele โ an allele that is expressed in the phenotype even when only one copy is present
- Recessive allele โ an allele that is only expressed in the phenotype when two copies are present (homozygous recessive)
- Homozygous โ having two identical alleles for a gene (e.g., BB or bb)
- Heterozygous โ having two different alleles for a gene (e.g., Bb); the dominant allele is expressed
- Genotype โ the combination of alleles an organism carries (e.g., Bb)
- Phenotype โ the observable characteristic resulting from the genotype (e.g., brown eyes)
- Carrier โ 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) โ 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 โ these carry method marks even if the grid is wrong
- Saying "dominant = more common in population": dominant means expressed when one copy is present โ 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" โ always state what the phenotypes are
- Treating Punnett square results as certainties: results show probability for each pregnancy independently โ 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 7โ9: Pedigree Charts
- Circles = female; squares = male; shaded = affected; horizontal line = mating; vertical line = offspring
- Two unaffected parents โ 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) โ neither allele is dominant over the other; written as I^A I^B or C^R C^W
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.
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
What is a recessive allele?
An allele that is only expressed when two copies are present (homozygous recessive). Represented by a lower-case letter (e.g. b). It is 'hidden' by a dominant allele.
What is a dominant allele?
An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g. B). Think of it as the 'bossy' allele.