B4: Inheritance, Pedigrees and Genetic Crosses
Official Cambridge & Imperial ESAT specification requirements for Inheritance, Pedigrees and Genetic Crosses on the Biology paper. Understand the syllabus boundaries, spot distractor traps engineered by examiners, and master the 2-step Sub-90 shortcuts that compress solve time well below the 88.8-second cutoff.
Official Syllabus Subtopics
Requirements specified in the UAT-UK framework
- 1Monohybrid crosses, homozygous vs heterozygous genotypes, dominant and recessive alleles
- 2Test crosses with homozygous recessive individuals to determine unknown parent genotype
- 3Codominance and multiple alleles (e.g. human ABO blood system: Iᴬ, Iᴮ, Iᴼ)
- 4Sex-linked inheritance: X-linked recessive conditions (haemophilia, red-green colour blindness)
- 5Pedigree charts and conditional carrier probability calculations
- 6The "2/3 Carrier Trap" for unaffected siblings of autosomal recessive individuals
- 7Lethal alleles causing altered 2:1 phenotypic ratios
Sub-90 Exam Speed Tips
How top candidates solve this spec under the 88.8s clock
The 2/3 Carrier Trap: If an individual is born unaffected from two carrier parents (Aa × Aa), the `aa` genotype is ruled out, so the probability of being a carrier (Aa) is 2/3, NOT 1/2.
Males cannot be carriers for X-linked recessive conditions; with one X chromosome (XY), they express the condition directly.
Examiners engineer distractor options using common algebraic expansions and sign slips. Always test structural ratios or invariants before calculating arithmetic values.
Tested Topics in Specification B4
Master the individual question patterns and Sub-90 shortcuts for this specification.
Autosomal & Sex-Linked Pedigree Probability Trees & Conditional Carrier Calculus (⅔ Trap)
Pedigree 10-second rules: (1) Two unaffected parents -> affected child = RECESSIVE (both parents are obligate carriers). (2) Unaffected father + affected daughter = AUTOSOMAL (cannot be X-linked recessive). (3) Unaffected sibling of aa child has P(Aa) = 2/3.
Non-Mendelian Ratios (2:1 Lethal Alleles, Codominance, Dihybrid & Multi-Cross Phenotypic Ratio Tables)
Spot non-standard ratios immediately: 2:1 live offspring ratio = homozygous lethal allele (1/4 die in utero); 1:2:1 phenotypic ratio = codominance/incomplete dominance (e.g. Iᴬ Iᴮ or coat colour); in multi-cross tables, identify the homozygous recessive test-cross row first.
Ready to Master Specification B4 Under Exam Conditions?
Practice timed questions under the authentic Pearson VUE interface with live split stopwatch pacing, dry-wipe scratchpad, and distractor forensics.