UAT-UK SPEC CODE: B4 (Inheritance)ASSUMED MATHS 1 PREREQUISITE: M7.7FREQUENCY: VERY HIGH
Autosomal & Sex-Linked Pedigree Probability Trees & Conditional Carrier Calculus (⅔ Trap)
Deconstructed from 23 past UAT-UK questions (9.87% of all historical Biology papers). Students using standard A-Level classroom methods spend an average of 160s on this archetype—blowing past the 90s ESAT budget. Using the 2-line Sub-90 heuristic below compresses solve time to 45s (saving 115s per question).
Historical Frequency
23 Past Qs
9.87% of Biology
Classroom Trap Time
160s
+70s over 90s budget
Sub-90 Target Time
45s
Banks +115s on the clock
Dominant UI Format
5-Option A–E
Maths 1 Skill: M7.7
Card 1 & Card 2 Preview: The Classroom Time Trap vs. Sub-90 2-Line Rule
Excerpted from our interactive 6-Step Topic Lesson for ARCH-B-04.
CARD 1 OF 6 • THE CLASSROOM TIME TRAP (160s)
Why Students Bleed Time:
Since I-1 and I-2 are unaffected and have an affected child II-1 (), both parents must be . The Punnett square gives . Because II-2 is unaffected, eliminate , leaving , so (~115s).
In 23 past UAT-UK questions (9.87% of Biology), candidates lose 115s by since the sibling is phenotypically unaffected, the aa (1/4) genotype is already excluded, leaving 2 aa carriers out of {aa, aa, aa} = 2/3! Instead, apply B4 with Maths 1 skill M7.7.
CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<45s)
Line 1 (Obligate Carrier Parents): Unaffected parents + child .
Line 2 (Condition on Unaffected): Cross out from (20s).
Sanity & Limit Check: Verify units/signs against
fence_post_or_domain_trap (Using P(carrier) = 1/2 (2/4) instead of 2/3 for an UNAFFECTED sibling of an autosomal recessive sufferer (from Aa x Aa parents)) and factor_of_two_or_stoichiometry_trap (Forgetting to multiply by 1/2 when the question asks for the probability of an affected SON (or DAUGHTER) specifically).Examiner Distractor Blueprint for ARCH-B-04
FOIL #1fence_post_or_domain_trap
Using P(carrier) = 1/2 (2/4) instead of 2/3 for an UNAFFECTED sibling of an autosomal recessive sufferer (from Aa x Aa parents)
Since the sibling is phenotypically unaffected, the aa (1/4) genotype is already excluded, leaving 2 Aa carriers out of {AA, Aa, aA} = 2/3!
FOIL #2factor_of_two_or_stoichiometry_trap
Forgetting to multiply by 1/2 when the question asks for the probability of an affected SON (or DAUGHTER) specifically
Omitting P(male) = 1/2 when the stem specifies the sex of the child rather than 'given that the child is a boy'.
FOIL #3multi_statement_linchpin_foil
Failing to rule out X-linked recessive inheritance when an unaffected father has an affected daughter
An affected X-linked recessive female (X^r X^r) MUST inherit one X^r from her father, so her father MUST be affected (X^r Y).
FOIL #4intermediate_stop_trap
Outputting the parental carrier probability instead of multiplying by 1/4 for two carriers to have an affected child
Stopping at the parent genotype step before completing the final Punnett cross.
Try 1 Live Sample Question Under the 90-Second Stopwatch
Press W to open the 2-line dry-wipe scratchpad, select A–E, and lock in to reveal the Dual-View solution.
sub90 ESATARCH-B-04: Autosomal & Sex-Linked Pedigree Probability Trees & Conditional Carrier Calculus (⅔ Trap) (Sample)Biology
00:00.0
SUB-90 ZONE (<45S)
0.0s45s Sub-9090s ESAT Budget150s Classroom
Right-click any option to cross out • Keys
A–E + EnterQuestion 1 of 1 • 90s Target
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The question stem, diagram, and answer options are hidden behind this pre-start gate. When you click below, the question will reveal and your 90-second split stopwatch will immediately begin ticking.
Biology•ARCH-B-04•Standard 5-Option
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