ARCH-B-04: Autosomal & Sex-Linked Pedigree Probability Trees & Conditional Carrier Calculus (⅔ Trap)

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-B-04: Why the 160s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~160s (Blows 90s Clock)

Since I-1 and I-2 are unaffected and have an affected child II-1 (

ff

), both parents must be

Ff

. The Punnett square gives

1\,FF : 2\,Ff : 1\,ff

. Because II-2 is unaffected, eliminate

ff

, leaving

\{FF, Ff, fF\}

, so

P(Ff) = \frac{2}{3}

(~115s).

The Fast ShortcutTarget: 45s (Saves 115s)

Line 1 (Obligate Carrier Parents): Unaffected parents +

ff

child

\implies \text{I-1} = Ff,\,\text{I-2} = Ff

.
Line 2 (Condition on Unaffected): Cross out

ff

from

\{FF, Ff, fF, ff\} \implies P(Ff \mid \text{unaffected}) = \frac{2}{3}

(20s).

Where Students Lose Time: 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.

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