ARCH-B-05: Non-Mendelian Ratios (2:1 Lethal Alleles, Codominance, Dihybrid & Multi-Cross Phenotypic Ratio Tables)

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-B-05: Why the 130s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~130s (Blows 90s Clock)

Two living yellow mice must both be heterozygous (

Yy \times Yy

), because

YY

is lethal. The zygote ratio is

1\,YY : 2\,Yy : 1\,yy

. Since

YY

embryos die in utero, the surviving live-born ratio is

2\,Yy\text{ (yellow)} : 1\,yy\text{ (grey)}

, so

\frac{2}{3}

of live offspring are yellow (~105s).

The Fast ShortcutTarget: 35s (Saves 95s)

**Line 1 (Lethal

2:1

Signature):**

Yy \times Yy \to \cancel{YY}, Yy, yY, yy

(

3

survive).
Line 2 (Live Fraction):

\frac{2\text{ yellow }(Yy)}{3\text{ live}} = \frac{2}{3}

(20s).

Where Students Lose Time: In 5 past UAT-UK questions (2.15% of Biology), candidates lose 95s by group a has a-antigens on rbcs and anti-b antibodies in plasma (having anti-a would agglutinate its own blood!). Instead, apply B4 with Maths 1 skill M7.7.

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