X-linked recessive disorders affect males predominantly due to hemizygosity on their single X chromosome.

Correct Answer

2. All sons of an affected mother should show the trait

Why Option 2 is Correct

Affected mother = homozygous recessive (XᵃXᵃ). All her eggs carry Xᵃ. Sons inherit her Xᵃ + father’s Y → 100% affected sons (XᵃY). Daughters get Xᵃ + father’s Xᴺ → all carrier daughters (XᵃXᴺ).

All Options Explained

1. All sons of a normal mother should show the trait

Normal mother = either XX (no allele) or XᴺXᵃ (carrier). Carrier mothers produce 50% affected sons only. Sons get random X from mother → 50/50 chance.​

❌ Incorrect: Only 50% risk from carriers.

2. All sons of an affected mother should show the trait

Affected mother (XᵃXᵃ) passes Xᵃ to ALL offspring. Sons = XᵃY (affected). Classic pattern in hemophilia pedigrees.

✅ Correct: 100% transmission to sons.

3. Many more females than males should show the trait

Opposite true: Males affected >> females. Females need homozygous XᵃXᵃ (rare); males need only XᵃY (single copy).

❌ Incorrect: Males >> females affected.

4. All daughters of a normal mother should show the trait

Normal mother passes normal X to half daughters at most. Carrier daughters = heterozygous (carrier, not affected). Affected daughters require affected father + carrier/affected mother.​

❌ Incorrect: Daughters rarely affected.

X-Linked Recessive Pedigree Patterns

*If father normal; 100% affected if father affected.

Exam Mnemonic: Affected Mom → All Sons Affected (AMASA). Classic for hemophilia, color blindness, Duchenne MD.​