Cue Column | Note-Taking Column

Carrier female genotype? | $X^A$ $X^a$ — heterozygous. Phenotypically normal. Can pass $X^a$ to 50% of each child. Key rule: carrier mothers produce affected sons and carrier daughters in 50:50 ratio.

Affected female requirements? | $X^a$ $X^a$ — homozygous recessive. Requires $X^a$ from BOTH parents. Father must be affected ($X^a$ Y) and mother must be carrier ($X^A$ $X^a$) or affected ($X^a$ $X^a$).

Father passes to which children? | X chromosome → ALL daughters (daughters always get father's X). Y chromosome → ALL sons (sons always get father's Y). Consequences: X-linked traits NEVER pass from father to son; all daughters of affected father are minimum carriers.

Key cross 1: Carrier female × Normal male | $X^H$ $X^h$ × $X^H$ Y → 25% $X^H$ $X^H$ (normal ♀), 25% $X^H$ $X^h$ (carrier ♀), 25% $X^H$ Y (normal ♂), 25% $X^h$ Y (haemophilic ♂). Result: 50% sons affected; 50% daughters carriers; no daughters affected.

Key cross 2: Carrier female × Affected male | $X^H$ $X^h$ × $X^h$ Y → 25% $X^H$ $X^h$ (carrier ♀), 25% $X^h$ $X^h$ (affected ♀), 25% $X^H$ Y (normal ♂), 25% $X^h$ Y (haemophilic ♂). Result: 50% children affected (¼ affected ♀ + ¼ affected ♂). Affected daughters possible.

Key cross 3: Affected female × Normal male | $X^h$ $X^h$ × $X^H$ Y → 50% $X^H$ $X^h$ (carrier ♀ — ALL daughters carriers), 50% $X^h$ Y (haemophilic ♂ — ALL sons affected). All sons affected; all daughters carriers.

Summary

The three key X-linked recessive crosses and their unique outcomes must be memorised. Carrier female × normal male is the most common NEET tested cross. Carrier female × affected male can produce affected daughters (rare combination). Affected female × normal male guarantees all sons are affected and all daughters are carriers.