Sex-Linked Inheritance

Genes on X (most school examples) show in males if recessive, because males have one X (hemizygous). Colour blindness and haemophilia named as X-linked recessive. A carrier mother (XHXh) x normal father: half sons affected, daughters usually not. Affected father (XhY) passes Xh to all daughters (carriers if recessive), none to sons (sons get Y). X-linked recessive traits appear more frequently in males because they have only one X chromosome. Pedigree analysis uses this pattern to identify inheritance mode.

SEX-LINKED INHERITANCE is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.

CRISS-CROSS HINT: Father to daughter to grandson is a common path for X recessives. Pedigree: more squares shaded, no father-to-son of an X gene. Examples: Haemophilia named; Colour vision named; No treatment doses. WORK A CARRIER CROSS: XHXh x XHY. Daughters: XHXH or XHXh. Sons: XHY or XhY. Phenotype: all daughters typical; half sons show the trait. Write X superscripts: X^H X^h (carrier female) × X^H Y (normal male). Sons: X^H Y or X^h Y. Daughters: X^H X^H or X^H X^h. State phenotype for each. Examples: Write XH not just H; Y has no allele in the school grid; Rare female affected if XhXh. Worked: carrier mother x typical father. Mother gametes: XH or Xh. Father gametes: XH or Y. Sons: XHY typical, XhY shows the trait. Daughters: XHXH or XHXh (carrier).