Practice question

Paternal transmission difference distinguishes sex chromosome inheritance: fathers contribute Y chromosome to sons and X chromosome to daughters, never X chromosome to sons. Genes located on X chromosome therefore cannot be transmitted from father to son; sons receive their single X exclusively from mother. Autosomal dominant and recessive traits can show father-to-son transmission via autosomes, and Y-linked traits by definition show obligatory father-to-son transmission. Absence of father-to-son transmission in pedigree serves as diagnostic clue for X-linkage, whether dominant or recessive, helping exclude autosomal modes.