Y-linked

Y-linked traits are determined by genes on the Y chromosome. These traits are passed exclusively from father to son, since only males carry a Y chromosome. A Y-linked gene will only affect male offspring and cannot be transmitted by females (who do not have a Y chromosome).

• Only males are affected by Y-linked traits (females have no Y chromosome). If a trait is Y-linked, an affected father will pass it to 100% of his sons and 0% of his daughters.
• Y-linked genes typically relate to male-specific functions (for example, genes involved in testes development or sperm production). An example of a Y-linked condition is Y chromosome infertility, where a mutation on the Y leads to male infertility and is passed from an affected father to all his sons.
• Because the Y chromosome has relatively few genes, Y-linked disorders are rare. When they do occur, the trait will appear in every generation along the direct male line (father -> son -> grandson, etc.), as long as males have children.

• A pedigree that shows a trait strictly being passed from fathers to all their sons (and never appearing in any females) is the hallmark of Y-linked inheritance. For instance, if a grandfather, father, and son all have the same condition and no female relatives are ever affected, think Y-linked.
• Exam questions may directly mention "a gene on the Y chromosome" - remember that means only male-to-male transmission. If you see male-only transmission with no skips, identify it as Y-linked.