X-linked recessive Punnett square.

How X-linked recessive inheritance works

In humans and many other animals, females have two X chromosomes and males have one X and one Y (OpenStax Biology 2e, 12.2). A father passes his Y to his sons, so he cannot pass an X-linked recessive trait to a son; an affected son inherited the gene from his mother (Wikipedia). A son needs only one recessive X to be affected, a daughter needs two, which is why the trait shows more often in males. OpenStax lists some forms of colour blindness, hemophilia and muscular dystrophy as human X-linked conditions.

Worked example. A carrier mother (X^AX^a) and an unaffected father (X^AY): each son has a 50% chance of being affected, matching Wikipedia's 50% for sons of a carrier mother, no daughter is affected and 50% of daughters are carriers. An affected father (X^aY) with a non-carrier mother: no son is affected (0%) and every daughter is a carrier (100%). Wikipedia adds that carriers can sometimes show features because of random X-inactivation, and that some scholars suggest dropping the dominant and recessive labels for X-linked traits; this tool uses the classroom convention.