X-linked recessive
Males have a peculiar vulnerability here: they carry only one X chromosome, so for a gene on the X they have a single copy with no backup. If that lone copy carries a recessive allele, the trait shows — there is no second X to mask it.
Females have two X chromosomes. A female with one altered and one ordinary copy is usually an unaffected carrier, because the working copy is enough. She passes the recessive X to half her children: sons who receive it are affected, daughters who receive it become carriers. Affected fathers cannot pass the trait to their sons (a father gives a son his Y, not his X) but pass the allele to all of their daughters, making each a carrier.
On a pedigree this produces a telltale signature: the trait mostly affects males, often skips generations, and travels from an affected man through his unaffected carrier daughters to his grandsons. Carrier females can occasionally show mild signs because of how X-inactivation randomly silences one X in each cell.
Hemophilia and red-green color blindness are X-linked recessive, which is why both are far more common in males than in females.
One X in males means no backup; the trait surfaces in sons and passes silently through carrier daughters.
A key clue is the absence of father-to-son transmission: a father never passes his X to a son, so an X-linked trait cannot travel directly down the male line.