A pedigree shows mostly affected males with unaffected parents and no male-to-male transmission. This pattern suggests

Core Concept

An X-linked recessive trait often affects more males because a male needs only one recessive allele on his single X chromosome. There is no father-to-son transmission because fathers give sons a Y chromosome, while heterozygous mothers can transmit the recessive allele to sons. The status of a mother's brothers is not required to identify this inheritance pattern.