Which one of the following observations would rule out a sex- linked trait in an extended family pedigree?

Sex-linked traits are passed via the X chromosome. Males transmit their Y chromosome to their sons and their X chromosomes to their daughters. Thus, an affected male cannot transmit a mutated X allele to his son, so the presence of male-to-male transmission in a pedigree categorically eliminates X-linked transmission as the genetic pattern of inheritance.