Final answer:
The trait described is likely an X-linked recessive trait, which appears more frequently in males since they have only one X chromosome, and a child expressing it indicates that both parents may be carriers.
Step-by-step explanation:
When looking at a pedigree, if the traits are only in males and a child has it but not the parent, it is likely an X-linked recessive trait. Since males have only one X chromosome, any recessive allele present on it will be expressed, making them more susceptible to X-linked recessive disorders. In the case where the child has a trait that is not present in either parent, it indicates that both parents may be carriers of the recessive allele, with the genotype Aa, meaning they have one normal and one affected allele, but do not show the trait because it is recessive.