In a pedigree, when all affected males have affected mothers and all affected fathers have only affected daughters, what type of inheritance is likely occurring?

The scenario described in the pedigree suggests the presence of X-linked inheritance, specifically an X-linked dominant trait. When all affected males have affected mothers, this indicates that the mutant gene is located on the X chromosome passed from the mother to her sons. Since males have only one X chromosome (inherited from their mother), if they are affected, their mother must have at least one copy of the affected X chromosome.

Moreover, the fact that affected fathers have only affected daughters supports the idea of X-linked inheritance, as fathers pass their X chromosome to their daughters and the Y chromosome to their sons. Since all affected daughters also have affected fathers, it reinforces the notion that the trait is dominant. The daughters inherit the affected X chromosome from their father (and possibly from their mother), which explains the appearance of the trait in the female offspring.

In summary, the characteristics of this pedigree strongly indicate X-linked dominant inheritance, where the affected males inherit the trait from their mothers, and the affected fathers can only pass it to their daughters. This pattern distinctly separates it from other inheritance types such as autosomal dominant, autosomal recessive, or X-linked recessive.