In X-linked dominant conditions, which statement holds true regarding inheritance?

In X-linked dominant conditions, the principle of inheritance significantly differs from that of X-linked recessive conditions. The statement that all affected individuals have an affected parent is true because an affected individual carries at least one dominant allele on their X chromosome.

In X-linked dominant inheritance, both males and females can be affected, but affected males will always pass the condition on to their daughters, since they contribute their X chromosome to them. Thus, any daughter of an affected male will also be affected. Affected females can pass the condition to both sons and daughters. This means that if an individual has the condition, it is likely they have inherited it from an affected parent, confirming the truth of the statement.

The other options do not accurately represent the nature of X-linked dominant inheritance: only males being affected is a characteristic of X-linked recessive conditions, while male to male transmission is not applicable since males pass on their Y chromosome to sons. Lastly, while females can be carriers, being a carrier does not mean that all female offspring of an affected individual are carriers; some may express the phenotype if they inherit the affected X chromosome. Hence, the correct answer reflects a fundamental aspect of inheritance in X-linked dominant conditions.