In a Sex-Linked Pedigree, what must be true if a daughter has a condition?

In a sex-linked pedigree, if a daughter has a condition that is inherited in a sex-linked manner (typically X-linked), it is important to recognize how inheritance works for traits associated with the X chromosome. Daughters receive one X chromosome from their mother and one X chromosome from their father. For a daughter to express a recessive condition linked to the X chromosome, she must have received the affected X chromosome from one of her parents.

If the condition is recessive and located on the X chromosome, the father must possess the affected allele because he has only one X chromosome to pass to his daughters. Therefore, if the daughter has the condition, she inherited the affected X from her father, who must exhibit the trait. Hence, it is true that if a daughter is affected by a sex-linked condition, her father must also have the condition to provide the X chromosome carrying the trait. This understanding reflects the nature of inheritance in sex-linked traits, specifically the pattern of expression in males and females.