Micheal Jackson and his then wife, Debbie Rowe apparently conceived their two children by first having the sperm (not Jackson's) and the egg (not Rowe's) fertilized outside the womb before having each implanted. This means neither is the biological parent of their two children. As for the yourngest child, the same procedure was followed with his surrogate mother.
I worry what this revalation is going to mean for the children as they get older. Even now, the two oldest are likely old enough to comprehend at least part of what will be said about their parents now.