More specifically, the WTS at one point claimed that the "life" of someone comes from the father, the sperm. Christ was born without a human father and thus did not inherit sin.
I did a little search and did not come up with a direct quote for this concept, so I'm not sure if they've backed away from that. What they seem to be saying now is that Jehovah could create a perfect human if he wishes, becoming the father of Christ as he was the father of Adam without the usual need for a human birth from a mother and father.
But perhaps they backed away from this as this would imply that the Nephillum, the children of rebel angels and humans might have been perfect, not inheriting Adamic sin. They were mortal, but still perfect and thus might have had an extended lifespan until God killed them.
Thus the question is valid whether or not an imperfect ovum from Mary would produce an impefect child. Maybe you need the father to be imperfect in order to produce transfer an imperfect life. This, of course, suggests that even though Eve ate of the forbidden fruit, that had Adam remained faithful, Eve still could have born him perfect children though she herself was imperfect after she ate of the forbidden fruit from the tree of knowledge.
But it is a moot point, as whatever it took, Jesus was born a perfect human, not one with inherited sin from Adam. Thus for his first coming he is represented by unfermented cakes, meaning his body was without sin, however God accomplished that. But the second time he comes, he is represented by a leavened loaf from the house, meaning he is not in a perfect body and his 2nd coming is not for the sake of a sacrifice, but for the sake of being a king. So the next time he appears, he is in an imperfect body, not needing to be born as an infant nor a perfect man. That is, he simply uses the body of one of his followers at the second coming, an imperfect man.
So that's an important point. He only had to be perfect the first time he arrived, but not the second.
Of course, God could have cured the ovum of imperfection before feretilizing it. He could have used the DNA from Mary after removing the imperfection. We just don't know. All we know is the result was a perfect human child.
Again, the WTS used to take the position that the "life" comes from the father and so Christ was born perfect, not having a human father. But that suggests that the children of the fallen angels were perfect. Apparently they thought about this because I did see a reference where they make it a very clear point that the angel Gabriel did not impregnate Mary. That is, no materialized angel had sex with her, which I guess might have produced a perfect human if the state of the father was the critical determining factor.
My interpretation leans now toward Jehovah removing the imperfection from Mary's ovum or just using the DNA from that ovum and placing it in a perfect one, creating a perfect child. Thus nothing was imperfect about Jesus, though he did share Mary's DNA, who was a descendant of king David. The second time he arrives, he must come through the line of Joseph, but he will be implanted into an imperfect body.