The Y chromosome was Charlie’s responsibility...assuming he was Harry’s biological father.
[...the “Y” chromosome is his responsibility.]
I believe there is also a timing issue. I’ve been told that a male sperm is faster but dies sooner than a female sperm. If the egg is waiting around for a sperm to arrive, there is a higher chance it will be a boy. If the sperm is waiting on the egg to drop, it will probably be a girl. This was told to my wife by her OB. Following that advice we are 2 for 2.