Japanese Imperial male-only succesion

The main difficulty is that there are 16 countries (including the UK) that have the British monarch as head of state. If the change of law to allow equal primogeniture were made in some but not all of these realms, the eventual result would be that one person would be Queen of the UK, Canada and New Zealand, while one of her close relatives would be King of Australia, Jamaica and the Bahamas.

The split between equal primogeniture and male-first primogeniture will arise on the death of Prince William if his eldest child is a daughter and at least one younger child a son.

Unless Prince William’s firstborn child is a daughter I doubt there’ll be any serious attempt at changing the line of succession. Getting sixteen different countries, with varying degrees of suppor for republicanism, to pass identical legislation is a logistical nightmare.

The geneticist in me must say… Y-chromosome from the male determines male gender. If the male donates an X-chromosome, the baby is female.