I used a whole centimetre of ink out of my best coloured pens doing it, but I think I have proved it's possible for there to exist a pair of kotters which will always produce ottaroo offspring when mated. Biology isn't my field of specialisation, though, let alone genetics ..... I had to make some assumptions that I don't know how reasonable they are in practice, not having intimate knowledge of kangaroo or otter genomes. But the maths seems to work out right, and you don't argue with mathematics.
So if anyone is thinking of buying a breeding pair of kotters, it might be prudent to insist to see a video of their first litter being born. Just to make sure. An unscrupulous breeder could