Hello!

I'm an undergraduate in a genetics lab and my PI, who works on meiosis in S.pombe just gave me the following puzzle;

http://imgur.com/qKif2ZH

He wants to know how an organism with the following four asci (with the - being none of the three markers) would be possible from a single generation. I have a feeling that it's going to involve inducing diploid organisms to mate, but I can't for the life of me come up with a coherent answer and was hoping some outside inspiration would help!

Many thanks!

edit: Haha of course it's crossover! I've been looking at this for hours trying to think of some super complicated way of producing all of these asci and it's just two crosses! Thanks everyone for alerting me to my daft oversight!