As I understand it, this is how it works. An auto has two recessive genes for autoflowerin (aa), a photo has two dominant genes for photperiod (AA). The photo gene is dominant, and from what I’ve read, the auto/photo gene is an all or nothing gene, not graded, like skin color. So if you cross an auto (which is aa) with a photo AA, you will get photos every time (Aa, or aA). If you then breed those offspring you have a 1 in 4 chance of getting an auto , the possible combinations are Aa, aA, AA, or aa, only aa will express autoflowering. Caveat: I took genetic a while ago!
I just had two volunteer plants come up in some soil I used for my last grow, all the plants were fem autos but one had a bad case of nanners so it got moved outside, the seeds are from that plant, and should be fem autos.