Here’s the full explanation
For this representation aa will be auto since it’s recessive
AA will be photo since it’s dominant
Aa and aA are gonna be fast finish autos
Let’s be begin shall we?
Cross a photo period AA with an auto aa
What results is the f1 seeds from that cross being ALL fast finished photos
So next we take those fast finish photos and cross them together again for the f2
Now u will expect in the f2 offspring to have 25% photoperiod, 50%fast finishing photos, and 25% autos
Let’s now cross the 25% autos found in the f2 and make our f3 generation
What u now have is 100% autos
Now for another curveball let’s take our f2 seeds and cross a fast finish photo with the autos we found
What u will have is 50% fast finishing photos and 50% autos
As far as breeding programs go, the f3 typically gets accomplished to get the desired autos isolated then one more run of em is done taking them to f4 and that’s typically the point the companies and breeders will make the seed stock available as they should be 100% auto at that time and stabilized for the traits they were chasing
Hope this helps to answer and give a complete understanding of what to expect
Cheers yall