If the yellowing is hapening between the veins then it could be downy mildew. Sweet basil if very susceptible and the disease can be transmitted by seed as well as by spores in the air. It is worse in humid conditions.
It is hard to grow sweet basil now. I can't grow it. I tried Eleonora, but it is not immune. It just lasts a little longer. Holy basil, African basil don't seem to show signs of disease. Lemon basil and thai basil will show the disease but to a lesser extent. I have tried preventive anti fungal sprays, but I can't keep up when it rains a lot. It might be better to grow it in the summer when there is less rain. As far as I know there is no GMO basil so it will be a long time for the breeders to breed one with resistance that still tastes good.
https://www.extension.umn.edu/garden/yar ... ny-mildew/