Sansa was developed in the 1970s by the Morioka Research Station in Japan, in collaboration with researchers in New Zealand, as a straightforward cross between Akane and Gala varieties.
Sansa appears to have good disease resistance, although it was not developed specifically for this purpose. The disease resistance is likely to come from the Akane parentage, since Akane is itself naturally disease resistant whereas Gala is susceptible to several common apple diseases.
In terms of flavor, Sansa is essentially an early-ripening Gala. It inherits Gala's inherent sweetness, but with more acidity - as might be expected given its earlier ripening season.