I don't know if it's a genetic thing, but my experience has been that white cats *seem* to shed excessively. Perhaps it's just that their hair stands out so much visually. You need to brush your cats daily, not only to pick up those loose hairs, but to help prevent hairballs.
Unfortunately, cats shed year-round; in the summer from the heat, and in the winter from the dry air generated by heaters and fireplaces. There are supplements you can give them that claim to help prevent shedding, but I haven't seen any testimonial experience to back that up.