blood cells could be modified to better carry oxygen, which would help the athlete's muscles to not get sore as fast. lactic acid wouldn't build up as fast either, causing less muscle soreness. this would affect muscular system and respiratory system. another modification would be the skeletal structure. you can change the bones of the athlete to make them better suited for their sport. you could strengthen, lighten, or elongate bones. for example, if the athlete is basketball player, they could use gene doping to become taller.