Season chicken breasts by rubbing with cumin, chili powder, salt, pepper and 1 tbs. olive oil. Marinade chicken breasts for 30-40 minutes. While chicken is marinating, slice the squash lengthwise, slice onion into 1/4 inch rings, and toss squash, onions and whole peppers with remaining olive oil. Grill these items and then allow to cool. Peppers will take slightly longer, they should be grilled until they are black. Once peppers are blackened, rinse away the charred skin, and then slice into strips. Slice squash halves into 1/2 inch pieces. Grill chicken next, and once done, allow to cool, slice into strips. To serve, place grilled vegetables and chicken into a whole wheat tortilla and top with cilantro yogurt dressing.