"如果和环境完全没有关
系，父母两人都不近视，则子女近视的可能性有6～15％就不好解释了。"

It's not difficult to explain:

If the gene is recessive in one's parents, it will not exhibite itself in the parents. But the parents still carry the gene nonetheless. But when the gene becomes dominate in the child, the child is myopic.