Myopia is highly myopic if it exceeds 600 degrees and has a genetic predisposition, but the mode of inheritance of myopia is complex and there is no definitive answer as to whether myopia is caused by a single gene or by multiple genes. If both parents have high myopia, the likelihood of having a child with myopia increases, with the exact likelihood varying depending on the mode of inheritance. Outdoor activity has a definite preventive effect on myopia, i.e., two hours a day or a cumulative ten hours a week of outdoor activity can prevent the development of myopia.