May in fact suggest that the degree of myopia is more likely to be influenced by environmental factors than the degree of hyperopia. Indeed, in reality, I cannot think of anything (like reading habits, light) that cause hyperopia except people are born of that.

On the other hand, if you look at the data closely, there is not much flip-flop in the degree of myopia between twins. if one is myopia, the other is also myopia though may at a different degree.

Using dichotomous variable in myopia is justified if one is answering this question: is myopia inheritable? But the dichotomizing SphE is insufficient if the research question is about the effect of genetic and environmentable factors is on the degree of myopia.

To me, the conclusion is myopia has a large component of genetic influence, but with proper precaution, one can postpone or even mitigate the myopia. During the critical periods such as early age or adolescence, it is important to prevent myopia. After the age of 30, the eye structure is more fixed and less likely develop a severe myopia even if I have the genes.