Complex sentence: The lactase persistent phenotype involves high mRNA expression, high lactase activity, and thus the ability to digest lactose, while the lactase nonpersistent phenotype involves low mRNA expression and low lactase activity.
Split into simpler sentences: The lactase persistent phenotype involves high mRNA expression, high lactase activity and thus the ability to digest lactose. On the other hand, the lactase non-persistent phenotype involves low mRNA expression and low lactase activity.