Complex sentence: The majority of people around the world remain lactase non-persistent, and consequently are affected by varying degrees of lactose intolerance as adults--though not all genetically lactase non-persistent individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase non-persistence allele.
Split into simpler sentences: The majority of people around the world remain lactase non-persistent, and consequently are affected by varying degrees of lactose intolerance as adults. However, not all genetically lactase non-persistent individuals are noticeably lactose intolerant, and not all lactose intolerant individuals have the lactase non-persistence allele.