Complex sentence: In most of the world's population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine, which causes the common symptoms of adult-type hypolactasia, or lactose intolerance.
Split into simpler sentences: In most of the world's population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine. Diminished lactase expression causes the common symptoms of adult-type hypolactasia, or lactose intolerance.