What is lactose intolerance gene?

I want to know about lactose intolerance gene, How it damage one's body?

As I know lactose intolerance gene is common now a days and Primary lactase deficiency is the most common cause of lactose intolerance worldwide. This type of lactase deficiency is caused by an inherited genetic fault that runs in families. Primary lactase deficiency develops when your lactase production decreases as your diet becomes less reliant on milk and dairy products.