一、阅读表达（15分）
Scientists guess that 80% of a person’s weight gain comes from genetics and 20% comes from the person’s environment. By 1998, more than 130 genes（基因）and gene neighbors were found to have some link to weight gain or weight loss in people. These different genes can either work all together or work in groups. This means that different people may _________or lose weight with difficulty for different reasons.
