Answer with Explanation:
Genetics' influence on the happiness of a person has long been studied. Some people say that the genes of a person affects the person's mood. For example, "5-HTTLPR" is a gene that is regarded to be connected to depression. This is why it is called "the depression gene." It is also associated with sleep disorders like "insomnia." When someone has trouble sleeping, it disrupts his way of life thereby affecting his happiness.
On the contrary, the genes exhibit themselves according to external environments. This is also one reason why a person's happiness may also be determined by his success in life, material possession, relationships with people and so on. So, a person's situation in life partly influences his happiness.