If a gene is recessive, it must be paired with another recessive gene before the trait will be expressed. A single gene may be responsible for a trait, e.g, eye color, but most often characteristics are polygenic, or determined by numerous genes. This doesn't mean that environment isn't important. There is an interaction between the two. Hereditary differences may appear immediately after birth.
Infant temperament is an example.
Mothers who take morphine, cocaine, methadone, etc. may give birth to drug addicted babies. Expectant mothers now advised to no consume alcohol in any amount. Maternal smoking leads to reduced oxygen supply to fetus, higher infant death rates, lower scores on tests of language and general mental ability. Lamaze , natural, or prepared childbirth now preferred. Early social development lays foundation for later relationships with peers, family, etc.