By most traits, are you taking behavior? Humans have been proven to inherit facial expressions. Mice whose mothers are easily stressed tend to end up getting easily stressed. However, the babies of easily stressed mice, when raised by calm mothers do not get as easily stressed out. Most appearances or genetic tendencies are hard to change through nurture. Someone who is born to tall parents will probably be taller than average. If they are not well fed as they grow up and develop their bones, they won't reach their genetic potential, though their children may if they grow up in ideal circumstances and be taller adults. Genes predispose all living things towards certain traits. A maple tree will never become a conifer, but if it is grown in harsh winter conditions, it may grow in a gnarled, stocky shape. Similarly, the same tree in different types of soils may have different colored leaves depending on the nutrients in the ground and how those influence the tree's growth. Flamingos eat a certain food to make them pink, so while that pink-ness is in their genes, they need to eat the right food to not be an off white color!
As to why, we are about as much a product of our environment as our genes. A child raised listening to many languages may become a lingual genius but just about every child has that potential... Many genetic mental and health disorders are influenced by health and stress levels, as well as by how strong of a support system a person has. Some people predisposed to bipolar disorder never develop the disease because they never have a traumatic event that triggers it.
