Here's why:

* Polygenic Traits: Many traits are influenced by multiple genes interacting with each other. For example, height, skin color, and intelligence are all influenced by many genes, each contributing a small effect.

* Environmental Influences: Even traits influenced by genes are also affected by the environment. For example, a person's height is partially determined by their genes, but nutrition and health also play a role.

* Epigenetics: Gene expression can be influenced by factors other than the DNA sequence itself, such as chemical modifications that can switch genes on or off. These modifications can be influenced by environmental factors.

Exceptions:

While most traits are complex, there are some exceptions where a single gene can have a major effect:

* Monogenic Traits: Some diseases are caused by a single gene mutation. Examples include cystic fibrosis, sickle cell anemia, and Huntington's disease.

* Simple Mendelian Traits: Certain traits, like eye color, are often controlled by a single gene with a dominant and recessive allele.

In conclusion: The vast majority of characteristics are not controlled by just one gene. They are typically influenced by a complex interplay of multiple genes and environmental factors.