Some traits do not obey Mendel's law. For example, people with red hair tend to also have pale skin. Why might this be the case?

Answer: Polygenic inheritance

Explanation:

Polygenic inheritance is a phenomenon that explains how a character like skin color show a range of more or less continuous variation due to many genes controlling it. And this is unlike Mendel traits that are controlled by single genes.

So, skin color in an individual is expressed as red hair and pale skin.