Khan Academy
Khan Academy: Polygenic Inheritance and Environmental Effects
Learn about traits that are controlled by multiple genes and/or influenced by the environment. Understand penetrance and expressivity.
Columbia University
Columbia University: Thomas Hunt Morgan
The site promotes the importance of Morgan and his experiments in genetics with fruit flies on the field of biology.
Talk Origins Archive
The Origin of Species, Chapter 14
The entire text of Charles Darwin's book, The Origin of Species is available on this site. This link is specifically for the chapter that discusses embryology as evidence for evolution.