Suppose that a large river separates two populations of mice. one population is all black, and the other all white. a beaver dam that is built allows the mice to cross the river. over time, many gray mice are born. what is this an example of?

Thank you for posting your question here at . I hope the answer will help you. Feel free to ask more questions.
The gray mice would be an indication that the genetic traits for black fur and white fur in these mice are codominant. If you define black mice as BB and white mice as WW, when they breed together they produce gray mice, BW. 

The principle of independent assortment describee how different genes independently separate from one another when reproductive cells develop.