Q Why do some chickens lay white eggs and others lay brown eggs?

— Rafael Garcia-Dove, Madison, Wis.

A Ron Kean, poultry specialist at the University of Wisconsin-Madison:

Genetics is the simple answer. Some chickens lay white shell eggs and some lay brown shell eggs, similar to the way hair color varies in people.

Shell color is not associated with the internal components of the egg.

If you break a brown egg and a white egg, you can see there’s very little difference between them.

The nutrition of the hen could cause a few minor changes, especially in yolk color, but the color of the egg shell is not related.

From turkey eggs to emu eggs, there is a lot of variation in bird egg colors.

Shell color is not typically determined by a single gene. A research study at UW-Madison showed that multiple genes are involved.

Researchers crossed two chickens that laid white eggs, with pure lines, and ended up with offspring that laid brown-shelled eggs. They found that different genes were involved with different breeds.

Apart from brown and white, some chickens even lay blue or green eggs. In this case, a single dominant gene is responsible for the blue and green color and traces back to breeds from South America.

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.