Most eggs come in two colors - or rather only should come in two colors: white or brown. But there's another type that are known to be blue. A new study has just figured out why that's so.

The Araucana chicken from Chile and the Dongxiang and Lushi chickens from China are known to lay pale-blue eggs.

A new study found that a single gene, called callee oocyan, is responsible for the blue dye, and it turns out that the chickens have a high incidence of a particular retrovirus called EAV-HP.

Instead of transcribing DNA into RNA and then into protein, retroviruses operate backwards, retroviruses have RNA, which they use to make DNA, and then integrate that DNA into the DNA of their hosts. HIV is an example of a retrovirus.

The EAV-HP makes the eggs look blue and while it is not necessarily harmful, they've been eaten widely. So you don't have to worry!

You now know that they're not sad eggs.

[via Virology Blog]