If a child has blue eyes and both of her parents have brown eyes, what does this tell you about the allele for
blue eyes?

Blue eyes are caused by a recessive allele.

Both parents had a recessive gene for blue eyes, but both parents also had a dominant gene for brown eyes. The two genes for blue eyes happened to meet each other and give the child blue eyes.

Also, put this in the biology section next time.

the answer is simply just 6n add the two together as they have an n