The Punnet Square

To discover the possible trait compilations a child may gain from their parents scientists use a handy little tool called a Punnet square. It's quite simple and very helpful.
You start with the parents Genotypes - or Gene combinations- Lets say we are looking at a singular trait of eye color. B will stand for brown eyes and be the dominate trait, and b will stand for blue eyes. So say Dad has The traits Bb so his eyes are brown, And mom has the same.
We begin with and empty square.

On the top we put Dad's Gene. and on the left side we put Mom's. Remember that the dominant trait will always go first. Now we carry the B from dad down and place it in the two boxes bellow, do the same with b. With mom's we go to the right.

Now lets look at what our kids could look like.

When you want to know what someone will look like on the outside you will for the Phenotype- or physical appearance- The phenotype is only the first letter. So if our child has BB or Bb than they have brown eyes, they will only have blue eyes if they have bb. 

Now we find the ratio or chances of what our child will have. 

The Genotypic Ratio here is 1(BB):2(Bb):1(bb)  

The phenotypic Ratio here is 3(brown eyes):1(blue eyes)

And that is how you fill out a Punnet Square