so a punnet square is used for putting together possible gene traits that one could have, let's say you have a black angus and a Red angus, red angus have the recessive gene black angus have both pronounced genes,
so for the black you have BB
and for the red you have Br
The first square would be BB
the second square would be BB
The third square would be BB
and the fourth square would be Br
so out of the 4 possible babies that you could have only 1 of them carries the recessive gene of the red angus coloring: the punnet square will look like this