How to draw a punnett square in microsoft word?

You want to open your columns, erhm, I believe it'll be 3 x 3

Insert > Table > Choose width and height needed.

You'll need to merge the first two horizontal and vertical, to put in your functions e. g., Aa, B, BB.

or you can do

Click on Tables > Move the table into a 3 x 3 table (for monohybrid) or a 5 x 5 (for dihybrid). So just key in the gametes in the first row and first column, leaving the first cell blank. After that, key in the genotypes of the offspring in the table.