Aa can produce two kinds of BB and two kinds of Bb.

If combined together, AaBb can produce four kinds, namely ABABABABABABABAB.

AaBbCc can produce 8 kinds, in fact, the above 4 kinds of gametes plus a C are 4 kinds, and adding a C is the other 4 kinds, so the total number of * * * is 8 kinds;

By analogy, if one Dd gene is added to the above eight gametes, one D is eight, and one D is eight, *** 16.

So every extra pair of alleles has to be multiplied by 2, and finally there are 2 N species.

This is understandable!