What is 2^n rule in genetics and how is it used to solve questions ?

the 2^n rule is applicable for finding the number of distinct genotypes of sperm or egg. The reasoning is that by independent assortment, a true heterozygote AaBbCc will have: (2 possible genotypes for A) x (2 possible genotypes for B) x (2 possible genotypes for C). You should exclude homozygous genes from the n count, as these only result in one possible genotype. If you want to look at the progeny, you would then multiply the number of possible genotypes or phenotypes of the sperm or eggs by looking at the minipunnet results of each gene.