Independent Alleles and Mendel's second law

One of the Rosalind problems deals with Mendel’s second law according to which traits are inherited independently one of each other. Let us assume we have two heterozygous organisms with genotypes Aa and Bb. Such independant assortment implies that the probability of an aa BB offspring is equal to $\Pr(\text{aa})\times\Pr(\text{BB})=\frac{1}{4}\times\frac{1}{4}=\frac{1}{16}$. The value $\frac{1}{4}$ comes from the fact that the probability of any offspring of this subtype is evenly the same for each crossing: