Consider a gene with four alleles a1, a2, a3, and a4. in the cross a1a2 × a3a3, how many offspring genotypes are possible?

With allele crossing we can represent it for an easier understanding by a table like the following:
                  Parent 1
              |    |a1___|a2
              |a3|    a1a3      |      a2a3
Parent 2|__||          
              |a3|    a1a3      |      a2a3
              |    |                  |

It is possible to form two genotypes. When the there is one homozygous individual, the variety in the combination will depend solely on the variation of the other parent.

It is evolution, because we all have some of the same traits like eye color or shape or ears.