The great variety of possible gene combinations in a sexually reproducing species is due in part to the

Sorting of genes as a result of meiosis.

Maybe talking about Mendel's law of segregation and law of independent assortment, I suggest looking those up.

It could also be due to "Crossing over" between homologous chromosomes--which occurs in late prophase.

If you've heard of "Metaphase", I'd suggest looking up the former and explaining that to your teacher!