What alleles do the f1 offspring have and why?

The alleles of the f1 offspring will depend on the alleles of the parents. In theory all of the alleles in the parental genotypes could be present in the f1 generation.

To work out which combinations of alleles will be present in the f1 generation/the proportion with one allele etc. you would need to draw some kind of cross.

AA x Aa

A A

A AA AA

a aA aA

So the f1 offspring have both the A and a alleles, because the two alleles from each parent are separated into the gametes

AA gives two gametes both with 'A' allele

Aa gives on gamete with 'A' and one with 'a'