The best compatible genotype for marriage is when AA marries an AA. With that you save your future unborn children from having worry about genotype compatibility.
If AA marries an AS genotype. You will definitely give birth to children with AA and AS which is good. But at times all the children will be AS.
AS and AS genotype should not marry, because they will definite give birth to SS which are Sickle cell diseases (a recessive disorder).
AS and SS should not try to marry each other.
SS and SS must not try and marry since there is no chance of escaping having a child with the sickle-cell disease.
Kindly study table below;
AA + AA = AA, AA, AA, AA
AA + AS = AA, AS, AA, AS
AA + SS = AS, AS, AS, AS
AA + AC = AA, AA, AA, AC
AS + AS = AA, AS, AS, SS
AS + SS = AS, SS, SS, SS
AS + AC = AA, AC, AS, SC
SS + SS = SS, SS, SS, SS
AC + SS = AS, AS, SC, SC
AC + AC = AA, AC, AC, CC
According to the table above, a person with the genotype AA can marry any other genotype. AA marrying SS can only give birth to AS children. They can give birth to AA but no danger attached.