If the carriers are not affected by the trait what percentage of offspring in the second generation are affected

The percentage of offfspring depends on whether both or only one of the parents is a carrier.

For example, Allele t represents a disease. Allele T is the dominant allele.

           T              t

T         TT          Tt

t           Tt           tt

When both of the parents are carriers, then there will be a 25% chance that the offsprings will be affected.