Hemath screening is important , this graphic very quickly shows why. A carrier would not de any symptoms of a condition. So unwittingly without testing you can place 2 carriers together for breeding, from a litter of 8, 4 could be expected to be afflicted.
Some conditions take time to manifest. So using an affected dog with a clear dog introduces a litter of 100% carriers into the gene pool.
The rest of the permutations are shown in the graphic. This is why good breeders must test.