Many genetic defects are simple recessive traits, meaning it takes two bad alleles (versions of a gene) to see the condition. If an animal is tested free (for instance DDF means tested free for DD), the animal does not have one of these defective alleles. If the animal is a carrier (DDC) then that animal has one normal copy and one defective copy of the gene. If an animal is affected (DDA) then both copies of the gene are defective. Typically we would expect these affected animals to show the signs (phenotype) for that genetic condition. The only way to get affected animals is to mate a carrier bull to a carrier dam. In carrier to carrier matings, we expect 25% of the progeny to be free, 50% to be carriers and 25% to be affected.
In the case of DD, some animals have both defective copies of the gene but don’t show any sign of the genetic condition (this is called incomplete penetrance). With incomplete penetrance some other factor is needed for the phenotype to display. This trigger may be environmental (nutritional, stress, etc.) or genetic (a second gene involved in the pathway).