All dogs carry defective genes.
Since genes are always carried in pairs, most of these abnormal genes are carried in a only single dose , so that their presence is completely concealed by the other, normal gene. A useful analogy is that a gene is like a set of instructions given to a particular workman doing a small job on a very big construction site. Each workman gets two sets of plans. If one set is damaged, he still has one good set, and the job can proceed. But if both sets are damaged, the job will not be finished, or it will be done wrong.