Unfortunately, only very few traits are determined by a single gene, as is the case with autosomal recessive inheritance. Often several genes are involved - this is known as polygenic inheritance. 

In this case, many genes are responsible, each of which can be present in a dominant or recessive form. If enough recessive gene pairs meet, the so-called "threshold value" is exceeded and the disease develops.

Examples of inheritance: 
6 gene pairs have been identified in a disease. With 6 gene pairs, there are 2 to the power of 6 distribution possibilities for each parent - in this case that would be 64. 

With 7 gene pairs there would already be 128 possibilities and with 10 gene pairs there are already 1024. 

This large number alone shows that polygenic inheritance is not so easy to get a grip on in terms of breeding. 

Dog breeds are artificial populations and are kept in closed stud books. Although the total number of dogs has increased enormously over the years, no new genetic material has been added. If a breed is selected for a specific characteristic (such as the Rhodesian Ridgeback's perfect ridge or as few or no white markings as possible), genetic diversity becomes even more limited.

As the degree of relationship increases, the risk of finding identical gene pairs also increases and the population becomes more homozygous. If a significant number of dogs are removed from breeding, this inevitably means that the occurrence of new hereditary diseases is encouraged. 

It is by no means the case that disease-causing genes can only be considered in isolation. It is quite possible that genes for very important traits are located in direct proximity on a chromosome. These would be eliminated at the same time, without wanting to. 

A genetic test for a polygenic inheritance, i.e. diseases caused by the interaction of several genes and environmental influences, is complex and not as simple as for monogenic hereditary diseases (a single gene mutation). There is no simple "test" that provides a clear probability of developing such a disease.