Genetic testing usually refers to the analysis of the gene itself. But because genes are often thousands of units long, it's costly to read every nucleotide. To save time and money, genetic testing labs use one of several methods that look for the most common variations in a gene. While generally accurate, these methods do introduce the possibility that a particular variant is not among those tested. For decades, doctors have been able to test for the products genes encode, looking at proteins and smaller chemicals for defects.
Some diseases arise because of a variant in a single gene.
When the cell reads the variant genetic code, it produces a variant protein. Some 4000 of the genetic diseases we know about today arise from variants in single genes. We know about these not because they are common nor because they represent the bulk of diseases influenced by genes, but because these are the easiest genetic diseases to find and understand. Most genetic tests available today detect variants in the single genes that cause these diseases. But most diseases are not as clear cut.