In the old days, genetic disease diagnoses were identified by the disease first, using the trait that the person had. If you couldn't stop bleeding, it was hemophilia. Black urine meant alcaptonuria. It was not until recently that genetic diseases could be identified by sequencing the DNA first, finding the mutation, and then notifying the person of their disease. (1) 

In today's world, genetic diseases can be identified far more quickly and well before someone is exhibiting the associated trait. In fact, they can be made in under a day. 

How quickly can a genetic disease be diagnosed? 

Scientists at the Rady Children’s Institute for Genomic Medicine...