They say things that as scientists they have to know can't possibly be right. A few years ago Aguirre and several others decided to put on their scientists' hats and apply the tools of modern biology to the study of the dog genome. These diseases, characterized by night blindness followed by progressive deterioration of daytime vision, bear a striking similarity to the human condition known as retinitis pigmentosa. Fortunately for Aguirre and others in the field, dogs are particularly good for genetic studies. Unlike human beings, who tend to intermarry widely, dog breeds have been kept separate through inbreeding.
Genes are the sequences of DNA on a chromosome that actually direct the body to do something.
But long stretches of DNA between genes have simply accumulated over the course of evolution. Mutations in parts of the genome that do something important are often harmful, and tend to get weeded out of the population quickly, whereas mutations in the junk DNA just pile up, in random variations that can be used to distinguish one family line from another. What all this means is that researchers can use junk DNA sequences as flags or markers to help them find their way around the genome. If a beagle is crossed with an elkhound and then their offspring are crossed, some of those dogs will suffer from retinal degeneration and some won't. If all disease carriers have marker A at a particular spot on a chromosome whereas no noncarriers do, the disease gene must be close by.