What is a "marker"? A marker is a unique sequence of nucleic acids found on a strand of DNA. Geneticists are currently finding DNA markers evenly dispersed in the canine DNA. These markers are polymorphic meaning they tend to look slightly different in different individuals. This means that we can tell by looking at a particular dog which grandparent contributed a certain part of its DNA.
Genes are arranged on chromosomes in a stable spatial relationship. During reduction division of the germ cells, a phenomenon called "crossing over" occurs, in which parts of analogous chromosomes are exchanged. The liklihood that two genes are separated by crossing over depends on how closely spaced they are on the chromosome. Closely spaced genes tend to stay together during the final division of the germ cells (meiosis). When 2 genes (or a marker and a gene) are close enough in their spatial arrangement on the chromosome to be transmitted together we call them "linked".
If we knew that a particular grandparent carried a disease, we could determine what part of the DNA is probably responsible for the disease. The certainty with which thet probability can be established depends on searches of large numbers of pedigrees. Here is an example that illustrates the basic procedure:
In this example the colors represent the markers unique to that grandparent each numbered position. Now imagine that grandparents G1 and G4 and puppies P1 and P4 have the disease and the disease has a recessive mode of inheritance. We don't know which position (1-9) to look at, that might be near the gene responsible. But when we look at the DNA markers we see that only at position 7 do both the affected pups, P1 and P4, have the unique DNA markers from both affected grandparents G1 and G4 (orange and purple). This means that that the disease gene is likely to be located somewhere near that position.G1 G2 G3 G4 Grandparents 123456789___123456789 123456789___123456789 chromosomes 123456789 | 123456789 123456789 | 123456789 mom dad Parents 123456789______________123456789 123456789 | 123456789 P1 P2 P3 P4 Puppies 123456789 123456789 123456789 123456789 123456789 123456789 123456789 123456789 * *
This example is very simplistic, but it is the general way markers are used. Right now, canine geneticists have found sufficient markers to have an ~85% probability of finding a simple recessive trait. This means that there are enough markers now, that a disease gene has an 85% chance of being close enough to a marker that we can detect linkage.