One method for finding gene mutations is to cut up the DNA into pieces and determine the sizes of the DNA fragments.
For example, lets say that a gene of interest is 100 base pairs long. If you cut the gene in half at a specific point halfway across, you would get two fragments that are 50 base pairs long. Restriciton endonucleases are proteins that cut DNA at certain, specific points.
However, lets say there was a mutation in the gene at the exact spot that the restirction endonuclease was going to cut the DNA. Since it is unable to cut, you would still have the single 100 base pair gene instead of the two 50 base pair fragments.
In this way we have found a mthod for differentiating a normal gene for a mutant gene. We can use this technique on DNA that we obtain from an embryo biopsy.