The thickness varies because of diffusion and concentration differences. As the DNA bands corresponding to different DNA fragment lengths migtate through a gel, the bands get thicker the farther they travel because the rate of thickening due to diffision is inversly proportional to DNA fragment length. read more
This means there is more DNA within that region. if by thick you mean the band is “smeared” this means the DNA of different lengths are merging together. To sort this you can either run the fingerprint for longer or dilute the DNA with a higher concentration of gel to lower the overall density of DNA across the gel. read more
When exposed to radioactivity a pattern of more than 30 dark bands appeared on the film where the labelled DNA was. This pattern was the DNA fingerprint. To compare two or more different DNA fingerprints the different DNA samples were run side-by-side on the same electrophoresis gel. read more