Twenty-six letters of the alphabet are the basic units of our English language. Words and sentences are formed from these 26 letters. Genes on the other hand are formed from only 4 chemicals, or bases. We can measure the length of the gene fragments by the number of bases they have.
All children have a biological mother and a biological father.
For each gene in the child, there is one genetic variant size that came from the father and one genetic variant size that came from the mother. He could be the child's biological father. The probability that he is the child's biological father depends on the frequency with which this gene fragment is found in the male population. The fewer men who have this gene fragment size, the greater probability that the tested man is the biological father of the child. This process is repeated several times and each time a different gene is analyzed.