Locus (plural: loci) is a term used for the DNA markers that are tested and reported on your DNA Testing results. 

On each report there are generally 16 markers displayed, 15 of which are used to help determine your DNA Test results.  

In the case of a DNA Paternity Test, the alleged father must match at least one of these numbers at each of the 15 markers. 

Without matches at each position he may be excluded as the biological father of the child. Having the biological mother included with each DNA test will strengthen the results of your test.