Genetic loci, loci being the plural of locus, are the segments of chromosomes containing functional genes that are taken for analysis in a DNA test. Biological traits that we exhibit as humans are done through gene mapping. ISO accreditation laboratories will test for 21 genetic loci, sometimes referred to as genetic markers; the higher the number of markers the more accurate the results. Some companies will test for less to save on costs and time.
The human body contains thousands of genes- some studies say around 44,000 genes and other studies have claimed it is as high as 70,000. Genes analyzed in a DNA test are not selected at random but are specifically located. Comparing the same fixed points on DNA between people will tell whether a genetic relationship exists. In the case of a paternity DNA test, all 21 markers tested will have to be identical between father and child if the alleged father is the biological father.