A child inherits half of the genes from his mom and half from his dad i.e. 23 chromosomes from his Mom and 23 chromosomes from dad which will result in 46 chromosomes total. Paternity testing identifies the specific DNA sequence at each of the 24 DNA markers tested in the Mom, Dad and Son. If both parents are the biological parents of the child, then both parents would have had to pass on one of their alleles at each DNA markers to the child.
