What is DNA?

DNA is the genetic "blueprint" which contains all the information necessary to make a living being. 

DNA is wound into tight thread-like structures called chromosomes, which are found in every cell in the human body (e.g. blood cells, skin cells, hair cells etc.). 

Humans have 46 chromosomes in total;

half of these are inherited from the mother (in the egg)
half of these are inherited from the father (in the sperm)

How is DNA Used in Paternity Testing?

Certain regions of human DNA show variations between people. 

At each of these regions a person possesses two genetic types (known as alleles), one inherited from each parent. 

By looking at a number of these variable regions in a person a DNA profile is produced. 

By comparing the DNA profiles of the mother and child it is possible to determine which half of the child's DNA was inherited from its mother (maternal alleles). 

The other half must therefore have been inherited from the child's biological father (paternal alleles).

The alleged father's profile is then examined to determine whether he has the DNA types in his profile that match the paternal alleles in the child.

If the man's DNA types do not match the child's, he is excluded as the biological father. 

If the DNA types match, then he is not excluded as the father.

Also read

Paternity Testing

DNA Testing