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The Science of Paternity

The mother and father each donate half of their DNA to their children. Of course, the parents receive half from each of their parents, and so on. For now we will focus on just parents and children.

In figure 1,  the mother has donated her red half to two of the children and her yellow half to the other two. She could have, just as easily, donated yellow to all four or red to all four. The same holds true for the father's DNA.

"But we don't look alike"

How your child looks may be a poor indicator of paternity for many reasons, but will still make you question the relationship. If the child looks nothing like you or has black hair when you have blond, it may be time for a DNA test but don't be surprised if they are still your child based on the DNA test. In figure 1, you can see from the illustration how two siblings can range from very similar to nearly nothing in common, and still be from the same parents. This is why a DNA test is so important. 

My personal barcode

Your DNA profile for paternity is made up of numbers (also called alleles). These numbers represent the genetic contribution passed down from each generation of parents. In figure 2, if we focus on 1 genetic location, you see the father and mother both have two numbers. Each received one of the numbers from their mother and one from their father.  Just as with the colors, there are four possible outcomes for the children.

You can see that if the child's DNA type was a [13,15] at this genetic location, it would not be possible for the man to be the father. It is possible for two offspring to have the same two numbers if we only look at one genetic location and for this reason we examine 16 different locations or genetic systems.

Figure 3

Figure 4

Figure 1

Figure 2

Possible Children Types

(10,13)
(10,14)
(12,13)
(12,14)

When he is the father

In figure 3, you can see that the mother and father have contributed equally to the genetic profile of the child. The report would read "the alleged father can not be excluded from being the biological father and the probability of paternity is 99.99%".

You notice the Amelogenin marker has letters instead of numbers. This is because the Amelogenin marker indicates the gender of the individual. XX is for female and XY is for male.

When he is NOT the father

In figure 4, it is impossible for the man who was tested to be the biological father of this child. He does not share a number (allele) in common at each of the genetic systems tested.

Numbers highlighted in red are DNA types that the alleged father could not have donated to the child. The report would read "the alleged father is excluded as the biological father of the child". Exclusions are always confirmed by a complete second analysis.