ABO blood typing can give some preliminary indications about possible paternity. Its use is limited, however, because there are only four possible types, and the vast majority of people in any population has only two of those types (A and O). This means that a man may have a type consistent with paternity and still not be the father of the tested child. More modern genetic tests, such as DNA typing, will lead to a more reliable conclusion regarding paternity. 

Because the gene for the O type can be masked by the genes for A or B, inheritance of blood type can be unclear to the layman. The charts below may help. In most instances, the charts accurately reflect the blood typings which will result from the indicated scenarios. However, there are instances when a rare mutation may arise. When it does, the typings may not hold in circumstances of genuine parentage. Consequently, these charts should be used as general starting information only. 

		And the Father is:
		A	B	AB	O
	A	A or O	A, B, AB, or O	A, B, or AB	A or O
	B	A, B, AB, or O	B or O	A, B, or AB	B or O
	AB	A, B, or AB	A, B, or AB	A, B, or AB	A or B
	O	A or O	B or O	A or B	O

 
 

		And the Child is:
		A	B	AB	O
	A	A, B, AB, or O	B or AB	B or AB	A, B, or O
	B	A or AB	A, B, AB, or O	A or AB	A, B, or O
	AB	A, B, AB, or O	A, B, AB, or O	A, B, or AB	Ordinarily not possible.
	O	A or AB	B or AB	Ordinarily not possible.	A, B, or O

To use either chart, find the row containing mother's blood type along the left side of the table, and the column containing the blood type of the other individual of interest, either father or child. The block at the intersection of the column and the row will give the blood types of the third individual. 

For example, if the mother is blood type B and the father is type A, use the first table. Go to the row containing the bolded letter "B" and the column containing the bolded "A." Where the column and the row intersect is the block containing "A, B, AB, or O." These are the possible types of a child which could be produced by the two adults. 

The second table gives the blood types of men who could produce a child of a known blood type with a woman whose blood type is also known. Use of this table is is similar to use of the first one. As an example, if a woman is known to be of blood type A and she has a child of type A, the blood types of the possible father can be found by looking in the block in row A under column A, and seeing the answer as "A, B, AB, or O." 

-- Fairfax Identity Laboratories