Child Blood Type Calculator

Blood Type Genetic Basis:

In genetics, blood type gene has two alleles, each allele has genotype A, B or O. 
The A and B are dominant, and O is recessive. So allele A combined with allele O is type A.
Similarly, BO is type B, AA is type A, BB is type B, OO is type O, and AB is type AB.
If both parents have type A blood, then the alleles could be AA or AO, thus the allele A frequency is 75%, allele O frequency is 25% for both parents.
So the chance of alleles OO is 25% × 25% = 6.25%, 
alleles AA is 75% × 75% = 56.25%, 
alleles AO is 75% × 25% = 18.75%, 
alleles OA is 25% × 75% = 18.75%.
Since AA, AO and OA are blood type A, and OO is blood type O, thus their child has 6.25% chance to be blood type O and 93.75% chance to be blood type A. 

The +/- is called the rhesus factor, with + being dominant, and - being recessive.
So if both parents are -, the kids are always -, otherwise the kids might be + or -. 


Child Blood Type Estimate Table:

		Father's Blood Type
		A	B	AB	O
Mother's Blood Type
	A	A/O	A/B/AB/O	A/B/AB	A/O
	B	A/B/AB/O	B/O	A/B/AB	B/O
	AB	A/B/AB	A/B/AB	A/B/AB	A/B
	O	A/O	B/O	A/B	O

Parent Blood Type Estimate Table:

		Child's Blood Type
		A	B	AB	O
One Parent's Blood Type
	A	A/B/AB/O	B/AB	B/AB	A/B/O
	B	A/AB/O	A/B/AB/O	A/AB	A/B/O
	AB	A/B/AB/O	A/B/AB/O	A/B/AB	Impossible
	O	A/AB	B/AB	Impossible	A/B/O

Source: http://www.endmemo.com/medical/bloodtype.php