Question

Ms. Pratt is claiming that Mr. X is the father of her child. Ms. Pratt is O-. Her baby boy is A+. Mr. X is blood type B+. Could he be the father of her child? If not, what blood type would he be expected to be?

Answer 1

Nope....
BTW, I take it you meant "If so, what blood type...".
If he isn't the father, there's nothing you can say about his blood group). If he WAS the father, the child's bloodtype would be either `O^+` or `B^+`.

Explanation:

There are 4 main blood types (8 if you include the Rhesus-factor)
It is a question of antigens and antibodies:

Bloodtype A has A-type antigens on the erythrocytes, and anti-B-type antibodies in the serum;

Bloodtype B has B-type antigens on the erythrocytes, and anti-A-type antibodies in the serum;

Blood type O erythrocytes don't carry antigens but the plasma has both anti-A and anti-B antibodies.

For the effects of pregnancy, the combinations and results are as follows:

`O and O rarr O`
`O and A rarr O or A`
`color(red)"O and B " rarr color(red)" O or B"`
`O and AB rarr A or B`
`A and A rarr O or A`
`A and B rarr O or A or B or AB`
`A and AB rarr A or B or AB`
`B and B rarr O or B`
`B and AB rarr B or A or AB`
`AB and AB rarr A or B or AB`

Therefore, if the baby is bloodtype A, the father can't possibly be mr. X....

The Rhesus factor is a bit more complicated, as it actually involves around 50 different factors, but suffice to say that, if one of the parents is `Rh^+`(positive), the child will be, no matter whether the other parent is `Rh^+` or `Rh^-`...

Therefore, if mr. X would be that father, the child would have either `O^+` or `B^+`...