There are many different antigens on the membranes of red blood cells. Some of these can be used to determine paternity (e.g. the ABO group, the MN series and the Rhesus factor). This is a relatively quick, cheap and easy process that can be carried out on a simple blood sample.
1) In which of the following families might doubt be cast on the paternity of a child and why?
Family # Father Mother 1st child 2nd child
1 A O A O
2 A B O A
3 AB A B O
4 AB. M A. MN B. N B. MN
5 O. MN AB. M AB. MN A. M
6 B. M A. MN A. N O. MN
DNA is a better measure to determine paternity but based on the question, Family # 5 would have to be the family upon which most doubt could be cast as to paternity as an O and AB parental combination can ONLY result in an A or B or O. The mother (in this case) would only pass on EITHER the A or the B. Therefore the first child's paternity would be questionable but the second child's IS possible.
The MN factor of families 4,5 and 6 is possible in each case.
Family #1 passed on A and O to the first child (resulting in A which is obviously AO)and O and O to the second as they are apparently AO and OO respectively.
Family #2 passed on OO to the first and AO (respectively to the second
Family #3 is also not possible in that the second child can be a blood type of O when the father has to pass on either an A or a B. In order for somebody to have a blood type of O, they must have an OO genotype.
The pattern continues.
ANS: Family #3 and #5 will have the most probable doubt cast on the paternity of their first child.