If you know the children's blood type, you could calculate Parent's Blood Type
The result +/- is called the rhesus factor, with - being recessive, and + being dominant. When parents are -, then kids are -, otherwise the kids might be + or -.
The table below shows the possible:
| 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 | |