How blood types are inherited
Your blood type is really two separate results: the ABO group (A, B, AB or O) and the Rh factor (positive or negative). Each comes from its own gene, on different chromosomes, so a child inherits them independently.
For ABO, there are three versions of the gene: A, B and O. Everyone has two copies, one from each biological parent. A and B are both dominant, and O is recessive, so it only shows when there is no A or B next to it:
| Blood group | Possible gene pairs |
|---|---|
| A | AA or AO |
| B | BB or BO |
| AB | AB |
| O | OO |
The Rh factor works the same way with two versions, D (positive) and d (negative). Rh positive people are DD or Dd, and Rh negative people are dd. That is why two Rh positive parents can have an Rh negative child, but two Rh negative parents cannot have an Rh positive one.
How the calculator works
Each parent passes on one of their two copies at random, like a coin toss. The calculator lines up every possible pair in a grid called a Punnett square and adds up the chances for each group.
There is one unknown: you can see your blood group on a donor card, but not whether a group A parent is AA or AO. The calculator fills that gap with typical European gene frequencies (A 28%, B 8%, O 64% of all ABO copies, and 40% of Rh copies negative). From those, about 82% of group A people carry a hidden O, about 94% of group B people do, and about 57% of Rh positive people carry a hidden Rh negative copy. If you tick "Not sure" for the Rh factor, it uses the average, where about 16 people in 100 are Rh negative.
A worked example
Take a parent with group A and a parent with group B. The group A parent passes on A about 59% of the time and O about 41% of the time. The group B parent passes on B about 53% of the time and O about 47%. Multiplying those out gives:
- AB: about 31%
- A: about 28%
- B: about 22%
- O: about 19%
So every group is possible. If both parents are Rh positive, each passes on the hidden negative copy about 29% of the time, so the baby has about an 8% chance of being Rh negative. Put together, the most likely single result is AB positive, at about 29%.
Some pairs leave no doubt at all. Two group O parents always have a group O child. A group AB parent and a group O parent have a child with group A or B, half and half, and never AB or O.
Which groups are possible
| Parents | Possible for the baby | Not possible |
|---|---|---|
| O and O | O | A, B, AB |
| O and A | A, O | B, AB |
| O and B | B, O | A, AB |
| O and AB | A, B | O, AB |
| A and A | A, O | B, AB |
| A and B | A, B, AB, O | none |
| A and AB | A, B, AB | O |
| B and B | B, O | A, AB |
| B and AB | A, B, AB | O |
| AB and AB | A, B, AB | O |
The Rh factor in pregnancy
The Rh factor matters most when the person carrying the baby is Rh negative and the baby is Rh positive. Small amounts of the baby's blood can reach the pregnant parent's bloodstream, especially at birth, and their body may then make antibodies against Rh positive blood. This usually does not harm the first baby, but in a later pregnancy those antibodies can attack the red blood cells of an Rh positive baby.
An anti-D injection prevents those antibodies from forming. Blood group and antibodies are checked at the first pregnancy blood test. If you are pregnant and Rh negative, you will usually be offered anti-D in the third trimester (often around 28 weeks), again after birth if the baby turns out to be Rh positive, and after any bleeding, fall or blow to the bump. In some countries a blood test first checks the baby's Rh factor from the pregnant parent's blood, so anti-D is only given when it is needed.
Limits of this calculator
The model covers the two main blood group genes and nothing else. Rare variants, such as the Bombay phenotype, cis-AB or weak D, can produce results the grid says are impossible, and the percentages are only as good as the European averages behind them. It is not a parentage test either. For your own pregnancy, your midwife or doctor will test your blood group and Rh factor and tell you whether anti-D is right for you.