If my parents are blood group A, why am I group O?

05 February 2006



My blood group is O-negative, my parents are Group A, and my brother is Group A. I was wondering how I came to be this blood group? I've also been told that I can give my blood to anyone else, but I can't have anybody else's. Is that right?


'O' is referred to as a recessive allele. In other words, if you have group O, your cells don't make any markers on their surface. These markers are like little flags that the body uses to recognise them. People who are Group A can be Group A because they have one gene for making 'A' markers, and another gene that doesn't make any markers. So someone can be Group A if they have one gene for A and one gene for O, because you have two copies of these genes. One is from your mother and one is from your father. The other way someone can be group A is if they have two copies of the A gene. If both your parents are Group A, they must have genes A and O each. When a sperm with an O gene in it meets and egg with an O gene in it, you end up with two O genes and blood group O. It's also right that you are the universal donor. You can only receive blood from a person like you: O rhesus negative. Only 15% of the population has this blood. The rhesus factor is an additional gene or flag on the surface of the cell. This is known as the D allele. If you have that, then your cells just make an additional marker. This becomes important is if a pregnant lady is O rhesus negative and has a husband who is rhesus positive. The baby can be rhesus positive and the result is that some of the baby's blood can mix with some of the mum's blood when the baby's born. This can make the mum make antibodies against rhesus blood. That's fine for the first pregnancy, but if you get pregnant again, those antibodies can end up in the baby and cause all sorts of problems. It is normal for antibodies to be passed from mum to baby, wither through the blood or through breast milk. However, if you've produced antibodes to something in the baby's blood, then the antibodies latch onto the blood cells and damage them. That's why you end up with a baby that looks bruised. This is just where the blood cells are breaking down. Luckily, this can be dealt with by giving the mother a dose of antibodies at birth. Overall, your blood is so useful because it has no markers on it, so when it's put into someone's body, they have nothing to latch onto.


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