The inheritance of ABO blood group is governed by specific laws of genetics.These laws are simple to understand and using it you can calculate the blood group of an individual if you know the blood group of their parents.There are many uses of this method.Couples can guess or sometimes even confirm the blood group of their future child.Other uses include, finding the true biological father and thus solving the cases of disputed paternity.Though in majority of cases you cannot confirm the blood group, you can surely get the most probable groups.

How to calculate the blood group?

To study this method of calculating blood group, you must first understand some basic facts in genetics.If you take any character of an individual, for example; height, eye color, blood group or anything, there are two levels of consideration.The outward appearance, or which is expressed is called PHENOTYPE.For example, when i say i am tall, that 'height is tall' is the phenotype.Eye color is blue or black is a phenotype.My blood group is B positive is a phenotype.The next level of consideration is the genetic level, which means the genes coding for that phenotype.It is called GENOTYPE.It represents the genes responsible for a character.For example, two genes are responsible for a blood group.In other words there are two genes in our body, so that one particular blood group is expressed.Similarly behind any phenotype you take there will be genes responsible or genotype.Here a point you must understand is that, there can be many genes in a population, but in an individual only some of them (a fixed number for one character) will be present and they give a particular phenotype.

Here we deal about ABO blood group.There are 3 genes altogether in a population which codes for the phenotype called ABO blood group.But in an individual only 2 genes will be present.They can be called for simplicity as A, B and O genes.Here i am not considering the Rh factor which gives positive and negative for a blood group, since if you understand the inheritance of ABO blood group Rh factor is easy to understand.So three genes in a population, but only two of them are present in an individual.Now the next important point we have to understand is the Law of Dominance.It states that A and B gene are dominant and O gene is recessive.I will explain the meaning with an example.Suppose i am A group.So there must be two genes inside me which makes a phenotype called A group.Those two genes can be either AA or AO.Here in AO combination one gene is O, but still the phenotype is A group, which means O is recessive and that if one A gene is present the blood group will be A group.Here we can understand one more important point.For the blood group to be O group(phenotype), both the genes must be O.ie.OO, because O group is recessive and so if only two of the genes are O then only the blood group will become O group.So lets summarise.

1.When you say one blood group(phenotype), it is actually controlled by two genes(genotype), which is represented here by 3 letter A, B and O.

2.A and B gene are dominant and O genes is recessive.

3.So A group phenotype can have AA or AO genotype; B group phenotype has BB or BO genotype; O group phenotype has OO genotype only and AB phenotype has AB genotype only(since both are dominant, both express).

If you understand these basic rules, you can easily find out the blood group of an individual, if you know the blood group of both parents.But before we start calculation we have to know one more basic fact.We know that all human beings are formed by the fusion of sperm and egg.In both these gamets (sperm or egg) only one gene from the person will be present.Why? So that when fusion occurs, the cell formed will have two genes just like any normal individual.So in gamete formation any one of the gene only pass down and fusion restore the normal condition of two genes in the new cell.Now let us take examples and start calculation.

Suppose father is A group and mother is O group.Both are phenotype.You can write the genotype of the father as AA or AO and that of mother only OO for the reason i stated earlier.So any one of the gene from the two genes take part in fusion.It can be any one based on probability.Now look at the picture.

Here understand that i have used Parent 1 and Parent 2, because it really don't matter if its father or mother.In any way you can calculate and will reach the same result.So the child of parents with A group and O group can be either A group or O group.But here you must never make a conclusion that child will have blood group of their parents only.You have to calculate and reach a conclusion.

Lets take another example with Parent 1 as A group and Parent 2 as B group.So A group can have a genotype AA or AO and B group can have a genotype BB or BO.Now look at the picture.

Thus any of the four blood group.ie. A group, B group, O group or AB group can come for the child and this cross proves that child's blood group can be different from the parents.So never make any rules.We have to calculate.

The next case i am going to consider is something you can confirm!!!.Yes.Here both parents are O group.So the genotype of both the parents are OO.So only one type of gene is taking part in the fusion which is O.So the child will get only the O gene and his blood group will be 100% O group.Confirmed!

Next cross is between A group and AB group.Here AB group occurs because both A and B gene are dominant.Here also we have all the genes.But if you observe we can draw one conclusion from this cross.We know that there is only one O gene.So the child's genotype will never be OO and so never O group.

Like this we can calculate anyone's blood group.You can replace A with B group in above examples and workout.But if you notice you can understand that so far we can confirm only one case, that is of O group.In all other cases there are more than one possibility.If you want to confirm other cases too, must make a family tree.Yes.you must know the blood group of the child's grandparents.Lets take your case.Suppose you want to confirm the blood group of your child.First you have to understand your genetic makeup.For example, suppose your father is B group and mother is O group and you are B group.Here out of two possibility of B group.ie BB and BO, you can confirm that you are BO.Why?, because your mother is OO.(see the cross picture once more with B in place of A).Thus you have a confirmation.Like that if you trace your husband's or wife's family tree you can confirm their genotype also and thus the calculation of your child's blood group will be more accurate.This is a usual method used to confirm our calculation.Thus i believe you have understood the inheritance of ABO blood group.

Now lets take Rh factor.Since you have understood ABO blood group, its easy to understand Rh inheritance.Same is the case with Rh factor also.The rule is Rh positive gene is dominant and Rh negative gene is recessive.So Rh positive means +- or ++, but Rh negative genotype will be only --.Compare it with dominant A gene and recessive O gene in the above example.Here also we have a confirmed result.If both parents are Rh negative, the child will be Rh negative only.Now lets see the cross.

Thus you can calculate the possible blood group of any person.One thing you must understand is that, these are all probability.A father might be BO and mother OO, but for 100 children born to them, the blood group may be BO only and OO may never come.Its all probability.It happens in random when gene pass into gametes.But in any way you can speculate the blood group of your child and help your curiosity to know.I have seen many couples who want to know the probable blood groups of their child and i hope this article really helps them...