Genetics of your blood group

Perhaps you are aware of Gregor Mendel‘s garden pea experiment(1856-1863 ) and the subsequent formulation of the basic laws of genetics. While formulating these laws, Mendel had for the first time used mathematics to solve a problem of biology which had raised eyebrows of skepticism at that time. Furthermore, his work did not receive the accolade it deserved but was later accepted and he is now known as the ‘father of genetics’.

English: Example of a Punnett square. In this ...
English: Example of a Punnett square. In this example in peas, the color yellow is determined by the dominant allele Y and the color green is determined by a recessive allele y.(Photo credit: Wikipedia)

I decided on making this a fun post. I’ll not go into all the conundrum of scientific overhaul. In fact  it was just a couple of days back when I was revising my basics of the principles of genetics, that I decided that a post was due on it too.

We were told that Mendel had been lucky in his findings as he had stumbled upon seven different traits in the pea plans, which expressed their characters independent of one another and had complete dominance over their “recessive” alleles. For example, a result of a cross between tall (TT) pea plants and dwarf (tt) pea plants gave rise to tall plants (Tt) in the F1 generation. The plants were not of intermediate height so there was no mixing of traits and that the dominant trait was independently expressed (here the “tallness“).

Oh, and the “TT” and “tt” are conventions used to denote the dominant and recessive characters respectively. “T” is written twice as we have two copies of a gene (the alleles) which codes for the same trait, with or without variation as we receive a copy of a gene from both our parents. So, the F1 plants, in spite of having a “Tt” genotype, were tall because the “T” dominates its allele variety “t”.

Now coming to the basics of blood group genetics, I’d like to mention two variations with the simple results Mendel obtained. The ABO classification or blood grouping shows multiple allele inheritance (there are 3 different variations in the genes, i.e., the alleles code for three different products) and co-dominance (two specific alleles when present express both their traits and do not dominate/ mask the other).

Here’s how:

In the ABO grouping, suppose we call the gene responsible for a particular blood group type as “I” and its alleles are “Ia” “Ib” and “i”.  
These alleles code for/control the different sugar polymers present in the plasma membrane of the red blood cells. So, Ia codes for Type A, Ib codes for Type B and when only “i” is present, we get no sugar polymer on the surface of the cell and the blood type is O. And when both Ia and Ib are present, we get the Type AB.


English: ABO Blood Group System
English: ABO Blood Group System (Photo credit: Wikipedia)

Case 1. Parents blood group types: AB  and O

Genes:                                            IaIb    and    ii

Gamete                                                 /               /

(in sperm/egg cell)                    Ia      Ib         i    i

Punnett square:

AB ↓/ O→ i i
    Ia Ia i Ia i
    Ib Ib i Ib i

Kids either have a Type A or a Type B Blood group.

Kids’ blood groups: Iai Iai Ibi Ibi (50% type and 50% B type)

Case 2. Parents: A and B

We can have 4 different possibilities here based on the parents’ genotypes.


Ia Ia and Ib Ib : progeny all with AB TYPE.


Ia i (heterozygous) and Ib Ib

Progeny: IaIb IaIb Ibi Ibi (50% chance of being AB Type and 50% of being B type)


Iai and Ibi

Progeny:IaIb Iai Ibi and ii (25% chance of each kind of blood type, AB, A, B, O)


IaIa and Ibi

Progeny: IaIb Iai IaIb Iai (50% chance of being ab type and 50% of being a type)

I recently came across a case of a couple with blood group A & O and their baby had the blood group B. Let’s see if it’s feasible by applying the simple laws of genetics:

Parents: A and O


Ia Ia and ii

Progeny: Iai Iai Iai Iai (only heterozygous A type possible)


Iai and ii

Progeny: Iai Iai ii ii (50% chances of A type and 50% chances of B type)

Therefore, a Type B is not possible. So the baby was either switched in the hospital (sounds incredible) or the guy is not the real dad.
But this cannot be taken as the sole criteria for paternity tests. Sometimes, due to phenotypic differences, the actual genotype does not show up. For example, the O in one of the parents might be due to the presence of Bombay Phenotype, as these individuals may show up O type blood even when it is not so. They are unable to express the A or the B alleles in their phenotype. These cases are extremely rare and normal transfusion in case of an emergency does not work for them.

Hope you enjoyed the post. This wasn’t one of our mainstream posts but then, who doesn’t like change once in a while. Have a great week ahead!

Note: This article being a science piece should not be treated as the ultimate source of the information it is trying to impart. This post just deals with the basics of genetics and shows how to predict the outcomes of likely crosses for determining blood group in humans.

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©The Idea Bucket, 2013.



  1. Mendel’s experiment was one of the first things I learnt in a course I took on evolutionary psychology and remains one of the most beautiful and elegant experiments I’ve come across in my undergrad studies. Awesome stuff you have here.

    Thanks for your Like!

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