What Determines The Blood Type of a Person?

Several factors play a role in determining the blood type or group of a person.  Blood, the life-fluid, occurs as different types based on the presence or absence of certain substances on the red blood cells. Four such types exist under the ABO blood typing system. Now, what causes a person to have a specific blood type?

Blood Type

Blood types exist as A, B, AB or O. This is based on the most common blood typing system– the ABO blood typing. The blood type is determined by a person’s genetic make-up; therefore, one is born with a certain blood type and it does not change throughout life. Knowing one’s blood type is important lest an event crops up whereby one would need blood transfusions as fast as possible – prior knowledge of the blood type would greatly accelerate the procedures, specially in emergency situations where time is limited and precious.

GENETICS & Blood Groups

Blood type is determined by genetics, more specifically by the genes coding for the blood types in question. Genes are the pieces of DNA which carry the genetic information which code for some of our characteristics, like eye color, skin color, increased risk for diseases, just to mention a few.

Genes may exist in different forms known as allelles. Allelles are but mere variations of the same gene – A, B, AB, O represent the different allelles of the gene coding for blood type. Generally, in a pair of allelles, one of them is dominant: that is, one of them has a more pronounced effect, and hence, it is the one determining the trait to be inherited. Or, what is also possible is to have a pair of allelles in which both of them are equally dominant, which is called co-dominance.

Understanding alleles

To illustrate the meaning of allelles, let us have an example. Consider the following three allelles coding for colour of flower petals in a plant:

  • A
  • a
  • B

‘A’ is the dominant allelle, coding for the color red while ‘a’ is the recessive one (the opposite of dominant), coding for colour blue, and ‘B’ is the dominant allelle coding for color white.

A –> Red

a –> Blue

B –> White

So, if the color of the flower petal is coded by the pair of allelles ‘Aa’, the petal will be red in colour, because the dominant allelle A is the one which determines the characteristic (the colour of the petals). If the colour of the petal of the flower is coded by the pair of allelles “AB”, both being dominant, the color will be determined by both: a mix of white and red, and hence, the color may become pink.


Relating this concept to blood type, the allelles A and B are dominant while allelle O is recessive.

Firstly, it is to be noted that the offspring obtains one allelle from the mother and the other one from the father.For example, if a child receives allelle A from the mother, and allelle B from the father, his blood type will be ‘AB’, because both allelles are dominant – codominance.

If he receives the allelle ‘O’ from the mother and ‘A’ from the father, his blood type will be ‘A’, because ‘A’ is dominant over ‘O’.

For a child to have the O blood type, he would have to inherit the allelle O from both parents.


Blood type is determined by another factor: the Rhesus factor. This is an antigen found on the surface of red blood cells. If people have the Rhesus factor, they have a positive blood type, and, if they don’t have the Rhesus factor, they are said to have a negative blood type.

For blood transfusions, knowing the blood type required is primordial because people with certain blood types reject blood which is of another type. However, O negative is a universal donor, as it has nothing that would potentially be foreign to someone else’s blood. Those with type A or B positive cannot receive receive blood of the negative type, else their body would initiate an immune response and attack the cells from the blood being transfused because they recognize the latter as foreign bodies which have to be targeted and eliminated.

An ABO blood test is done to determine a person’s blood type.

The following shows which blood type can be transfused to someone of a specific blood type.

Blood Type Compatibility

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