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A blood group comparison is not enough to indicate paternity or maternity because your biological child may not have the same blood group as yours. Let’s see why.

The most used blood group system is the ABO system, which divides people into 4 blood groups; A, B, AB and O. These blood groups are defined by antigens that are attached on your red blood cells. Imagine your red blood cell is a strawberry, antigens are the little yellow strawberry seeds that are attached all over the outer surface of this fruit. People with blood group A have A-antigen, group B have B-antigen, group AB have mixture between A-antigen and B-antigen, while group O have no antigens at all.

The ABO histo-blood group gene on the 9th chromosome dictates what antigens a person has.  This gene can be written as “i” and it has 3 forms (or alleles). The first 2 are “iA” and “iB”, which are codominant alleles and cause the appearance of A-antigen and B-antigen. The last form is “iO”, which is recessive allele and cannot produce antigens. When dominant and recessive alleles are paired, the dominant allele will always suppress the expression of recessive allele. This phenomenon is called Co-Dominance.

As all chromosomes occur in pairs, the 9th chromosome has 2 chromosomes bound together and both of them have an ABO histo-blood group gene each. The combination of 2 genes on each chromosome cause difference in blood group appearance:
1. People with blood group A may have iA & iA (homozygous – same alleles), or iA & i (heterozygous – different alleles).
2. People with blood group B may have iB & iB (homozygous – same alleles), or iB & i (heterozygous – different alleles).
3. People with blood group AB have iA & iB.
4. People with blood group O have iO & iO.

The following image shows the possibilities of combinations of alleles for each blood group.
“Genotype” is the set of genes (or alleles of a gene) responsible for a specific trait.
“Phenotype” is the observable or physical characteristics used to describe that trait; in this case, the blood group. 

Child always inherit 1 chromosome from Father and another one from Mother. The inheritance of ABO histo-blood group gene appears randomly and how a child’s blood group will look like depends on the blood group of the parents.

For example, if one parent has blood group B (homozygous) and another parent has blood group A (heterozygous). The chance for a child to have blood group B is 50% and to have blood group AB is also 50%. There is no chance the child will have blood group A or O. See the image below:

The table below summarizes the various blood groups by genotype that children may inherit from their parents.

The table below is a simplified version of the table above showing only the phenotype (Blood type).

If in doubt after checking these blood tables, and you would like to do a paternity test, come speak to us at PTC Laboratories Thailand. We guarantee a minimum accuracy of paternity of 99.99% to help you answer one of the most important questions of your life!

https://www.slideshare.net/nirmalajosephine1/biology-form-5-chapter-5-51-inheritance (See slide 63-74 )