The ABO Blood Group System

Blood Groups

Antigens on the blood cells are used to group the blood type of a person. The antigens are proteins and are called chemical markers. There are two types of antigen; A and B, these two can form a new blood type called AB. In blood type O there is no antigen present. This causes the antigen A or B to be dominant over O. To show these groups on a grid you should use the symbol I^A, I^B, I^O. The genotypes would be shown as such:

Geno-type                                   Blood Group

I^A I^A or I^A I^O                       A

I^B I^B or I^B I^O                       B

I^A I^B                                           AB

I^O I^O                                          O (No antigen)

In math the symbol “^” means powered by so there is no need to put this symbol just write the A, B and O in small size and in the upper right corner position.


Pedigrees are like a tree of blood types in a family and can be used to easily display the blood type of all the family. It is as such:


The circle in a pedigree means female while the square is for male. It is fairly easy to use and if one genotype is missing, as if one can be missing from 3rd generation, you can deduces it by looking at the generation before it.

Determination of Gender in Humans

In a gamete of a human there are 23 chromosomes which is half of the chromosome number in a normal cell. Out of 23 chromosomes 22 are completely normal chromosomes and are called autosomes due to this reason. One chromosome of the gamete is different. It can be XX or XY depending on the gender of that particular person. In a male gamete, the chromosome is XY and in female it is XX. X means female while Y means male. This shows that if fertilization is to take place then there can be a 50% possibility that the baby born is male or female. The grid that is formed from this supports this possibility:

    X           X

X        XX        XX


Y        XY         XY

As the grid shows there is 50% chance that the off spring is a boy or a girl. The chromosomes of the gametes can also be shown in a Keryotype:

It is completely normal if a family has two boys and two girls and is not a slight bit unusual. Some parents might have one girl and two boys.


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