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ABO Blood Groups

Siya

4/23/2022

what is it?

The ABO blood group system, is the classification of human blood based on the inherited properties of red blood cells (determined by the presence or absence of the antigens A and B, which are carried on the surface).

Because of this, an individual may have type A, type B, type O, or type AB blood.

what

significance

The ABO blood group system in humans is an example of co-dominance. It is of great medical importance: before blood is transfused, it is vital to find out the blood group of a patient and ensure that it is matched. Unless this is done, there may be complications due to coagulation of red blood cells.

significance

genotypes

genotypes

When representing blood group alleles, the letter i is used to represent the different antigenic forms (isoantigens)

A allele = I^A ; B allele = I^B ; O allele = i (recessive)

The genotypes for the different blood groups can be summarized as follows:

table

Neither I^A

nor I^B

is dominant over the other

allele (resulting in blood type AB, and co-dominance).

inheritance

The reasons for two

alleles being co-dominant and the other allele

being recessive:

--> All three alleles produce a glycoprotein in the membrane of red blood

cells.

--> I^A alters the glycoprotein by addition of acetyl-

galactosamine. This altered glycoprotein is

absent from people who do not have the allele I^A

so if exposed to it they make anti-A antibodies.

--> I^B alters the glycoprotein by addition of

galactose. This altered glycoprotein is not

present in people who do not have the allele I^B

so if exposed to it they make anti-A antibodies.

inheritance

inheritance continued...

The genotype I^A I^B causes the glycoprotein to be altered by addition of acetyl-galactosamine and galactose. As a consequence neither anti-A nor anti-B antibodies are produced. This genotype therefore gives a different phenotype to I^A I^A and I^B I^B so the alleles I^A & I^B are co-dominant.

continued...

The allele i is recessive because it causes production of the basic glycoprotein: if either of the I^A or I^B alleles is also present the glycoprotein is altered by addition of acetyl-galactosamine or galactose. I^A I^A and I^A i therefore give the same phenotype (and so does type B).

overview

summary

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