ABO system: chart, blood types, exercises

O ABO system classifies blood groups into types A, B, AB and O. This system was described in 1900 by Karl Landsteiner, who initially described groups A, B, and O. The AB phenotype was described just a few years later by Alfred Von Decastello.

Read too: Blood

→ Genetics of the ABO System

ABO blood groups are determined by three alleles different from a single gene:I^THE, I^B Hey. These three alleles are responsible for ensuring the presence of four phenotypes: blood A, blood B, blood AB and blood O.

These four groups are characterized by the presence or absence of agglutinogens in your Red Cells and agglutinins at the blood plasma (read more below). the I allele^THE is responsible for ensuring that the blood has agglutinogen A, while the I allele^B is responsible for agglutinogen B. The i allele is not responsible for producing agglutinogen.

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The ABO system is a classic example of multiple alleles (or polyallelia) it's from codominance. It is a case of multiple alleles, as it has three different alleles of a single gene (I

^THE, I^B Hey). Codominance, in turn, occurs because between the I alleles^THE Hey^B there is no dominance relationship.

That's why we have individuals I^THEI^B with AB phenotype, that is, with the production of two types of agglutinogen, A and B. It is worth noting that, although there is co-dominance between I^THE Hey^B, there is dominance in relation to the i allele.

So we have to^THE Hey^B exert dominance over i, but among the I alleles^THE Hey^B there is codominance. With that we have to genotypes of the blood types are:

Blood types (phenotype)	Blood Type Genotype
THE	I^THEI^THE or I^THEi
B	I^BI^B or I^Bi
AB	I^THEI^B
O	ii

• Example of crossing involving the ABO system

Julio and Aline are married and have, respectively, O blood and AB blood. What is the probability of the couple having a child with type AB blood?

Resolution: Julius has type O blood, so his genotype is ii. Aline has AB blood, so her genotype is I^THEI^B. Note below the Punnett's painting (sort of table in which it is possible to separate the possible gametes and discover the genotypes of the descendants) showing this cross:

i	i
I^THE	I^THEi	I^THEi
I^B	I^Bi	I^Bi

After performing the crossing, we can see that there is no possibility for this couple to have children with blood type AB, only blood type A is possible (I^THEi) and B (I^Bi).

See too:Alkalosis and acidosis - decrease or increase in CO₂ in the blood

→ Agglutinogens and agglutinins in the ABO system

In the ABO system four distinct blood types are observed: A, B, AB and O. They are characterized by the presence or absence of agglutinogens and agglutinins in plasma. You agglutinogens are substances found in plasma membrane of the red blood cells, while the agglutinins they are antibodies present in blood plasma that act against certain agglutinogens.

They exist two types of agglutinogens, A and B. Agglutinogen A is present in blood A, while agglutinogen B is in blood type B. People with AB blood have both agglutinogens: A and B. Blood type O people do not have agglutinogens in their red blood cells.

In the figure we can see the agglutinogens and agglutinins present in different blood types.

As well as agglutinogens, there are two types of agglutinin: anti-A and anti-B. The person with blood type A has anti-B agglutinin, while the person with blood type B has anti-A agglutinin. People with type AB blood do not have agglutinin in their plasma, unlike people with type O blood, which is characterized by the presence of anti-A and anti-B agglutinins.

→ who donates to whom

Considering only the ABO system, we can make some important statements regarding the blood transfusion:

• Type O blood individuals they can donate to anyone, as they do not have agglutinogens in red blood cells. For this reason, they are known as universal donors. These individuals, however, can only receive type O blood, as they have anti-A and anti-B agglutinins.

• Individuals with AB blood they can receive blood of any type, as they do not have agglutinins in their plasma. For this reason, they are called universal receivers. These individuals, however, can only donate to individuals with AB blood, as they have A and B agglutinogens in their red blood cells.

• You blood A individuals they cannot donate blood to B blood individuals, as the red blood cells would be attacked by anti-A agglutinins. They can donate to people with A blood and people with AB blood. People with A blood can receive donations from people with A blood or O blood.

• Blood B individuals they cannot donate to A blood cells, as their red blood cells would be attacked by anti-B agglutinins. They can therefore donate to people with B blood and people with AB blood. People with B blood can receive donations from people with the same blood or O blood.

Read too:Blood donation

See below a scheme that illustrates what was exposed here:

→ ABO System Table

ABO system
type A blood	Type B blood	type AB blood	blood type O
Genotype	I^THEI^THE or I^THEi	I^BI^B or I^Bi	I^THEI^B	ii
agglutinogen	THE	B	A and B	does not have
agglutinin	Anti-B	anti-A	does not have	Anti-A and Anti-B

→ Exercise solved

(Mackenzie) Regarding blood groups, it is correct to state that: a) An individual belonging to type O does not have agglutinins. b) An individual with type B agglutinin cannot be the offspring of a type O parent. c) Individuals belonging to type AB cannot have children who belong to type O. d) A Type A man married to a Type B woman cannot have Type AB children. e) The absence of agglutinogens is characteristic of individuals belonging to type AB.

Reply: The correct alternative is letter c, as individuals of AB blood have genotype I^THEI^B and a person of blood O has genotype ii. To have a ii child, the presence of the i allele in both the father and mother is essential.

Also access:what are platelets?

By Ma. Vanessa Sardinha dos Santos

Would you like to reference this text in a school or academic work? Look:

SANTOS, Vanessa Sardinha dos. "ABO System"; Brazil School. Available in: https://brasilescola.uol.com.br/biologia/sistema-abo.htm. Accessed on June 28, 2021.