Blood Type Genetics

Instructor: Patricia Noel
In this lesson you'll gain an overview of blood type genetics. Find out how you inherit your blood type and explore what the different blood types mean.

How Is Blood Type Inherited?

Do you know your blood type? How about the blood type of your parents? Blood type is an inherited trait. Therefore, the blood type of your parents directly impacts what your blood type is.

When we inherit genes we inherit one gene from our mother and one from our father. Therefore you have two alleles (alternate forms of a gene) for blood type. You inherited one allele from your mother and one from your father. The possible alleles that you could inherit are A, B, or O. Additionally, if you know your blood type, you may also know that your blood type can be positive (+) or negative (-), though we'll save discussion about this for another lesson.

Now, before we can understand blood type inheritance we must remember that some genes are dominant and some are recessive. Dominant alleles will mask recessive alleles. In the case of blood type, A and B are both dominant over O. Therefore if you inherit one A or B allele and an O allele, your blood type will be the dominant type (A or B). This also means that the only way for someone to have type O blood, is if they have two O genes.

Two more important terms when discussing inherited traits are phenotype and genotype. A genotype refers to the gene a person inherits, while the phenotype refers to how the traits appear physically. It is possible for a person to have a recessive allele in their genotype but never show that trait in their phenotype. The table below lists all of the possible genotypes and resulting phenotypes for blood types in humans.

Genotype and Phenotype Chart
geno phenotype chart

Predicting Blood Type

If you know the blood type of two individuals you can easily predict the possible blood types of their children using a Punnett Square. The example below shows a cross between an individual with type A blood (notice this individual carries the recessive O allele) and an individual with type B blood (who also carries the recessive allele).

Remember a Punnett Square shows us all of the possible genotypes of hypothetical children of two individuals. The parents' genotypes are listed on the side and top of the chart and the four genotypes inside the box represent the possible genotypes of the offspring.

Punnett Square Example
example 1

You can see from the resulting Punnett Square that these two individuals can have children with any of the four possible blood phenotypes.

Why Does Blood Type Matter?

The genes for blood type that you inherit actually code for antigens that will be present on your red blood cells. An antigen is a substance that can trigger an immune response if it is foreign to that individual. The 'A' gene for blood type codes for type A antigen, the 'B' gene codes for B antigens and the O gene codes for no antigens.

Our immune system is trained to recognize foreign substances in our body. Unfamiliar antigens can cause our body to have a reaction in which it tries to destroy these new and unfamiliar cells. This is why it is important that when a person receives blood, that the receiver and the donor have compatible blood types.

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