Blood Type Genetics

Instructor: Amanda Robb

Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

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. Updated: 07/21/2020

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 that determines which kind of blood cells we produce. 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 (-). These terms refer to the presence of another protein called the Rh factor. Positive blood types produce Rh protein and negative blood types do not. Thus, there are actually eight different blood types: A+, A-, B+, B-, AB+, AB-, O+ and O-.

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. Type A and B are codominant, meaning that neither is dominant over the other. Thus, a person who inherits an A allele and a B allele will have type AB blood and make both A and B blood cells.

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 for human blood types
The genotype and phenotype chart for human blood types

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, a genetic tool that helps us predict what genes the offspring will inherit. 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 for human blood types
An example of a Punnett square for human blood types

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, the receiver and the donor have compatible blood types.

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