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
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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.
For example, if individuals are born with type A blood, their immune systems will not attack blood cells with type A antigens, but any blood with type B antigens could cause an immune reaction. These individuals can safely receive type A blood and type O blood. Similarly, a person with type B blood can safely receive type B blood and type O blood. This is because type O blood does not have either A or B antigens and therefore will not cause an immune reaction.
In addition to AB blood, individuals with type AB can receive type A, type B, or type O blood because their bodies are familiar with both A and B antigens. The immune system of a person with type O is not familiar with either A or B proteins and can only receive type O blood. In addition, Rh-negative blood is typically given to those with Rh-negative blood types, but those with Rh-positive blood types can receive either Rh-positive or Rh-negative blood types. Thus, type O- blood is considered the universal donor and can give blood to any other blood types.
Lesson Summary
We inherit our genes for blood type, the kind of blood cells we produce, from our parents. We each receive one gene for blood type from our mother and one from our father. The possible versions or alleles of this gene are called A, B, and O. A and B are both dominant alleles and therefore will mask an O allele. For example, if a person inherits an A gene along with an O, then the A will mask the O, and the person's blood type will be type A. These genes all code for antigens on red blood cells. Antigens are substances in the body that can trigger immune responses.
An individual with type A blood will contain A antigens, type B will contain B antigens, type AB will contain both A and B antigens, and type O blood has no antigens. It is important to know your blood type because the antigens present on your blood cells determine which blood types would be compatible to your own. A genetic tool called a Punnett square can be used to predict which genes the offspring will inherit, including those that govern blood type.
