Blood Types

Biology Tutorial

Blood Types


Blood types are one of the most interesting and applicable examples of genetic inheritance. However, they can be a bit tricky to wrap your head around. This problem challenges you to apply a basic understanding of blood type genetics to a real-life problem (which is something that teachers love to do on tests!)

Sample Problem

Sally, who is a regular blood donor, knows that she has type A+ blood. Her husband, Tim, does not know his blood type. Sally and Tim have three children, who have blood types of O+, O-, and A+. What is Tim’s genotype?


The first thing to recognize in this problem is that blood type involves TWO different “pieces”, which are inherited in different ways.

The first “piece” of blood type is the LETTER. A person can be A, B, AB, or O. These blood types are generated by three different alleles: A, B, and O, which are usually denoted by IA, IB, and i respectively. Here’s the tricky part: IA and IB are co-dominant to each other, but both of them are dominant to i, which is recessive. Let’s break down what that means in terms of blood types.
Someone that is type A can have the alleles IA and IA, or IA and i.
Someone that is type B can have the alleles IB and IB, or IB and i.
Someone that is type AB will always have the alleles IA and IB.
Someone that is type O will always have the alleles i and i.

The second “piece” of blood type is the Rh Factor. A person can be Rh-positive (usually denoted “+”) or Rh-negative (usually denoted “-“). These types are generated by only two alleles, Rh+ and Rh-, which follow a classical pattern of inheritance where Rh+ is dominant and Rh- is recessive.

This means that in any blood type problem that specifies both a letter and the Rh factor, we need to consider the two separately.

Let’s start with the letter “piece.” Sally is A- and her kids are O+, O-, and B+. If we translate that into “allele” terms (remember, we’re only looking at letters!)
Sally is IA and unknown.
Her first kid is i and i.
Her second kid is i and i.
Her third kid is IB and unknown.
That means Sally’s husband Tim MUST have an IB allele, or it would be impossible for the third kid to be type B (note: even though we don’t know one of Sally’s alleles, it can NOT be IB or she would be type AB, not A!). What about his second allele? Because two of the kids are type O and carry two i alleles, they MUST have got one from each parent, so Tim MUST have one.
That means Tim’s alleles are IB and i.

We’re halfway done! Now for the Rh factor “piece” of blood type, which is a bit simpler. Sally and Tim’s children are a mix of both positive and negative. We know that Sally is negative, which means her alleles must be Rh- and Rh- (since Rh- is recessive). Therefore, for any of the kids to turn out positive, Tim MUST carry an Rh+ allele. What about his second allele? Because one of the kids is negative, Tim MUST also carry an Rh- allele, or it wouldn’t be possible for him to produce a child with two Rh- alleles. That mean’s Tim’s alleles are Rh+ and Rh-.

Overall, the solution to this problem is that Tim’s genotype is IB/i, Rh+/Rh-.

Any questions? Comment! 🙂

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