Solving ABO Blood Type inheritance problems (interactive tutorial)

1. Blood type: An Introductory Slideshow

Start by viewing this slideshow.

Note that if you can’t view the slideshow below, don’t worry: it’s all covered below. It’s something in your District’s network settings that’s blocking your view (and as much as I’d like to share this with you, I can’t. Sorry!).

2. Reading: Blood Type and blood transfusions

The ABO blood type system is based on glycoproteins (proteins attached to polysaccharides) found on the surface of red blood cells.

• If your red blood cells have the A glycoprotein, your blood type is type A.
• If you have the B glycoprotein, you’re type B.
• If you have both glycoproteins, you’re type AB.
• If you don’t produce either glycoprotein, you’re type O.

The presence or absence of these membrane glycoproteins is of no selective advantage or disadvantage –unless you’re getting a blood transfusion. Here’s how it works.

Our immune system can recognize molecules and decide if those molecules belong to us or if they indicate the presence of an infection. Molecules perceived as foreign are known as antigens. In response to an antigen, your immune system will counterattack with proteins called antibodies. One of the effects of antibodies is to create big clumps of invading cells. These clumps are later swallowed up and disposed of by another part of the immune system – white blood cells.

In the case of the ABO blood type system, this makes some blood transfusions helpful, but others deadly. If you’re type O and receive blood with the A or B glycoprotein, your immune system will perceive these cells as invaders. The subsequent clumping (called agglutination) causes shock and death.

People with blood type AB have both glycoproteins on their blood cells. As a result, any type of transfused blood (A, B, AB, or O) can be received without a reaction.

Having read that, see if you can complete the chart and reading below.

3. The Genetics of ABO Blood Type

The ABO system is under the control of a gene found on chromosome 9. Three alleles can be found on the locus for the blood type gene.

• The A allele codes for the A glycoprotein.
• The B allele codes for the B glycoprotein.
• The o allele codes for nothing. (Note that I wrote “o” in lowercase on purpose, as we’ll see below).

Three alleles? You might be thinking that we only have two parents. How can you inherit three alleles?”

The answer is that you can’t. As with any system of alleles, in the ABO system, any individual can only have two alleles. But in the human population, three alleles exist. Any individual is only going to possess two of those alleles. But other people may have the third allele.

This type of genetic situation is called multiple alleles. And while, based on the genetics that you’ve learned so far, multiple alleles might seem weird or exceptional, it’s common (and might be the norm). However, in most introductory biology courses, the ABO system is the only multiple allele scenario that you’ll learn about.

In addition to multiple alleles, the ABO system has one more feature that might be new to you. In a typical Mendelian trait, alleles are either dominant or recessive. In a heterozygote, one allele shows up in the phenotype and one is completely masked. But in the ABO system, the A allele and the B allele are co-dominant. That means that both alleles get expressed in the phenotype. That’s what happens if you inherit an A allele and a B allele. You express both genes, and, as a result, your red blood cells will have both the A glycoprotein and the B glycoprotein.

At the same time, both the A allele and the B allele are dominant to the o allele, which is recessive. For example, in a heterozygote who inherits both an A and an o allele, the blood type will be Type A–just as much type A as in an individual who is homozygous A (AA).

Having read what’s above, let’s see if you can complete the chart below:

4. Extending Your Learning

There’s a lot more to learn about blood type. For example, none of what you read about above deals with the Rh system, which is connected to blood types like A⁺, B-, O⁺, etc. To learn more about Blood type and its inheritance, follow these links (or other ones that you find on your own: let me know if you find something good)

• American Red Cross on blood type
• Wikipedia article on blood types

Record what you’ve learned in your student learning guide.

5. Solving Blood Type Problems

You solve genetics problems involving blood type in pretty much the same way as you solve any other type of problem. The same six steps I advocated in the last tutorial apply here (click on the link if you need to review).

Link to

• Solving Genetics Problems involving sex-linked alleles
• Genetics Menu