In The Gno

Blood type is a special type of inherited trait that is governed by multiple alleles, or gene variants.  Most often referred to as the ABO blood group, these three alleles (A, B, and O) can produce four phenotypes from six genotypes.  Of the three possible alleles, A and B are always dominant to O.  In addition, A and B are codominant; this means if inherited together, both alleles will be expressed.  Like any other inherited trait, only one allele will be donated from each parent.  Possible combinations (genotypes) are AA, AO, BB, BO, AB, and OO.  The phenotypes that result from these are (1) Type A blood for AA and AO genotypes, (2) Type B blood for BB and BO genotypes, (3) Type AB blood for the AB genotype (remember the codominant alleles), and (4) Type O blood for the OO genotype.

Another interesting snippet about blood types is how they are physically expressed.  A blood type is expressed by the presence of certain sugars on the surface of red blood cells.  Individuals with Type A blood have only carbohydrate A on their red blood cell surfaces.  Likewise, red blood cells with carbohydrate B at the surface will dictate Type B blood.  Type AB blood contains both carbohydrates and Type O has neither.

In this manner, blood types are extremely important in situations such as transfusions where foreign blood will enter the body.  Donated blood must only contain blood cells that match the type of carbohydrate present in the recipient’s blood.  If donated blood cells contain a foreign carbohydrate, the recipient will make antibodies against donor cells causing them to clump together, or agglutinate, putting the recipient at risk.  Clumping can eventually cause a blockage that could lead to reduced or nonexistent blood flow and possible death.  The laboratory process of blood typing is performed in much the same way, except a blood sample is first withdrawn from the individual, then the antibodies for type A and B are mixed with the sample.  Blood type is then determined based on clumping reactions.

Most people know what their blood type is, but if you don’t, you can still try to figure it out with a nifty visual tool called a Punnett square.  The Punnett square is named after its inventor, a pioneer in the genetics field, Reginald C. Punnett.  This tool is used to determine possible combinations of alleles; alternatively, it is used to predict the probability of certain genotypes that can result from a genetic cross.  Here’s how it works:

This is a simple Punnett square that shows a monohybrid cross, a cross involving only one trait (in this case, blood type).  The first parent, represented horizontally at the top of the Punnett square, has Type A blood with an AA genotype.  The second parent, represented vertically along the left side of the Punnett square has Type B blood with a BO genotype.  Again, this parent has Type “B” blood because the allele B is always dominant to O (as is A).

Back to our Punnett square…  You can see the combinations from this cross show that there is a 50% chance that the offspring will be Type AB and a 50% chance that the offspring will be Type A (A masks O).  This is also expressed as a 1:1 ratio between AB and AO genotypes.  Any combination of alleles can be depicted by drawing a Punnett square.  If you know your parents’ blood types, this is one method you could use to figure out your own possible blood type.

One unique situation is for crosses resulting in the OO genotype, which is the only allelic combination that yields Type O blood.  The only time a person will end up with Type O blood is if each parent has at least one O allele to donate.  Obviously, if both parents have Type O blood, all of their children will have Type O blood as well.  However, parents with genotypes AO (Type A blood) or BO (Type B blood), can still produce offspring with Type O blood.  See the Punnett square to learn why:

As you can see, this cross predicts a 1:1:1:1 ratio between genotypes.  So, there is a 25% chance of a child with these two parents having Type O blood, even though neither parent actually has Type O blood.

For this same reason, parents can be carriers of a disease without being afflicted.  This is how recessive genetic disorders are passed from parents (with no disease) to their children.  Let me draw you one last Punnett square so it will be easy to visualize what I am trying to explain.  In this example, “r” will represent any number of recessive inherited diseases like Cystic Fibrosis, Sickle Cell Anemia, and Tay Sachs, and “R” will represent the dominant allele that will mask the disease.

Notice again, that both parents (just like in the blood type example) must have at least one copy of the recessive allele.  This means both parents are heterozygous for this trait (See Genotypes and Phenotypes).  If these two people mate, there is a 25% chance that their offspring will get the disease.  This is the reason why genetic counseling and prenatal screening is usually recommended if it is suspected that either.  On the other hand, if only one parent is a carrier (Rr), there is no chance their children will be afflicted.  Draw your own Punnett square and see if you can figure out why.  (Hint: the cross is RR x Rr and remember that in order to express a genetic disease, there must be two copies of the recessive allele.)

I’m Pyrimidiva, and I hope you have enjoyed my topic on blood types as much as I enjoyed writing it. Stay tuned for more posts on genetic diseases and remember, what you “know” is not as important as what you “gno.”