More Complicated Inheritance Patterns
So, yesterday's gene had two alleles. What happens if you have *more* than two alleles? For this, we're going to look at human blood groups, which are controlled by one gene with three alleles, which you'll be familiar with: A, B and O. Now, O is recessive, just like short pea plants, but A and B are what's termed "co-dominant". That is, each one can be expressed to produce its own phenotype, regardless of whether the other is present or not.
This is how we get the four blood groups. Remember that you have two alleles out of the three available ones, one inherited from your mother, the other from your father. They can be the same or different. Because A and B are codominant, but both dominant over O, if you have AO or AA, you will be group A, and if you have BO or BB, you will be group B. People in group AB must have AB, ie one A allele and one B allele, and as the O allele is recessive, to be group O you must be OO.
I am going to digress here to tell you why that has ramifications for blood transfusions here. Basically, A and B are both proteins known as antigens that sit on the surface of red blood cells. An antigen is a protein that is recognised by the immune system of another organism as foreign, which will result in the cell's destruction. O, however, is an allele that does *not* code for a functional antigen. Because your body doesn't want to recognise its own blood as foreign, if you have the A allele, your immune system does not produce anti-A antibodies, and exactly the same is true of people with B and anti-B antibodies.
So, if you have AB blood, you have no blood-cell-eating antibodies and can receive blood from any blood group. A and B can receive blood both from their own group and from O, and group O can only receive blood from other group O people. Note that this means group O blood can be given to anybody, which makes it very useful for transfusing into people with acute blood loss if there isn't time to test for their blood group.
But what about the rhesus factor, I hear you ask? Well, that's a separate gene, with two alleles, positive being dominant over negative. Rh+ people can receive Rh- blood, but the reverse is not true. So yes, more properly, it is O- blood that can be transfused into anyone. However, as a single gene with two alleles, it's not as interesting as the ABO system, so I mostly ignored it :p
Tomorrow: Examples of inheritance controlled by more than one gene!