Re: If Bloodtype O is recessive, why is it so prominent in the population?

Your question brings up a common misconception in the field of genetics: 
that dominant traits are the most common ones.  That is not necessarily 
true.  The gene for Huntington's disease is dominant, yet most people do 
not have it.  The gene for 6-fingered dwarfism is dominant, yet most people 
are clearly not 6-fingered dwarfs.  How common a particular trait is within 
the population depends not only on whether it's dominant or recessive, but 
what the frequency of the gene is within the gene pool.

As your students probably know, ABO blood type is determined by a single 
gene.  Each person has 2 copies, and there are three different alleles:  A, 
B and O (usually represented by i)  A and B are codominant to each other, 
and both are dominant to i.  So:

Type A can be AA or Ai
Type B can be BB or Bi
Type AB is AB
and type O is ii.

However, these alleles do not appear with equal frequency in the gene pool.  
i is much more common than A, which is more common than B.  It should be 
easy to see then, why more people have type A blood than type B.  However, 
there are so many more i's out there in the gene pool that the chances of 
getting ii are higher than Ai.  So O is the most common blood type.

To illustrate this, imagine a bag of marbles.  Let black marbles be the 
most common allele.  In the example of 6-fingered dwarfism, black would be 
the gene for 5 fingers and normal height.  6-fingered dwarfism is a very 
rare allele to find, so throw in a couple of red marbles to represent it.  
Then draw marbles out, two by two, replacing them in the bag each time.  If 
you have only a few red and hundreds of black, most of the time you will 
wind up with 2 black, which is a normal phenotype.  It only takes 1 red one 
to give a 6-fingered dwarf, but that is going to happen only rarely, 
because the reds are overwhelmingly outnumbered by blacks.

The illustration could be expanded to include blood type by making the 
black marbles i, red marbles A and green marbles B.  Your bag will have 
mostly blacks, some reds, and even fewer greens.  Reds and greens are not 
as rare here as they were in the previous example, but they are still less 
common than blacks, so that the odds of pulling out 2 blacks will be higher 
than pulling out a black and a red, or a black and a green, or two reds, or 
two greens, or a red and a green.  So

Type O > Type A (AA + Ai) > Type B (BB + Bi) > AB.

(Note that, in this case, BB (2 greens) would actually be the rarest thing 
to get.  Being homozygous for B is actually rarer than type AB blood, but 
since type B blood includes all Bi's and BB's, type B blood is more common 
than AB.)

If you know the frequencies of alleles in the population, you can calculate 
the frequency of the various geneotypes and phenotypes.  Perhaps an 
interesting problem for your students would be:  What does the frequency of 
a recessive allele have to be for the recessive phenotype to be more common 
than the dominant one?

Thank you for your question.

Louise Freeman