Once again, a true-life puzzle

I posted a question a while back asking if identical twins always have the same blood type. I asked that question because I read in a trivia magazine that they do not always have the same type. The explanation was that enviornmental factors influence blood type along with genetics. It seems that the person who wrote that fact was pulling my leg. Now I have another question about a trivia 'fact' I read in the same magazine:

The magazine said that aardvarks only give birth to identical quadruplets. Is this true?

Yes, it is indeed an amazing fact. Their egg always divides down natural fault lines to produce four gentically identical offspring. Occasionally a random division occurs before this process, resulting in octuplets.

I think I read this magazine too. The same issue also tells us there's no such word as 'gullible'.

Complete malarkey. Aardvarks generally bear single young.

_Armadillos_, on the other hand, usually bear identical quadruplets:
http://wildlife.tamu.edu/publications/TAEXWildlife/ WILDPUBS/A010.PDF http://www.flex.net/~lonestar/armadillo.htm http://pelotes.jea.com/armad.htm

...and so on.

I have a question (which probably belongs on some biology or statistics 'group):

If type O is recessive (A and B being dominant) then how did O+ get to be the single most common blood type in the 'States?

This just boggles me.

'Dominant' doesn't mean more frequent (in the gene-pool). Nor does 'recessive' mean infrequent. The frequency of the O allele is very high. However, when it encounters an A or a B, its effects are masked - i.e., it's recessive, whereas the A/B allele is dominant.