And so if we carry out the Punnett square, these are the nine different types of possibilities. So in this particular case, both of those chromosomes contains the alleles that code for protein A. In these two cases, one of the chromosomes contains that allele that doesn't code for anything, but the other one contains the allele that codes for protein A. And so this case, this case and this case will always produce only protein A on the membrane of red blood cells. And this is known as blood group A. Now, let's take a look at these two cases.