Subject: what makes diffrent things rare?

Hello.  Recently my class has been talking about blood types after talking 
about genes.  I always thought that O- was the rarest blood type and then I 
found out that I am AB.  When I told my teacher she told me that AB is the 
rarest and that red hair is as well.  I am a redhead with AB blood.  Why is 
red hair a rare thing, and what is the order of blood according to rarity?  
(Could you please give percentages of the positives and negitives for each 
type as well as the overall percentage for that type?) What makes these things 
rare?  And why?  Thank you in advance for taking your time to read and/or 
respond to this email.    ~joy~

Re: what makes diffrent things rare?