There are several interpretations to this theory.
Here are the general ideas of each blood type. The Rh factor plays no role in the blood type/personality idea:
Type O:
Type Os are outgoing, and very social. They are initiators, although they don't always finish what they start. Creative and popular, they love to be the center of attention and appear very self confident.
Type A:
While outwardly calm, they have such high standards (perfectionists) that they tend to be balls of nerves on the inside. Type As are the most artistic of the blood groups. They can be shy, are conscientious, trustworthy, and sensitive.
Type B:
Goal oriented and strong minded, type B's will start a task and continue it until completed, and completed well. Type Bs are the individualists of the blood group categories and find their own way in life.
Type AB:
Type ABs are the split personalities of the blood groups. They can be both outgoing and shy, confident and timid. While responsible, too much responsibility will cause a problem. They are trustworthy and like to help others.
Source: http://www.bellaonline.com/articles/art22988.asp
Going by this chart, it looks like Dev has O blood group. NT also appears to be O type. Both start tasks that they don't finish, both are charming and love to be the centre of attention.
Geet looks like she is B type. She is goal oriented, strong minded and she did find her own way of life.
Maan is definitely AB type going by this chart. He is both outgoing (when it comes to his business) but introverted (when it comes to his personal life). He assumes responsilibility and is dependable. We all know how many times he has helped Geet.
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