Have you ever wondered why you have five fingers?? We have been using our hands for more than twenty years and you may have the same question as i do which is why we have 5 fingers.
To better understand the reason, let’s look at a special case of “polydactyly“. In short, polydactyly refers to the situation where there are more than five fingers in a hand (or foot). There are two types of polydactyly which refer to Type A and Type B. In Type A, the extra finger can function properly like other fingers and have a complete bone structure. In Type B, the extra finger is non functional and seems to be “floating”.
Type A of polydactyl http://https://en.wikipedia.org/wiki/Polydactyly
Type B of polydactyly http://https://en.wikipedia.org/wiki/Polydactyly#/media/File:Polydactyly_Left_Hand.jpg
To answer our question in the beginning, we have to know the cause of Type A. The duplication of the entire finger is due to an abnormal function of a gene called “Sonic hedgehog” gene or”SHH” gene which plays an important role in the growth of digits on limbs and organization of the brain. During the fifth week of an embryo development, SHH gene will synthesize a signalling protein called Sonic hedgehog protein which act on the ulnar side of the “hand” (at the black dot in the picture) and fingers start to form.
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