Can anyone tell me the equation for the following series of numbers?

1,2,3,5,8,13,21,34,55,89,144,233 etc.

Fibonacci number - Wikipedia, the free encyclopedia

Thanks

I created this sequence when working out where the number of different origins of an X chromosome on each generation of a woman's family tree. I assumed there was some mathematical principle but could not think of one. Considering I did not know of Fibonacci numbers, if I had been born 850 years earlier it could have been named after me!

i.e. 1/1 (her self), 2/2 (her parents), 3/4 (her paternal grandmother and her mothers parents), 5/8, 8/16, 13/32, 21/64, 34/128, 55/256 etc.