There is no bible backing for this idea and it is interesting to note how Splane tries to overcome this problem. In his 2015 JW Broadcasting monthly broadcast he says
Now if you were asked by someone to identify a scripture that tells us what a generation is, what scripture would you turn to? I'll give you a moment to think about that. My choice is Exodus Chapter 1 verse 6 "Joseph eventually dies and also all his brothers and all that generation"
Note that he takes it for granted that the listener accepts this new concept but then invites them to try and find a scripture themselves that backs it up. Basically what's happening here is that the concept must be accepted and a search in the scriptures is required to support it. This itself is a backwards way of thinking when you profess to use the bible as a basis of your teachings, letting the bible interpret itself. Rather what they have done is come up with the idea and manipulated what the bible is saying to fit that teaching.
Then he tells us that 'his choice' is Exodus 1:6 and proceeds to try to make it fit to the idea. He's a GB member that supposedly collaborated with the other members to come up with this interpretation (or at the very least, supported the majority vote in accepting it) and yet when it comes to supporting it with scripture, he uses a text which is his own choice.
The bottom line is, the r&f must accept the idea but they are allowed to come up with or twist any scripture of their choice to make it fit. Brilliant, especially when there is no such scripture.
