Hi After maximising windows on my macbook air with yosemite os my minimise buttonn get disabled.
That's because the Green button is now the Full Screen button, by default, not the maximize button.
The green dot works differently in Yosemite. If you don't hold the Option key down when you press the Green dot, it goes to Full Screen mode by default:
https://support.apple.com/kb/PH18785?viewlocale=en_US&locale=en_US
http://www.macobserver.com/tmo/article/os-x-yosemite-what-does-that-little-green -button-do-now
In the future, you want to make sure that you post your question in the proper area. You posted to the Mac Pro area. The proper area would have been the Yosemite area.