Earlier, when I upgraded MacBook Pro from Mavericks to Yosemite then I faced many issues. Even WiFi connection was also dropping out continuously. But I got the solution to get rid from this issue.
Go to ‘Keychain access’ area and locate ‘Wireless Network‘. Now, double click on this network. Once you double click on it, two tabs will be shown here (Attributes & Access Control)
Go to ‘Access Control’ tab and ‘tick’ the check box “allow all applications to access this item”
Once you are done with the above mentioned steps, try to access WiFi connection. It will be resolved.