Before I respond to your inquiry, I must state that we have not spoken, I have not reviewed the relevant documents and facts, and I do not represent you. Therefore, my discussion below is not a legal opinion, but is informational only. Finally, my discussion applies only to issues to which Pennsylvania, New Jersey or Federal law apply, unless otherwise specified.
That being said, unless one subordinate is being designated to relay instructions due to race, gender, age, disability, religion or other similar issues, then there is no law prohibiting a supervisor from choosing the same subordinate each time. What you are asking does not appear to be a legal question, but a management question - you may be treated unfairly under the law, but it doesn't mean that it is 'right' or that it is good business. Unless you are being treated differently because of your race, gender, age, etc., if you are unhappy with the way you are being treated, you have to raise it with your employer, or find a new job.
If you would like to discuss this matter further, please feel free to contact me at the below address(es) or telephone number.
/Christopher E. Ezold/
The Ezold Law Firm, P.C.
One Belmont Avenue,
Bala Cynwyd, PA 19004
Please note that I am not admitted to the practice of law in Pennsylvania, so my comments are general and not considered legal advice.
Generally speaking, employers can run their business as they choose, making up stupid rules, relying on arrogant, unfair, abusive managers to run things, and making poor business decisions. The law does not say that employers need to be wise, fair or decent to their employees.
Antidiscrimination and other laws that protect employees only come into play when employers mistreat/fire/lay off employees who are in a protected class due to race, sex, age, disability, etc. Whistleblowers, those seeking time off for their own or a family member's illness and employees acting in the best interest of public policy may, under some circumstances, enjoy some legal protection. Employees being retaliated against for complaining about discrimination or pay issues or worker's compensation may enjoy some legal protection. Unless you believe your boss is acting toward you the way that he is because of one of these types of issues, while his/her behavior may be unproductive, hostile and even stupid, it is likely not illegal. If however, you believe you are being treated this way because of your age, gender, race, etc., you should speak to an employment lawyer in Pa. who handles these types of issues.
I agree with the over two answers, particular Mr. Ezold's. There are limited number of bases upon which you can claim discrimination. Unless one of those is the reason your colleague is being chosen over you, there is no claim.