Would it be too cruel if I had a retarded kid put up for adoption?

As it stands right now I am the only one in my family that can continue not only my family line but the legacy of surname as my surname is extremely rare. With the duty bestowed to me to continue my family I hope only to have healthy sons but if I have one daughter or so I think that would be alright as well as I plan to hopefully have 4 children. Hypothetically, if i was to have a mentally or physical handicapped child that not only would not be able to reproduce successfully but would bring down the integrity of my family I believe I will want to put him up for adoption.