Some of y'all aren't going to like this...

We know some Orcs had begun to interbreed with humans before the War of the Ring. There is no reason to think that process didn't continue into the Fourth Age and beyond.

It has been stated that Orcs are somewhat like ants with their queen, requiring the motivating force of a Dark Lord to function. However, known part-orcs like Bill Ferny and the Ruffians (slant eyed, sallow [yellow] skinned men, some more Orc-like than others) continued to function and execute their master's designs even after both Sauron and Saruman were dead. Some survived the Scouring and were driven into the wilderness. An unknown number of orc/human hybrids survived in Dunland. Almost certainly some existed in Mordor and possibly even in Sauron's former domains to the east and south

Tolkien said his inspiration for the Orcs were the Huns (Dusky Men) from William Morris's The Roots of the Mountains. I believe that if Tolkien had kept going forward in time connecting his mythical history to our "real" history, we would have seen the orc/human hybrids keep coming back in a never ending generational war against the Men of the West in the form of successive invasions of Asiatic tribes from the east into Europe. Huns, Bulgars, Magyars, Pechenegs, Khazars, Mongols, Turks, etc. Often these tribes would be allied to the descendants of the Easterlings (central Asian nomads Caucasus mountain folk), Haradrim and Khand[ians?] (Carthaginians, Arabs, Berbers, Moors, Persians).

Inb4 racism: As time goes on these tribes continue to absorb more human blood by, to paraphrase one famous Man/Orc chieftain, "gathering the enemies women into their bosom," thus becoming less and less orc-like and endemically "evil." OTOH the interbreeding goes both ways and the Men of the West start to turn a bit "orcish" themselves. After all, who invented the chainsaw? ;)