Some people say the existence of mixed-race people proves that race is not real. Actually, it proves the opposite: Race is real. After all, there has to be white people and black people in order to create mulattoes. Without the existence of real whites and real blacks, you cannot have mulattoes. You can't make a mulatto by having blacks breed with blacks or by having whites breed with whites. For there to be the mulatto, there has to be white people and black people. Also, only mixing whites and blacks will produce mulattoes. You can't create a mulatto by mixing whites and Asians. Likewise, you can only create a Eurasian by mixing white and Asian. You can't get a Eurasian by mixing black and Arab.
You can mix orange juice and apple juice to produce an orange-apple blend.
So, does the existence of the mixture prove that orange juice and apple juice are not real? Does it prove that orange juice and apple juice are mere social constructs? No, the mixture is possible only if apple juice and orange juice really exist in the first place. The mixture can only result from the blending of distinct juices.
Just like the blend of apple juice and orange juice isn't possible unless apple juice and orange juice really exist, mixed-race people cannot coming into being without the existence of distinct and separate races.