If race was biologically "fixed," either it would not be possible for different "races" to mate, or else the children of parents from different "races" would inherit only one "race" or the other. But "race" is not a fixed biological feature, so intermixing produces people with a mix of features from both parents.
I had a chance to observe this in my own family. My mother's parents were mmigrants from northern Germany. Her relatives had blond, red, and light brown hair. Most had blue eyes, but there were a few with brown eyes, like my mother who was a brown-eyed blond.
My father's mother was Seneca, Mohawk, and English. She had straight black hair, brownish black eyes, and creamy olive skin. My father's father was Algonquian and German Swiss. He had black hair, blue eyes, and bronze skin. They had 9 children. 3 of them (including my father) had blue eyes. Two of the blue-eyed ones (including my father) had black hair. The other one had pale brown hair,and very European facial features. She was the "white sheep" in the family.
My father's other 6 siblings had straight black hair, brownish black eyes, and skin tones from dark beige to light brown. They had the cheekbones and nose features associated with Native Americans. One of them looked like she had no European amcestry at all.
When both sides of my family got together for weddings, graduations, or funerals, the color variety was impressive.
So, in my generation, my sister was a brown-eyed blond like my mother, but she had golden beige skin, not my mother's pale pink beige. Her face shape and features were definitely from my father's Native side. One of my brothers had very dark brown hair and "pure" brown eyes (not hazel). He looked a lot like a distant Seneca cousin from the rez, but with lighter skin. The other brother had dark blond hair and blue eyes. Before it turned gray, my hair was between medium and dark brown. I am often mistaken for Italian American or French Canadian. To most people, I look like a White person of European descent but occasionally people recognize a mix of Native and European American.
Those variations of physical features within my family would not be possible if "race" was a fixed biological feature.
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