Final answer:
Race is not considered a scientific biological categorical system due to the lack of accurate ways to divide up human biological variation. Genetic markers do exist for some physical characteristics, but they do not align with the racial categories we use. If we were to change our social constructions and group people by different genetic characteristics, we would have a different set of races.
Step-by-step explanation:
Biological anthropologists, biologists, and geneticists argue that the concept of race itself is not scientifically valid. While there are genetic markers for physical characteristics, like skin color and hair texture that are evident in the DNA of each person, only a few of the many thousands of DNA markers align with the convenient categories we use to categorize into racial groups.
For example, only about 15 of our 45,000 genetic markers control for skin pigmentation. If we wanted, we could choose from thousands alternative genetic characteristics to classify people. If our social constructions were to change, and we suddenly decided to group people by height, fingerprint patterns, or blood type (rather than skin color, hair texture, and eye shape), we would have an entirely different set of races across the globe.