Final answer:
Scientists are limited in studying racial differences because race is not scientifically valid and there is more genetic variation within racial categories. Therefore, scientists focus on studying human genetic diversity based on ancestry rather than race.
Step-by-step explanation:
Scientists are limited in their ability to study racial differences because the concept of race itself is not scientifically valid. Genetic markers for physical characteristics, like skin color, hair texture, and eye shape, only align with the convenient categories used to classify racial groups to a limited extent. Additionally, there is more genetic variation within a given racial category than there is between racial categories. The use of race as a variable in genetic research can lead to confusion and misdiagnoses of diseases. Therefore, scientists focus on studying human genetic diversity based on ancestry rather than race.