Final answer:
The major constraint to human genetics research in the US is ethical and social considerations, including privacy and the appropriate use of genetic information.
Step-by-step explanation:
The biggest constraint to the study of human genetics in the United States is likely related to the complexity and variability of genetic traits within the population, as well as issues of privacy and ethics concerning the use of personal genetic information. While factors such as family size, knowledge of human biology, and the number of genes in the human genome might present challenges, they are not the primary constraints. The major constraint is the ethical and social considerations associated with accessing and using genetic information, which can impact the management of family records and how genetic data can be used in research and medicine.
Considerations of privacy and ethics are especially relevant in the context of genetically-inherited diseases, such as sickle cell anemia and Huntington's disease. There is also the challenge of differentiating between race and ancestry in studies, which impacts how we understand and categorize human genetic diversity. The resolution of such constraints would require careful handling of ethical, social, and medical issues surrounding human genetics, balanced against the potential benefits of medical advancements.