What is the crucial drawback in using family studies to try to determine the risk of a genetic disorder among relatives sharing a household?

Sharing both an environment and genetic background can make nature hard to distinguish from nurture.

Explanation:

The shared inherited characteristics can be responsible for the occurrence of a particular genetic disease. But the genetic disease may also result due to shared common environment.

For example, the lung cancer may be transferred as a genetic disease from the parents to the offspring but it may also spread through the environment where two or more members of the family smoke. Thus the determination of the fact that the disease is caused due to inheritance or due to environmental influence becomes difficult. Hence, it becomes difficult to distinguish between the influence of nature and nurture on the organisms.

children tend to take more after their grandparents more than their parents. it’s the dna you received that was passed down to you somewhere in your family. maybe one of your grandparents had/has straight hair and you might have gotten it from them.

that is wrong, its not thymus.