What is the crucial drawback in using family studies to try to determine the risk of a genetic disorder among relatives sharing a household?
a. relatives will be less likely to tell the truth to researchers in order to look better compared to other family members.
b. family studies are an unnecessary intrusion as the same information can be obtained from strangers sharing an environment, such as those living in an apartment building.
c. people with the same genetic background are unlikely to share an environment.
d. sharing both an environment and genetic background can make nature hard to distinguish from nurture.

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

Explanation:

Genetic disorders are not always monogenic, meaning you have the disease or you don´t based on your genotype. (E.g. polycystic kidney disease) and they are relatively rare in comparisson to other multifactorial and common diseases like diabetes, heart disease or cancer.

Nature refers to all the hereditary factors influencing the risk of a genetic disorder while nurture refers to all the environmental variables playing a role, like sharing a household. This means sharing the same food, same water, same social relationships, etc... all which can play a crucial role trying to determinate the causes of a disease.

This is why it is considered a drawback using family for genetic studies, even when we know they are sharing the same gene pool.

It will be harder to distinguish any differences found, if they come from the genetic shared background (nature) or shared environment (nurture).