Salaam alaikum. One of my friends is getting married to his cousin who happens to be in closer circle of his family. On the request of both the parties, medical Doctors conduct genetic counseling, in which blood samples of would-be groom and bride is taken (before marriage); those blood samples are kept under medical examination in the lab to understand if their children would inherit any disorder in future.
All this is done before marriage, this is also referred as 'Genetic Sex'. Is this practice permissible in Islam? Please guide us. Jazak Allah Khair.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is permissible, Allah willing, to carry out a medical examination before marriage. This becomes even more confirmed if it is predominantly believed that there are hereditary diseases in the family.

Shaykh Ibn Jibreen was asked, “What is the ruling on carrying out a medical examination for both spouses before marriage?”

He answered, “It is permissible to do so if an internal disease is feared which would affect one’s health, prevent the comfort of the spouses and the stability and tranquility of life. As it might be that one of them is possessed [by jinn] or has epilepsy, or has a chronic disease, even though the disease may be a simple one, like asthma, diabetes, bilharzias, or rheumatism. The same thing applies to being sterile and not being able to have children. However, if the spouses appear to be in good health and the environment and the society where they live has no such diseases or similar ones, then, in principle, they are not affected with any disease and one should not fear anything about them. In which case, there is no need for a medical examination to be carried out for both spouses.”

Allah Knows best.