Is Preimplantation Genetic Diagnosis (PGD) allowed in Islam for below reasons :
1) My wife has a genetic condition called HNPCC, if the gene is passed on to the child, the child when he or she reaches 40-45 years will develop cancer. We have seen many cases in the family. PDG is possible to elimante this gene.
2) I already have sons / daughters and want to have opposite sex child through PGD.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

This kind of diagnosis might require exposing the private parts of the woman; and the scholars of Fiqh state that it is impermissible to expose the woman’s private parts unless there is a compelling necessity or a dire need equal to necessity. That is the basic rule. But if this ailment exists among the family members and could most likely be inherited, and, at the same time, by that diagnosis the harmful gene might be detected and removed, with no harm to be inflicted on the fetus because of that operation, we see that there it is permissible, Allaah willing, to make that diagnosis and remove the harmful gene.

In this respect, it should be brought to attention that this operation should be done by a woman, and a Muslim woman is given priority over a non-Muslim woman. But if no woman is available, it is permissible that a man performs the operation, and a Muslim is given priority over a non-Muslim.

However, it is impermissible to expose of the private parts more than what is required by necessity. This pertains to the first question.

As for the answer to the second question, whoever could give birth by the usual way, it is impermissible for her to do such diagnosis to determine the gender of the fetus. For further information, see our Fatwa 83358.

Our advice is that one should accept what is predestined for him by Allaah The Almighty, and ask Him to endow him with the righteous offspring, and good would be decreed for him, by Allaah’s permission.

Allaah Knows best.