Should she marry a man who is a carrier of thelassaemia?

Praise be to Allah.

Firstly: 

Undoubtedly one of the aims of marriage is to produce righteous offspring and increase the numbers of the ummah of Muhammad (peace and blessings of Allaah be upon him), as it says in the hadeeth narrated by Abu Dawood (2050) from Ma’qil ibn Yasaar (may Allaah be pleased with him) who said: A man came to the Prophet (peace and blessings of Allaah be upon him) and said, “I have found a woman who is of good lineage and is beautiful, but she does not bear children. Should I marry her?” He said, “No.” Then he came again with the same question and he told him not to marry her. Then he came a third time with the same question and he said: “Marry those who are loving and fertile, for I will be proud of your great numbers before the other nations.” Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 1784.  

That also means producing offspring who are healthy and able to do the duties enjoined by Islam, and to bear the burdens of conveying the message. 

If the couple know that their marriage may produce children who are sick or who are carriers of a disease, then it is better for them not to get married in that case, so as to ward off expected harm and to reduce evil and harm in the ummah of Islam, and to protect themselves from hardship and suffering that may affect them when taking care of a sick child. 

From what we have read, it seems that if both spouses are carriers of this sickness, then each of their children has a 25% chance of being healthy, a 25% chance of being affected, and a 50% chance of being a carrier. But if one of them is healthy and the other is a carrier of the disease, then the danger is greatly reduced, as the possibility of the child being born healthy goes up to 50%, and the possibility of him being a carrier of the disease is also 50%, but there is no possibility of the child being born with the disease. 

These possibilities are based on experience and research, but the matter is entirely subject to the will and decree of Allaah. 

As that is the case, then it is better for you to marry a healthy person. This does not mean that you should put good health before religious commitment as you say; what you have to do is to look for a healthy man who is religiously committed, and there are many such, praise be to Allaah. 

If you give up the idea of this marriage for the sake of your children, and to reduce the sickness and prevent it spreading further in the ummah, then we hope that Allaah will compensate you with good, and reward you for that. 

We ask Allaah to help and guide you. 

And Allaah knows best.