Question Summary:
IF someone has an illness and due to avoid passing heridetary illness to new borns, avoids to having any children….

Question Detail:

IF someone has an illness and due to  avoid passing heridetary illness to new borns, avoids to having any children. Instead chooses to adopt children instead.

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatoh
Rasulullah (Sallalahu 'Alayhi Wasallam) said:
تزوجوا الودود الولود فإنى مكاثر بكم الأمم
Marry a woman who is very loving and can bear many children, for verily I will take pride though you over other nations. (Abu Dawud, Ahmad)
It is clear from the above narration that having many children is encouraged and praiseworthy.  To abstain from having children without a legitimate excuse is disliked in Shari’a.  However, if one has a genuine excuse recognized in Shari’a, then it will be permissible to avoid having children.
If one is diagnosed with a serious illness and medically it is proven that the illness is hereditary and it will pass on to the children, then it will be permissible to avoid having children.
Rad al-Muhtar (3/176) H.M. Saeed
( قوله قال الكمال ) عبارته : وفي الفتاوى إن خاف من الولد السوء في الحرة يسعه العزل بغير رضاها لفساد الزمان ، فليعتبر مثله من الأعذار مسقطا لإذنها .
It is permissible to adopt children in all circumstances, having children, not having children, and avoiding having children due to reasons valid in Shari’a. 
Hereunder are some rules pertaining to adoption:
1.  The adopted child should not take up the name of the adoptive parents and the lineage of the child should be maintained.  Rasulullah (Sallalahu Alaihi Wasallam) had adopted Zaid bin Harithah (Radhiyallahu Anhu) as a son before prophethood.  People called him Zaid, the son of Muhammad (Sallalahu Alaihi Wasallam).  Allah Ta’ala revealed the following ayahs.
مَا جَعَلَ اللَّهُ لِرَجُلٍ مِنْ قَلْبَيْنِ فِي جَوْفِهِ وَمَا جَعَلَ أَزْوَاجَكُمُ اللَّائِي تُظَاهِرُونَ مِنْهُنَّ أُمَّهَاتِكُمْ وَمَا جَعَلَ أَدْعِيَاءَكُمْ أَبْنَاءَكُمْ ذَلِكُمْ قَوْلُكُمْ بِأَفْوَاهِكُمْ وَاللَّهُ يَقُولُ الْحَقَّ وَهُوَ يَهْدِي السَّبِيلَ . ادْعُوهُمْ لِآَبَائِهِمْ هُوَ أَقْسَطُ عِنْدَ اللَّهِ فَإِنْ لَمْ تَعْلَمُوا آَبَاءَهُمْ فَإِخْوَانُكُمْ فِي الدِّينِ وَمَوَالِيكُمْ وَلَيْسَ عَلَيْكُمْ جُنَاحٌ فِيمَا أَخْطَأْتُمْ بِهِ وَلَكِنْ مَا تَعَمَّدَتْ قُلُوبُكُمْ وَكَانَ اللَّهُ غَفُورًا رَحِيمًا.
Allah has not made for any man two hearts in his (chest) cavity, nor did he make your wives whom you subjected to zihar, your mothers, no did he make your adopted sons you (real) sons.  That is (merely) a word uttered by your mouths.  And Allah says the truth and He shows the (right) way.  Call them by (the name of) their (real) fathers; It is more equitable in the sight of Allah.  And if you do not know their fathers, then they are your brothers in faith and your friends.  And there is no sin on you in the mistake you make, but in that which you do with intention of your heart and Allah is Most-Forgiving, Very-Merciful.
 In the above verses, Muslims are commanded not to call an adopted child by his adoptive parents.  Instead, he should be called by his real father’s name if it is known.  If not, then he is your brother in deen, but he is not your son.  (Ibn Kathir)
2. Strict purdah must be observed when the child reaches the age of puberty.  Purdah is necessary between the mother and the child if it is a boy and between the father and child if it is a girl.  As an alternative, the child may be adopted during infancy (before the age of two) and the mother breastfeeds the child.  In doing so, they will become the foster parents and mahram of the child and purdah will no longer be necessary.  Nevertheless, breast feeding will not make him a real son and his lineage will still be made separate.
3.  The child will not inherit from the adoptive parents.  They may bequest one-third of their estate to adopted child in their will if they wish to do so.
4.  The child may be adopted from any ethnicity group.  When the child has reached the age of understanding, he should be informed with wisdom about the reality of the matter.
And Allah knows best
Wassalam
Ml. Ehzaz Ajmeri,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah