Expand the following. (i) SCID (ii) ZIFT
(i) SCID is severe combined immunodeficiency. It is a primary immune deficiency. It is a genetic disorder characterized by the inhibition of development of functional B cells and T cells, caused by numerous genetic mutations. The gene coding for adenosine deaminase (ADA) which is necessary for the breakdown of purines get mutated and this defective now not able encode ADA. Lack of ADA causes accumulation of dATP that reduces ribonucleotides to generate deoxyribonucleotides. Without functional ribonucleotide reductase, lymphocyte proliferation is inhibited and such disorder is called as SCID.  (ii) ZIFT is Zygote Intrafallopian Transfer. It is a type of artificial reproductive technique. In this, egg cells are removed from the woman's ovaries and fertilized in the laboratory. The transfer of zygote and early embryo up to eight blastomere stage is then carried out into the fallopian tube.