Carriers of a single T allele would demonstrate no clinical manifestations of glycine encephalopathy though offspring who inherited 2 T alleles would be expected to exhibit a range of developmental delays without appropriate medical treatment.
"Glycine encephalopathy, also known as nonketotic hyperglycinemia (NKH), which is an inborn error of glycine metabolism defined by deficient activity of the glycine cleavage enzyme and, as a consequence, accumulation of large quantities of glycine in all body tissues including the brain." https://en.wikipedia.org/wiki/Aminomethyltransferase
It is also of interest that lowered expression of GLDC has been related to lower rates of various cancers. http://www.ncbi.nlm.nih.gov/pubmed/25855294 http://www.ncbi.nlm.nih.gov/pubmed/25496516 http://www.ncbi.nlm.nih.gov/pubmed/22225612