WHAT IS GENETIC IMPRINTING?
One of the ways Prader-Willi Syndrome can arise is through a defect in genetic imprinting. Imprinted genes are genes whose expression (whether or not they are transcribed and translated) depends on the parent they are inherited from. The human genome consists of roughly 30,000 genes. A diploid cell contains two copies of each of these genes, a paternal and a maternal. For most of the 30,000 genes, both the paternal and maternal copy of the gene are being expressed in the diploid cell. Scientists have discovered that about 80 of the 30,000 genes in the genome are imprinted. This means the paternal copy of the gene is expressed while the maternal copy is repressed or vice-versa. Imprinting occurs during the formation of the egg and sperm cells. If a gene is destined to be repressed, the TATA base sequence in the promoter of the gene is methylated. This basically blocks the RNA polymerase from binding to the promoter. If the RNA polymerase cannot bind to the promoter of the gene, the gene
