<prompt> Which chromosome is involved in Angelman syndrome?chr 13chr 15chr 18chr 21  <response>: Ans. is b i.e., chr 15 Genomic imprinting A person inherits two copies of their genes, one from mother and one from father. Usually both allels of gene are active in cells. However, in some cases, one of the allel either maternal or paternal is silenced and only the counterpart remains active. When the allel inherited from mother is silencedinactivated paternal allel is active, it is called maternal genomic imprinting. When paternal allel is silenced maternal allel is active it is called paternal genomic imprinting. Thus genomic imprinting leads to preferential expression of an allel depending on its parental origin. Important examples of genomic imprinting are1 Prader Willi syndrome It is due to paternal genomic imprinting in which there is deletion on paternal chromosome 15. They are characterized by hypotonia, obesity, short stature, mental retardation, hypogonadism, hyperphagia, and short hand and feet. There is decrease in GnRH, LH, FSH and GH. Ghrelin levels are raised.2 Anglemann syndrome It is due to maternal genomic imprinting in which there is deletion on maternal chromosome 15. They are characterized by hypotonia, mental retardation, seizures, ataxia, and inappropriate laughter Happy puppets. 