One of the following disorders is due to maternal disomy of chromosome 15 -
Ans. is `b' i.e., Angleman syndrome o Don't get confused by previous explanations, where I have explained the deletion of paternal chromosome 15 in Prader Willi Syndrome. o To understand this question, one should know genomic imprinting as well as uniparental disomy. Genomic imprinting o A person inherits two copies of their genes one_ form their mother and one from their father Usually both copies of each gene are active or "turned on" in cells.. o In some cases however only one of the two copies is turned on. o Which copy is active depends on the parent of origin. Some genes are normally active only when they are inherited from a person's father. Others are active only when inherited from a person's mother. o This phenomenon is known as genomic imprinting. In genes that undergo genomic imprinting, the parent of origin is often marked or "stamped" on the gene during the formation of egg and sperm cells. This stamping process called methylation is a chemical reaction that attaches small molecules called methyl groups to ceain segments of DNA. These molecules identify which copy of a gene was inherited from the mother and which was inherited from father. The gene that are imprinted or stamped are "silent"i.e.,they do not remain functional. Uniparental disomy (UDP) o Normally a person inherits one copy of each chromosome pair from biological mother and the other copy of the chromosome pair from biological father. Uniparental disomy occurs when a person receives two copies of a chromosome from the same parent and no copies from the other parent. o UDP can occur as random event during the formation of egg or sperm cells or may happen in early fetal development. Example of Genomic imprinting and UDP The classical examples are Prader -Willi syndrome and Angleman syndrome. Prader-Willi syndrome o Approximately 70% of affected individuals have a small deletion on long arm ofparenteral chromosome 15,i.e. paternal genomic imprinting. There is silencing of paternal gene due to deletion.. Paternal gene Silenced (Paternal genomic imprinting) Maternal gene Only functional alle present o The remaining 30% of patients have maternal uniparenteral disomy for chromosome 15. That is they have two otherwise normal copies of maternal chromosome 15 and no paternal 15. o Thus Prader Willi syndrome ( in both above situations) results from the absence of paternal contribution to this region of chrmosome 15, whether by paternal deletion ( paternal genomic imprinting) or maternal UPD. Angleman syndrome o Most of the affected individuals have deletion on maternal chrmosome 15,i.e. maternal genomic imprinting. There is silencing of maternal gene due to deletion. Maternal gene Silenced (Maternal genomic imprinting) Paternal gene Only functional allel present o The remaining patients have paternal uniparental disomy for chromosome 15, i.e., they have two otherwise normal copies of paternal chromosome 15 and no maternal 15. o Thus Angleman syndrome results from the absence of maternal contribution to this region of chromossome 15, whether by maternal deletion ( maternal genomicimprinting) or paternal UDP. Prader Willi syndrome o Paternal deletion (Paternal genomic imprinting) o Maternal uniparental disomy Angleman syndrome Maternal deletion (maternal genomic imprinting) Paternal uniparental disomy
Prader Willi syndrome
Angelman syndrome
Hydatidiform mole
Klinefelter's syndrome