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Broadly speaking, an individual’s genotype is their complete set of genetic material.

In the world of genetic disorders, the word also refers to the alleles or variants an individual carries in a particular gene or genetic location. (This is in contrast to one’s phenotype, which is the word used to describe observable characteristics or traits.)

Angelman syndrome is caused by a genetic difference on the UBE3A region of chromosome 15, but this problem can take different forms—which is why there are different kinds of AS genotypes. They are:

Deletion

These individuals AS are missing a piece of DNA in region 15q11‑13 on the maternal copy of chromosome 15. Go here to learn more, and connect with other Deletion families.

Mutation

These individuals have a small difference in the DNA in the UBE3A gene on the maternal copy of chromosome 15. A mutation can happen anywhere on the gene. Go here to learn more, and connect with other Mutation families.

UPD/ICD

These two genotypes are technically different, but functionally the same. An individual with UPD has two copies of chromosome 15 from their father, both of which are silent, instead of one each from the father and mother. Those with ICD have a defect on their imprinting center. The imprinting center's job is to reset imprinting when individuals make eggs and sperm. Because the imprinting center doesn't work, the chromosome 15 that the mother received from her father still says "paternal", which leads to no UBE3A expression in the neurons. Go here to learn more, and connect with other UPD families and go here to learn more, and connect with other ICD families.

Mosaic

This is an extremely rare genotype of Angelman syndrome where a small proportion of the body’s cells are expressing UBE3A. A person with mosaicism has a mixture of cells that are expressing UBE3A as well as cells that do not make UBE3A protein. Go here to learn more, and connect with other Mosaic families.

Clinical Diagnosis

There are also individuals whose testing for Angelman syndrome, including methylation and UBE3A sequencing, does not find any differences but who still meet the diagnostic criteria for AS. These individuals may have as yet unrecognized genetic variants that affect UBE3A or genomic imprinting on Chromosome 15. It is also possible that they have another condition with similar symptoms. Go here to learn more.

(It’s also important to note that there are several other syndromes that present like AS. Visit this page for more information.)

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