Uniparental Disomy (UPD)

Definition - What does Uniparental Disomy (UPD) mean?

Uniparental Disomy (UDP) occurs when two copies of a chromosome are inherited from one parent, and no copies are inherited from the other parent. Typically, only one copy in each chromosome pair is inherited from each biological parent.

During fertilization, the sperm and the egg unite to form a zygote, which should have 23 pairs of chromosomes—one in each pair from the egg and the other from the sperm. These chromosomes provide the roadmap for the zygote to continue the development into an embryo which will eventually become a fetus, then baby.

UDP can happen as the result of a random event during the development of sperm or egg cells, resulting in an egg or sperm have too many or too few chromosomes.

The consequences of UDP vary from no observable changes to potentially life-threatening disorders to spontaneous miscarriages.

FertilitySmarts explains Uniparental Disomy (UPD)

When both chromosomes come from one parent, there is an increased risk of developing a number of conditions. This is because in certain cases, only one copy of the gene pair is turned "on." This activation is decided by who the gene came from or the parent of origin. Known as genomic imprinting, certain genes are marked with molecules that identify a gene as originating either from the mother or father.

Because most genes are not imprinted, UPD may have no impact on health or development. In certain cases, it does make a difference if the gene comes from the mother or father, meaning that active copies of genes can be missing in the case of UPD.

There are two types of UDP; isodisomy and heterodisomy. In isodisomy, the two chromosomes are identical copies of the same chromosome, whereas, in heterodisomy, the two chromosomes are different copies of the same chromosome.

Prader-Willi syndrome and Angelman syndrome are disorders that can be caused by UPD which result from inheriting both copies of chromosome 15 from either the mother or the father respectively.

UDP can occur in nearly every one of the 23 chromosomes pairs, and as such, conditions like cystic fibrosis, retinoblastoma, osteogenesis imperfecta, and thalassemias have resulted from this phenomenon.

Cytogenic studies do not detect disorders caused by uniparental disomy because the number of chromosomes remains the same. In order to determine if someone's chromosomes have been copied, a molecular analysis which shows the parental lineage of the chromosomes must be conducted. Consulting a healthcare provider or a genetic counselor can give more insight in UPD.

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