All of the following are examples of uniparental disomy except
**Question:** All of the following are examples of uniparental disomy except
A. Monosomy X
B. Trisomy 18
C. Trisomy 13
D. Trisomy 21
**Core Concept:**
Uniparental disomy (UPD) is a genetic condition that occurs when a chromosome from one parent is present in an individual's cells in excess of the usual two copies. There are two types of UPD: isodisomy and heterodisomy. In isodisomy, both copies of a chromosome are identical (homozygous), while in heterodisomy, the chromosomes differ in their genes (heterozygous).
**Why the Correct Answer is Right:**
D. Trisomy 21 (Down syndrome) is a condition caused by an extra copy of chromosome 21, resulting from trisomy 21 (aneuploidy). In this case, both copies of chromosome 21 are identical (homozygous), which is not consistent with the definition of uniparental disomy.
A, B, and C are examples of uniparental disomy. In monosomy X (A), only one copy of X chromosome is present, resulting in Turner syndrome in females. Trisomy 18 (B) and 13 (C) are also examples of aneuploidy due to an extra copy of chromosomes 18 and 13, respectively, which is different from the definition of uniparental disomy.
**Why Each Wrong Option is Incorrect:**
A, B, and C involve aneuploidy (extra chromosomes), not uniparental disomy. In uniparental disomy, there is only one copy of a chromosome from one parent. Aneuploidy results in conditions like Down syndrome or Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).
**Why the Correct Answer is Right:**
D. Trisomy 21 (Down syndrome) is a condition caused by an extra copy of chromosome 21, resulting from trisomy 21 (aneuploidy). In this case, both copies of chromosome 21 are identical (homozygous), which is not consistent with the definition of uniparental disomy.
**Why Each Wrong Option is Incorrect:**
A, B, and C involve aneuploidy (extra chromosomes), not uniparental disomy. In uniparental disomy, there is only one copy of a chromosome from one parent. Aneuploidy results in conditions like Down syndrome or Edwards syndrome (trisomy 21) and Patau syndrome (trisomy 13).
**Core Concept:** Uniparental disomy involves having only one copy of a chromosome from one parent. Aneuploidy, like in trisomy 21, results in conditions like Down syndrome or Edwards syndrome and Patau syndrome.
**Core Concept:**
Uniparental disomy involves having only one copy of a chromosome from one parent. Aneu