Question

A 3 -year-old child exhibited some early indication of Turner syndrome, which results from a \(45, \mathrm{X}\) chromosome composition. Karyotypic analysis demonstrated two cell types: \(46, \mathrm{XX}\) (normal) and \(45, \mathrm{X}\). Propose a mechanism for the origin of this mosaicism.

Short answer

Question: Propose a mechanism for the origin of mosaicism in a 3-year-old child exhibiting early indications of Turner syndrome. Answer: The origin of mosaicism in this case can be attributed to an error during mitotic cell division, specifically nondisjunction, in early embryonic development. Nondisjunction occurs when sister chromatids fail to separate properly during anaphase, leading to one daughter cell receiving both X chromosomes (46,XX) and the other receiving none (45,X). The persistence and proliferation of both 45,X and 46,XX cells result in mosaicism in the child.

Step-by-step solution

Understand the chromosomal composition of Turner syndrome and normal cells.

In order to propose a mechanism for the origin of the mosaicism, we need to understand the difference between Turner syndrome cells (45,X) and normal cells (46,XX). Turner syndrome results from the absence or structural abnormality of an X chromosome, while normal cells have two X chromosomes (46,XX).

Determine the type of cell division involved in producing a mosaic individual.

Mosaicism can occur as a result of errors during cell division processes, such as mitosis or meiosis. Since the child is already born, the error must have occurred during mitotic cell division, which is involved in growth and development after fertilization.

Identify the possible point of error during mitotic cell division.

The possible point of error might have occurred during the formation of the 46,XX zygote or during early embryonic development, where an error in mitotic cell division produced 45,X cells.

Propose a mechanism for the origin of mosaicism.

One possible mechanism for the origin of the mosaicism in this case is called "nondisjunction." Nondisjunction is when sister chromatids fail to separate properly during anaphase of mitotic cell division. In this case, the nondisjunction event might have occurred in one of the early cell divisions, affecting one of the X chromosomes. Specifically, when the homologous pair of X chromosomes failed to separate during anaphase, one daughter cell received both X chromosomes (46,XX) while the other daughter cell received none (45,X). As the embryo continued to develop, cells with either 45,X or 46,XX chromosomal composition persisted, resulting in mosaicism.