A 3 -year-old child exhibited some early indication of...

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.

Key Concepts

Mitotic Nondisjunction

Mitotic nondisjunction is an error that occurs during cell division, where chromosomes fail to separate properly during mitosis. In an embryo that starts off normally, such an error can result in one daughter cell receiving an extra chromosome and another receiving one less, leading to a mosaic pattern of chromosomal numbers among cells.

Anaphase Lag

Anaphase lag is a process during cell division where one chromosome lags behind and does not get included in the nucleus of the daughter cell, effectively becoming lost. This is one mechanism that can result in mosaicism, as some cells may end up with a normal chromosomal complement while others are missing a chromosome.

Chromosomal Mosaicism

Chromosomal mosaicism refers to the presence of two or more populations of cells with different chromosomal compositions within an individual, arising from a genetic change that occurs after fertilization. This phenomenon is important because it can lead to variable clinical manifestations, depending on the proportion and distribution of the abnormal cell line(s) throughout the body.

Turner Syndrome

Turner syndrome is a chromosomal disorder typically resulting from the complete or partial loss of an X chromosome in females, leading to a 45,X karyotype instead of the normal 46,XX. The presence of mosaicism, where some cells are 46,XX and others are 45,X, accounts for the variability in clinical presentation seen in this syndrome.

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