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

Verified step by step guidance

1. Understand that Turner syndrome typically results from monosomy X (45,X), where one X chromosome is missing in all cells, but mosaicism means there are two or more different cell lines in the same individual.
2. Recognize that the presence of both 46,XX and 45,X cell lines suggests that the individual started from a single fertilized egg but developed two genetically distinct populations of cells.
3. Consider that the mosaicism could arise from a post-zygotic error, such as nondisjunction or anaphase lag, occurring during an early mitotic division after fertilization of a normal 46,XX zygote.
4. Specifically, anaphase lag could cause one X chromosome to be lost in some cells during mitosis, resulting in a 45,X cell line alongside the original 46,XX cells.
5. Summarize that the mechanism involves a mitotic error after fertilization leading to two cell populations: one normal 46,XX and one monosomic 45,X, explaining the mosaic karyotype observed.

Key Concepts

Turner Syndrome and Chromosomal Composition

Turner syndrome is a genetic condition characterized by the presence of a single X chromosome (45,X) instead of the typical two sex chromosomes. It leads to various developmental and physical features, and understanding its chromosomal basis is essential to interpret karyotype results and clinical manifestations.

Mosaicism in Genetics

Mosaicism occurs when an individual has two or more genetically distinct cell lines derived from a single fertilized egg. In this case, some cells have 46,XX chromosomes while others have 45,X, indicating post-zygotic chromosomal loss or nondisjunction, which results in a mixture of normal and abnormal cells.

Mechanisms of Chromosomal Nondisjunction and Anaphase Lag

Chromosomal mosaicism can arise from errors during cell division, such as nondisjunction, where chromosomes fail to separate properly, or anaphase lag, where a chromosome is lost during mitosis. These mechanisms explain how some cells lose an X chromosome after fertilization, producing the observed mosaic karyotype.