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.

Key concepts

Turner syndrome

Turner syndrome is a genetic condition affecting females, where one of the X chromosomes is partially or completely missing. Typically, females have two X chromosomes, but in Turner syndrome, the chromosomal composition is 45,X, meaning they have only one X chromosome. This condition affects various developmental aspects and can lead to features such as short stature, delayed puberty, and certain heart defects.

Despite its challenges, individuals with Turner syndrome can still live healthy lives with appropriate medical care. Early diagnosis and management, including growth hormone therapy and estrogen replacement, can help in overcoming some of the physical effects. Knowing about Turner syndrome can significantly influence a girl's ability to lead a fulfilling life.

It is crucial for families and healthcare providers to be aware of the condition's potential implications and available supportive measures.

nondisjunction

Nondisjunction is a key concept in understanding genetic anomalies like mosaicism. It is an error in cell division where chromosomes do not separate properly. This can occur during anaphase, a stage in cell division where paired chromosomes are usually pulled apart evenly to opposite ends of the cell.

When nondisjunction happens, one daughter cell might end up with an extra chromosome, while another might lack one. This error is significant in either meiosis or mitosis, leading to conditions like Turner syndrome when involving sex chromosomes.

• In meiosis, nondisjunction can lead to chromosomal abnormalities in the gametes, which are then passed to offspring.
• In mitosis, as seen in mosaicism, the error occurs during early embryonic development, creating cells with different chromosomal compositions.

The occurrence of nondisjunction, especially in cases like Turner syndrome, is crucial for understanding how mosaic genetic profiles can arise.

chromosomal composition

Chromosomal composition refers to the specific number and structure of chromosomes in an individual. In humans, the typical chromosomal composition is 46 chromosomes, consisting of 23 pairs. These include 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).

Understanding chromosomal composition is vital in diagnosing genetic disorders. In Turner syndrome, for example, the chromosomal composition is altered to 45,X instead of the typical 46,XX. This missing or incomplete X chromosome can result in various physiological changes and health challenges.

In a mosaic individual, such as in the case mentioned, cells have different chromosomal compositions, such as a mix of 45,X and 46,XX cells. This divergence can influence the severity and variety of symptoms experienced by the individual.

cell division

Cell division is an essential process for growth, development, and maintenance of all living organisms. It occurs in two main forms: mitosis and meiosis.

• Mitosis is responsible for producing two identical daughter cells from a single parent cell, facilitating growth and tissue repair.
• Meiosis, on the other hand, creates gametes (sperm and egg cells) with half the typical number of chromosomes, which is crucial for sexual reproduction.

During cell division, chromosomes need to be correctly replicated and separated to ensure each new cell gets the correct number and type. Errors, like nondisjunction, can disrupt this process causing mosaicism, as seen in Turner syndrome mosaics.

Understanding cell division helps in comprehending how genetic variations such as mosaicism occur, shedding light on the complexities of genetic development and the emergence of certain syndromes.