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In general, nondisjunction can occur in any form of cell division that involves ordered distribution of chromosomal material. Higher animals have three distinct forms of such cell divisions: Meiosis I and meiosis II are specialized forms of cell division occurring during generation of gametes (eggs and sperm) for sexual reproduction, mitosis is the form of cell division used by all other cells ...
Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. [3] Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo. [ 3 ]
Nondisjunction occurs when sister chromatids on the sex chromosome, in this case an X and an X, fail to separate. An XX egg is produced, which when fertilized with a Y sperm, yields an XXY offspring. This XXY chromosome arrangement is one of the most common genetic variations from the XY karyotype, occurring in approximately one in 500 live ...
An incident in chromosome separation during anaphase II (of meiosis II) called nondisjunction can result in sperm cells with an extra copy of the Y-chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y-chromosome in each of the body's cells.
Completely inactive mitotic checkpoints may cause nondisjunction at multiple chromosomes, possibly all. Such a scenario could result in each daughter cell possessing a disjoint set of genetic material. [citation needed] Merotelic attachment occurs when one kinetochore is attached to both mitotic spindle poles. One daughter cell would have a ...
The cause of the condition lies in conception or utero with the aggregation of two distinct zygotes or blastocysts (one of which expresses 46,XX and the other of which expresses 46,XY) into a single embryo, [4] which subsequently leads to the development of a single individual with two distinct cell lines, instead of a pair of fraternal twins.
Numerical errors can arise at either of the two meiotic divisions and cause the failure of a chromosome to segregate into the daughter cells (nondisjunction). This results in an extra chromosome, making the haploid number 24 rather than 23. Fertilization of eggs or insemination by sperm that contain an extra chromosome results in trisomy, or ...
Mitotic nondisjunction after conception would lead to mosaicism, and is discussed later. Some cases have been reported of people with Down syndrome having children with trisomy 21. [5] In these cases (all from mothers), the ovaries were trisomy 21, leading to a secondary nondisjunction during gametogenesis and a gamete with an extra chromosome 21.