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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 ...
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.
Nondisjunction usually occurs as the result of a weakened mitotic checkpoint, as these checkpoints tend to arrest or delay cell division until all components of the cell are ready to enter the next phase. For example, if a checkpoint is weakened, the cell may fail to 'notice' that a chromosome pair is not lined with the spindle apparatus. In ...
When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
Aneuploidy occurs when nondisjunction at a single chromosome results in an abnormal number of chromosomes. Aneuploidy is often harmful and in mammals regularly leads to spontaneous abortions (miscarriages). Some aneuploid individuals are viable, for example trisomy 21 in humans, which leads to Down syndrome. Aneuploidy often alters gene dosage ...
This is the result of non-disjunction, where the person receives two copies of chromosome 7 from the mother (maternally inherited) rather than one from each parent. Normal expression requires IGF2 genes to be received from both parents, this may result in the same condition found in those with normal chromosomal inheritance but duplication ...
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This process results in each gamete usually containing a mixture of chromosomes from both original parents. Improper chromosome segregation (see non-disjunction, disomy) can result in aneuploid gametes having either too few or too many chromosomes. The second stage at which segregation occurs during meiosis is prophase II (see meiosis diagram ...