- Failure of paired chromosomes to disjoin (separate) during cell division so that both chromosomes go to one daughter cell and none to the other. Nondisjunction causes errors in chromosome number such as trisomy 21 (Down's syndrome) and monosomy X (Turner syndrome).
* * *Failure of one or more pairs of chromosomes to separate at the meiotic stage of karyokinesis, with the result that both chromosomes are carried to the one daughter cell and none to the other.
* * *non·dis·junc·tion .nän-dis-'jəŋ(k)-shən n failure of homologous chromosomes or sister chromatids to separate subsequent to metaphase in meiosis or mitosis so that one daughter cell has both and the other neither of the chromosomesnon·dis·junc·tion·al -shnəl, -shən-əl adj
* * *n.a condition in which pairs of homologous chromosomes fail to separate during meiosis or a chromosome fails to divide at anaphase of mitosis or meiosis. It results in a cell with an abnormal number of chromosomes (see monosomy, trisomy).
* * *non·dis·junc·tion (non″dis-junkґshən) failure (a) of two homologous chromosomes to pass to separate cells during the first division of meiosis, or (b) of the two chromatids of a chromosome to pass to separate cells during mitosis or during the second meiotic division. As a result, one daughter cell has an extra chromosome and the other has one too few. If this happens in meiosis, after fertilization an aneuploid individual may develop, e.g., a child with trisomy 21 (Down syndrome).
Nondisjunction. Normal meiosis (A) is contrasted with failure of homologous chromosomes to separate in meiosis I (B) or of sister chromatids to separate in meiosis II (C).
Medical dictionary. 2011.