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【正文】 aic, having a large proportion of diploid cells. Medical Geics (3) Chromosome nondisjunction The failure of a pair of homologous chromosomes to separate properly during meiosis. The failure of homologues (at meiosis) or sister chromatids (at mitosis) to separate properly to opposite poles, that is two chromosomes or chromatids go to one pole and none to the other. Medical Geics ? Nondisjunction can occur during meiosis I or meiosis II. Medical Geics In meiosis I, the error occurs when the homologous pairs both travel into the same daughter cell. The result is two daughter cells that have two copies of the chromosome (called disomic cells) and two cells that are missing that chromosome (called nullisomic cells). Medical Geics Nondisjunction in Meiosis I: Medical Geics In meiosis II, the error occurs when the sister chromatids will not separate and thus travel into the same daughter cell. 單體型 —— 45,x Medical Geics Nondisjunction in Meiosis II: Medical Geics Fertilization following Meiosis I error: Medical Geics Fertilization following Meiosis II error: Medical Geics Medical Geics 三體型 —— 21三體 Medical Geics ? Nondisjunction occurs during early period of merogenesis,the result is mosaic Medical Geics ? Chromosome loss Medical Geics B. Chromosome Structural Aberration Structural aberration is a change in chromosome structure detectable by microscopic examination of the metaphase stage of cell division, observed as deletions and fragments, intrachanges or interchanges. Medical Geics (1) A chromosome deletion occurs when the chromosome breaks and a piece is lost. This of course involves loss of geic information and results in what could be considered partial monosomy for that chromosome. Medical Geics Medical Geics Medical Geics Medical Geics 缺失 a：末端缺失； b：中間缺失 46,XX(XY),del(1)(q21)/ 46,XX(XY),del(1)(pter→ q21) 46,XX(XY),del(3)(q21q31)/ 46,XX(XY),del(3)(pter→ q21:q31→ q31) Medical Geics (2) A related abnormality is a chromosome inversion. In this case, a break or breaks occur and that fragment of chromosome is inverted and rejoined rather than being lost. Inversions a
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