1、Chromosomal Abnormalities(染色体畸变 ) Cytogenetics (细胞遗传学 ) is the field of study involving examination of the genetic material of the cells. The genetic material (DNA) is contained in chromosomes. Each organism has a characteristic number and arrangement of chromosomes.Introduction A karyotype （ 核型 ） i
2、s a photograph of the chromosomes at metaphase, when they are most easily seen. In a karyotype, the chromosomes are arranged by pairs: alterations in chromosome size (structure) or number are seen here.Normal Human KaryotypeA normal human cell contains 46 chromosomes: 22 pairs of autosomes（常染色体） , a
3、nd two sex chromosomes.Chromosomal Abnormalities Chromosomal abnormalities can be of two types.(1) Structural(结构 ) abnormalities are caused by breaks in the chromosome being incorrectly repaired. “Ends” of broken chromosomes are very unstable in a cell. (2) Numerical(数目 ) abnormalities are caused by
4、 gain or loss of chromosomes.Common Structural Abnormalities(1) Deletions (缺失 )： loss of genetic material(2) Inversions (倒位 ): no change in the amount of genetic materialCommon Structural Abnormalities(3) Duplications(重复 ): gain of genetic material(4) Translocation(易位 ): no change in the amount of g
5、enetic material (if balanced)(一 ) Deletions 顶端缺失 (Terminal Deletions )： 指缺失的区段位于染色体某臂的外端。 中间缺失 (Interstitial Deletions )： 指缺失的区段位于染色体某臂 的中间。 (1)无着丝粒断片：最初发生缺失的细胞在分裂时可见落后染色体 无着丝粒断片，(2)顶端缺失有丝分裂出现因 断裂 融合 双着丝粒染色体 后期染色体 桥 。减数分裂联会时，有未配对的游离区段。(3)中间缺失减数分裂染色体联会时形成 缺失环 。注意：较小的缺失往往并不表现出明显的细胞学特征；缺失纯合体减数分裂过程也无明显的细胞学特征 。缺失的 细胞学鉴定
