TAILIEUCHUNG - Chapter 063. Chromosome Disorders

In humans, the normal diploid number of chromosomes is 46, consisting of 22 pairs of autosomal chromosomes (numbered 1–22 in decreasing size) and one pair of sex chromosomes (XX in females and XY in males). The genome is estimated to contain between 30,000 and 40,000 genes. Even the smallest autosome contains between 200 and 300 genes. Not surprisingly, duplications or deletions of chromosomes, or even small chromosome segments, have profound consequences on normal gene expression, leading to severe developmental and physiologic abnormalities. . | Chapter 063. Chromosome Disorders In humans the normal diploid number of chromosomes is 46 consisting of 22 pairs of autosomal chromosomes numbered 1-22 in decreasing size and one pair of sex chromosomes XX in females and XY in males . The genome is estimated to contain between 30 000 and 40 000 genes. Even the smallest autosome contains between 200 and 300 genes. Not surprisingly duplications or deletions of chromosomes or even small chromosome segments have profound consequences on normal gene expression leading to severe developmental and physiologic abnormalities. Deviations in number or structure of the 46 human chromosomes are astonishingly common despite severe deleterious consequences. Chromosomal disorders occur in an estimated 10-25 of all pregnancies. They are the leading cause of fetal loss and among pregnancies surviving to term the leading known cause of birth defects and mental retardation. In recent years the practice of cytogenetics has shifted from conventional cytogenetic methodology to a union of cytogenetic and molecular techniques. Formerly the province of research laboratories fluorescence in situ hybridization FISH and related molecular cytogenetic technologies have been incorporated into everyday practice in clinical laboratories. As a result there is an increased appreciation of the importance of quot subtle quot constitutional cytogenetic abnormalities such as microdeletions and imprinting disorders as well as previously recognized translocations and disorders of chromosome number. Visualizing Chromosomes Conventional Cytogenetic Analysis In theory chromosome preparations can be obtained from any actively dividing tissue by causing the cells to arrest in metaphase the stage of the cell cycle when chromosomes are maximally condensed. In practice only a small number of tissues are used for routine chromosome analysis amniocytes or chorionic villi for prenatal testing and blood bone marrow or skin fibroblasts for postnatal studies. Samples .

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