Cytogenetics Course PDF

Cytogenetics course n°5  Course n° 5: cytogenetics Definition: is the study of genetic phenomena at the cellular level, more specifically at the level of chromosomes without the need to extract DNA. - Chromosomal abnormalities: number, structure. -Recombination of chromosomes...

Cytogenetics course n°5  Course n° 5: cytogenetics Definition: is the study of genetic phenomena at the cellular level, more specifically at the level of chromosomes without the need to extract DNA. - Chromosomal abnormalities: number, structure. -Recombination of chromosomes. The human karyotype and its abnormalities: Definition: The karyotype allows the identification and classification of an individual's chromosomes. It is, therefore, the chromosomal configuration of a subject. Principle and technique of the karyotype: chromosomes are visible only in dividing cells, at the metaphase stage. Cells with a high division index. Cells in culture (low division index): blood lymphocytes. Normal human karyotype: Nomenclature 46 chromosomes. 22 pairs of autosomes (1 to 22 in decreasing size order). 1 pair of sex chromosomes or gonosomes: XY in males and XX in females. Chromosomes classified by their size and the position of the centromere. Chromosomes are divided into regions from 1 to 3, each region is divided into numbered bands, and some bands into sub-bands. Example: X q 2, 7, 3: long arm of chromosome X, region 2, band 7, sub-band 3 Main karyotype abnormalities in human pathology: Homogeneous: present in all the individual's cells, it occurs during paternal or maternal meiosis (gametogenesis). Mosaic: affects only a portion of the individual's cells, it occurs during the first divisions of the fertilized egg. Karyotype anomalies can occur de novo or be inherited from one of the parents. Karyotype abnormalities can occur de novo or be inherited from one of the parents. Numerical anomalies: Aneuploidies: Absence of a chromosome or the presence of an extra chromosome from the haploid set, which is caused by an accident during fertilization. Trisomy 21: it is the presence of a chromosome in 3 copies, with a total of 47 chromosomes. Polyploidy: it is the condition of having a chromosomal set equal to at least 3 complete sets of chromosomes (3n) or more. KLINEFELTER syndrome: affected men have the following karyotype: XXY, or more rarely XXXY, XXXXY, or a mosaic of XY/XXY. Structural anomalies: Spontaneous or induced by mutagens. Deletion: the loss of a chromosome segment. Cri du chat syndrome is a disorder caused by a deletion in chromosome 5, leading to intellectual disability and a malformation of the larynx. Duplication: even normal chromosomes contain gene sequences that are repeated from a few dozen to several thousand times. Inversion: a linear DNA segment within a chromosome is oriented in the reverse direction without any molecular loss. Translocations: in this case, the broken part of a chromosome attaches to a non-homologous chromosome. Most translocations are reciprocal, meaning that the two chromosomes exchange their broken segments. Robertsonian translocation: Also called centric fusion, it involves the transfer of an entire chromosome into another chromosome. Balanced with 45 chromosomes, but unbalanced gametes. The most common structural aberration (1/1000). Responsible for 5% of Down syndrome cases by translocation A. Autosomal anomalies 1.Trisomy 21: Dawn syndrome karyotype: 47XX, +21 or 47XY, +21 2. Trisomy 18: Edwards trisomy Karyotype: 47XX+18, 47XY+18 3. Trisomy 13: Patau syndrome Karyotype: 47XX+13, 47XY+13 4. Partial deletion 5p : Cry of cat’s syndrome Karyotype: 46 XY 5p – , 46 XX5p ̶ B. Gonosome anomaly (The aberration of sexuel chromosomes) 1. Turner syndrome Karyotype: 45XO 2. Klinfelter syndrome Karyotype : 47XXY