Karyotyping
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Karyotyping is a cytogenetic technique
Cytogenetics is the study of the structure and properties of chromosomes
Karyotype and ideogram are used to classify chromosomes
Karyotyping is done in plants and humans
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Cytogenetics studies the structure and behavior of chromosomes during mitosis and meiosis, chromosomal influence on the phenotype and the factors that cause chromosomal changes
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Eukaryotic chromosomes have a specific structure
They consist of nucleosomes, DNA double helix, coils, supercoils, and histones
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Chromosomes have sister chromatids, short and long arms, centromeres, and telomeres
During metaphase, chromosomes are duplicated
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Homologous chromosomes have similar structures
They have centromeres and are replicated from paternal and maternal chromosomes
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Chromosomes are classified based on the position of the centromere and length of chromosomal arms
They can be metacentric when centromere at the center
sub-metacentric when near the centre
acrocentric when close to the end
telocentric at the end
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Karyotype is prepared from microphotographs of metaphase chromosomes
Metaphase is chosen because chromosomes are condensed and visible under a microscope
Karyotype represents the complete set of chromosomes in a eukaryotic cell
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Karyotyping involves arresting cells at metaphase using colchicine
The cells are fixed and stained, then examined under a microscope
Chromosomes are cut out and arranged in homologous pairs according to size
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An ideogram or karyogram is a diagrammatic representation of the karyotype
It shows all pairs of chromosomes arranged by size
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Karyotyping is a technique used to examine and arrange chromosomes in pairs according to size
It is represented in an ideogram
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Karyotype is the complete set of chromosomes in a eukaryotic cell defined by the number and the appearance.
Karyotyping is the technique used to examine and arrange chromosomes in pairs according to size in an ideogram
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Karyotyping is important for locating changes in the number and structure of chromosomes
It helps solve taxonomic problems and represents evolutionary relationships
It is commonly used in clinical diagnosis of genetic disorders
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A chromosomal karyotype is used to detect chromosome abnormalities and diagnose genetic diseases, birth defects and certain haematological and lymphoid disorders
It is requested when a fetus is suspected of having a chromosomal abnormality, in cases of miscarriages or infertility, and to detect acquired chromosomal abnormalities in cancer patients
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Samples for karyotyping can be collected from blood, bone marrow, tissue, amniocentesis(amniotic fluid extraction), or chorionic villus sampling
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The banding pattern of chromosomes is characteristic for each pair
An ideogram represents the banding pattern and is an international system
Bands are numbered starting at the centromere (each arm)
Examination of bands at increased resolution reveals larger numbers of them visualized (observed at prophase or prometaphase)
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Karyotypes are described by listing the number of chromosomes, sex chromosome constitution, and anomalies in number or morphology
• Normal male: 46,XY; normal female: 46,XX
• Klinefelter Syndrome: 47,XXY
•Turner Syndrome: 45,X
•Female with Down Syndrome: 47,XX,+21
•Male with Angelman Syndrome (deletion between q11 and q13 on chromosome 15): 46,XY,del(15)(q11q13). q11 is read as "q one one"; q13 is read as "q one three".
•Female with a balanced reciprocal translocation: 46,XX, t(9;22)(q34;q11.2).This example is the Philadelphia chromosome translocation seen in chronic myelogenous
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The short arm of a chromosome is represented as "p" and the long arm as "q"
del Deletion
• der Derivative
• dup Duplication
• I lsochromosome
Commonly used cytogenetic nomenclature:
• r Ring chromosome
• t Translocation
• ter Terminal (may also be written as pter or qter)
• ins Insertion
• inv inversion
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"+" or "-" is used to indicate addition or loss of a whole chromosome
"+" or "-" can also indicate gain or loss of a chromosome part
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Interpretation of test results should be done by someone with specialized training in cytogenetics
Some findings may be straightforward, while others can be complex.
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Abnormalities that chromosome analysis may reveal:
Trisomy: presence of an extra chromosome
Examples: Down syndrome (Trisomy 21), Patau syndrome (Trisomy 13), Edward syndrome (Trisomy 18), Klinefelter syndrome (XXY)
Monosomy: absence of one chromosome
Example: Turner syndrome (X instead of XX)
Deletions: missing pieces of chromosomes/genetic material
Some may be small and difficult to detect
Duplications: extra genetic material
Can be present on any chromosome
Translocations: pieces of chromosomes break off and reattach to another chromosome
Balanced translocation: one-to-one switch, all genetic material present but in the wrong place
Unbalanced translocation: not all genetic material present
Genetic rearrangement: genetic material present on a chromosome but not in its usual location
Can coexist with duplication or deletion
Interpreting the effects of changes can be challenging
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Explaning Test Results/Karyotypes to Parents
Sample test result form for mosaic Down syndrome diagnosis
Goal is to help interpret the results
Test result form details:
Patient information
Specimen details
Indications for study: Clinical features of Trisomy 21
Metaphases counted: 21
Banding technique: GTG
Metaphases analyzed: 6
Number of cultures: 2
Banding resolution: 550
Metaphases karyotyped: 2
Test result: 47,XX,+21[15]/46,XX[6]
Abnormal karyotype, female
Interpretation: Cytogenetic analysis shows three copies of chromosome 21 (trisomy 21) in fifteen out of 21 metaphase cells examined. An additional 10 cells were scored for the presence of an extra copy of chromosome 21 and all 10 cells were trisomic. Consistent with the clinical diagnosis of mosaic Down syndrome.