Chromosome studies, Blood Chromosome Analysis, Cytogenetics, Chromosome Karyotyping
- The test can be performed on almost any tissue, including:
- Blood. Leucocytes are a good source and can easily get the sample from the vein.
- Bone marrow.
- Amniotic fluid (amniocentesis).
- Chorionic villous sample.
- Smears from the buccal mucosa are accessible but not as good as other sources.
- Tumor cells.
Purpose of the test of chromosomal karyotyping or chromosomal analysis (Indications):
- This test is done to find a chromosomal defect that may lead to or is a risk of the disease.
- Count the number of chromosomes.
- Look for structural changes in chromosomes.
- On a couple that has a history of miscarriage or infertility. Both parents need chromosomal studies.
- In the case of two consecutive abortions.
- When there is more than one organ abnormality in the infant.
- In the case of developmental delay, mental retardation, and growth retardation.
- To advise on ambiguous genitalia.
- In the case of mental retardation and or developmental delay when the cause is not known.
- In the case of the family history of chromosomal translocation.
- To examine any child or baby who has unusual features or developmental delays like:
- Mental retardation.
- Growth retardation.
- Delayed puberty.
- Primary amenorrhoea.
- Ambiguous genitalia.
- Prenatal diagnosis of congenital disease.
- Turner syndrome.
- Klinefelter syndrome.
- Down syndrome.
- In the case of spontaneous abortion, stillbirth, and neonatal death.
- The bone marrow or blood test can identify the Philadelphia chromosome, which is found in about 85% of people with chronic myelogenous leukemia (CML).
- The amniotic fluid test is done to check a developing baby for chromosome abnormality.
- Advised in ladies over the age of 40 years (advanced age). In these ladies, prenatal chromosomal testing is done. 1 out of 400 has a greater risk for chromosomal abnormality, biochemical abnormality, or single gene anomaly.
- Tissue from the spontaneous abortus, stillbirth, or neonatal death, to find the cause of death.
Effect of chromosomal abnormalities on pregnancy and fetus:
- There may be spontaneous abortion.
- 50% of the fertilized ova have some form of chromosomal abnormality, and >99% of these ova will die during gestation.
- There may be 50% chromosomal abnormalities in the fetus, and out of these,> 90% will die during the gestational period.
- 50% of abortions, 7% of stillbirth, and 0.5% of neonates have chromosomal abnormalities.
History of chromosomes:
- Strasburger first described chromosomes in 1815.
- Waldeyer used the chromosome term in 1888.
- Chromosomes appear in rod-shaped dark staining bodies during the metaphase stage of mitosis when cells are stained with special stains and can see under a light microscope.
Each human somatic cell has 46 chromosomes in pairs of 23 from each parent.
- These are diploid cells; the fetus gets one chromosome from each of the parents, one from the father, and one from the mother.
- It means the donation of one chromosome from each of the parents.
- 22 chromosomes are identical called homologous (autosomes).
- One pair consists of XX in females and XY in males.
- Chromosome number 1 is the longest.
- Chromosome number 22 is the shortest.
- Chromosomes can not be distinguished from their length because of the variation from person to person.
- Therefore the position of the centromere is used to classify chromosomes.
- Chromosomes are stained with the Giemsa stain, which gives distinctive chromosome bands.
Karyotyping refers to the arrangement and pairing of cell chromosomes in order from the largest (longest) to the smallest (shortest) to analyze their number and structure.
- Karyotyping is the number and appearance of chromosomes in the nucleus of a eukaryotic cell.
- Karyotyping refers to the arrangement of cell chromosomes, their number, and structure.
- Karyotypes describe the number of chromosomes and what they look like under a light microscope.
- The normal karyotype of chromosomes consists of:
- 22 pairs of autosomal chromosomes (XX).
- One pair consists of a sex chromosome, XY for the male and XX for the female.
- The somatic cell with more or less than 46 chromosomes is called aneuploidy.
- More than 46 chromosomes are called hyperploidy.
- Less than 46 chromosomes are called hypoploidy.
Chromosomal abnormalities may be:
- Chromosomes abnormalities are the common cause of mental retardation and miscarriage.
- Roughly 1 in 12 conceptions has a chromosomal abnormality.
- Most of these fetuses don’t survive to term and 50% of first trimester fetuses abort and have major chromosomal abnormalities.
- Karyotype (Chromosomal) abnormalities occur because of:
- Deletion denoted by del. This may take place on the short or long arm.
- Translocation denoted by t.
- Genetic rearrangement.
- Females = 44 autosomes + 2 sex chromosomes (XX) = Karyotype = 46, XX
- Males = 44 autosomes + 2 sex chromosomes (XY) = karyotype = 46, XY
The procedure of chromosomal analysis:
- The cells are stimulated to divide.
- Lymphocytes can be stimulated by phytohemagglutinin.
- Add Colchicine to arrest the mitotic division.
- A hypotonic solution is added to spread out the chromosomes.
- Fixation is done by Carnoy’s solution.
- Now put the cells on the slide.
- Cell membranes are ruptured, and chromosomes remain in well-defined groups.
- The chromosomes are stained by various staining agents like Giemsa or Fluorescent material.
- Giemsa banding (G banding) is the most common method for banding the chromosomes.
- The dark bands are rich in DNA bases adenine and thymidine.
- The light bands are rich in cytosine and guanine.
- Quinacrine banding (Q banding) is like Giemsa banding, and it is a fluorescent method.
- Karyotyping: Now, select cells with intact chromosomes.
- Chromosomes are counted, identified, and evaluated.
- This is the arrangement and pairing of cell chromosomes, starting from the largest to the smallest in size.
- Reporting: Count autosomal chromosomes, and these are numbered according to their size.
- The larger one is labeled as 1.
Reporting of Chromosomal analysis:
- Hyperploidy is an extra chromosome.
- Hypoploidy when the chromosomes are missing.
- Duplication is when part of a chromosome is duplicated.
- Deletion is when the part of a chromosome is missing.
- In addition, when the chromosome has the additional material.
- Insertion when the part of a chromosome is repositioned into a different area of the karyotype.
- Inversion is when the chromosome segment is reversed in the orientation.
- Mosaic when some cells have abnormal cytogenetic abnormality whereas the other cells don’t have it.
- Translocation is when there is a balanced exchange of the chromosome without any loss or gain of the material.
Chromosomal abnormalities and syndromes:
|Chromosomal abnormality||Disease or syndrome||Defect|
|Philadelphia||Chronic myelocytic leukemia||Ph t(9;22) and variants|
|t(6,9)||AML with basophilia|
|t(3;3), inv(3)||AML with thrombocytosis|
|Ph t(9;22)||B- cell ALL|
|t(1;14) and variants||T-cell ALL|
|t(8;14), t(2;8), t(8;22)||Burkitt lymphoma|
|One X (45 XO)||Turner syndrome||
|Trisomy 21 (47XX, G+ or 47XY,G+||Down’s syndrome||
|5 p deletion||Cat Cry|
|trisomy 18||Edward’s syndrome|
|Trisomy 13||Patau syndrome|
|Mutation 11||Sickle cell anemia|
|Hyperploidy||In some acute lymphoblastic leukemia|
|Hypoploidy||In some myelodysplastic syndrome|
|Trisomy 8 mosaic||Trisomy 8 syndrome|
|Trisomy 22q 11-pter||Cat-eye syndrome|
|Xq27.3||Fragile X syndrome|
|47, XYY||XYY syndrome|
|47, XXX||Tripple X syndrome|
|Xq13-21.1 sex-linked||Severe combined immune deficiency (SICD)|
|11q 22.3||Ataxia telangiectasia|
|X q222||X-linked agammaglobulinemia|
|6p21.3||Selective IgA deficiency|