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 get easily the sample from the vein.
- Bone marrow.
- Amniotic fluid (amniocentesis).
- Chorionic villous sample.
- Smears from the buccal mucosa are easy but not as good as other sources.
- Tumor cells.
Purpose of the test (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 in 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 be done to 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.
Effect of chromosomal abnormalities:
- There may be spontaneous abortion.
- There may be 50% chromosomal abnormalities in the fetus and out of these > 90% will die during the gestational period.
- 50% of the abortions, 7% of the stillbirth, and 0.5% of the neonates have chromosomal abnormalities.
- Each human somatic cell has 46 chromosomes in pairs of 23.
- 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 that 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.
- Chromosomes number 1 is the longest.
- Chromosomes number 22 is the shortest.
- Chromosomes can not be distinguished from the 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 chromosomes 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 the 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.
- 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 when part of a chromosome is duplicated.
- Deletion when the part of a chromosome is missing.
- 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 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 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|
|One X (45 X0)||Turner syndrome||sex chromosome is missing|
|Extra X in male (47 XXY)||Klinefelter syndrome||2 or more chromosome with one Y chromosome|
|Trisomy 21||Down syndrome||three copies of chromosome 21|
|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|