KARYOTYPING

HUMAN KARYOTYPING

          Chromosomal aberrations are abnormalities that are seen in the number or microscopically observable structures of chromosomes. A normal human chromosome set should contain 44 autosomes and 2 sex chromosomes. To study these, we apply a technique called karyotyping

KARYOTYPING; This is the process of studying chromosomes of organisms while the cells are undergoing mitosis. A karyotype is a layed down frame containing the whole layout or complete set of chromosome in an organism cell. Karyotype can be carried out for the following reasons;

·         To determine the sex of a fetus

·         To determine the number of chromosomes

·          To trace causes of diseases genetically

·         To  generally study the chromosomes

G-banding; This is one of the steps in a karyotype fo study.it is a staining method that involves treating the sample with trypsin ( to digest the protein ) and then staining with GIESMA. This is done because each homologous chromosome has a specific G-banding pattern and so this allows for easy study and identification.

Chromosomes consists of the short arm (P) and the long arm) and can be classified based on the position of their centromere  (separation point) into 3groups;

1.       METACENTRIC; When the centromere is toward the middle of the arms i.e. median. In this case the upper arm and the lower arm seem to be of the same length because they have been equally divided. Group A, F or E have their chromosomes with this characteristic

2.       SUBMETACENTRIC;When the centromere tilts more towards a particular side i.e. towards one of the arm and the lower arm (usually the lower arm) looks longer than the upper arm. Group B, X, X or E fall into this characteristic

3.       ACROCENTRIC; When the centromere tilts too much towards a side and making it look like the other arm is not present. Group D, G, and Y fall into this group

Based on the size they can also be grouped into; large, medium, short and very short.

Based on the combination of both size and centromere location they can be grouped into 7 classes ( looking at the human chromosome);

·         GROUP A ( LARGE METACENTRIC); They are the largest and have their cntromere towards trhe median plane

·         GROUP B (LARGE SUBMETACENTRIC); They are large and have their centromere tilting more towards a side of the arm

·         GROUP C (MEDIUM SUBMETACENTRIC); They are medium sized with submedian centromere

·         GROUP D ( MEDIUM SCROCENTRIC); They are medium sized with acrocentric centromere

·         GROUP E (SHORT META/ SUBMETACENTRIC ) Theyare short with either metacentric or submetacentric centromere

·         GROUP F (SHORT METACENTRIC); They are short with metacentric centromere

·         GROUP G (VERY SHORT ACROCENTRIC) ; They are very short with acrocentric centromere

·         Chromosome X ( MEDIUM SUBMETACENTRIC); They are medium sized with submetacentric centromere

·         Chromosome Y ( VERY SHORT ACROCENTRIC) ; They are very short with acrocentric centromere.

ARRANGING KARYOTYPE; In arranging karyotype,

Ø  Chromosomes are arranged with short hand facing upwards and long hands facing downwards

Ø  Regions and sub-regions that are differentially stained are labeled with numbers from the proximal  to the distal end of the chromosome arm

The best blood used for karyotyping is the peripheral blood and the lymphocytes in particular ( because the red blood cells lack nucleus). Nevertheless, the components in the lymphocyte is non-dividing so we use PHA (phytohemaglutinin) to stimulate cell division.

ALTERATION IN CHROMOSOMES

 This can be number alteration or structural alteration. Number alteration is caused by;

NONDISJUNCTION;This occurs as a result of faulty separation of homologous pair during anaphase 1 of meiosis or sister chromatids during anaphase 2. This leads to  aneuploidy

·         ANEUPLOIDY; This is a codition that arises as a result of the unity of gametes (during fertilization)  that have been victims of disjunction. The resulting cells have abnormal chromosome number. It could be trisomic ( having an extra chromosome e.g. Down’ssyndrome) or monosomic ( missing a chromosome eg Tuner’s syndrome)

Structural alterations could arise as a result of;

TRANSLOCATION; This results from the breaking away of a part of a chromosome and become joined to a chromosome whose part is also broken

DELETION; This results also from the breaking away of a part of a chromosome, but different from translocation in that it does not become attached to another or complemented by another.

INVERSION; This results from chromosomal breakage, flipping over and reattachment to the same chromosome. The  reattached segment is now and inverted sequence of the normal.

KARYOTYPE ANALYSIS

This involve four steps;

·         Collecting chromosomes; this has to do with the various steps in setting up the karyotype for study

·         Matching chromosomes; Chromosomes are matched with their homologous pairs. One of each pair is then numbered and the numbering is from the largest to the smallest. Matched pair have to have the same size, banding pattern and centromere location.

·         Diagnosis of chromosome; This involves the study of the chromosomes for any abnormality.

·         Researching abnormality; Here we take closer look and study of the abnormality observed.

STEPS IN CHROMOSOME DIAGNOSIS

NUMBER DIAGNOSIS; This is targeted at general look at the number for any deficit or excess

Ø  ARE THERE 46 CHROMOSOMES (FOR HUMAN)… IF YES GO TO  HOMOLOGUE DIAGNOSIS

Ø  IF THERE ARE NOT 46 (LESS OR MORE)…. GO TO NUMBER SPECIFIC DIAGNOSIS

NUMBER SPECIFIC DIAGNOSIS; This is targeted at  knowing what type of number alteration has occurred

Ø  IS IT A TRISOMY CONDITION ?

Ø  IS IT A MONOSOMY CONDITION ?

HOMOLOGUE DIAGNOSIS;  This is targeted at a general look at the matched pair for any mismatching

Ø  ARE ALL CHROMOSOME PAIRED WITH ITS DUE HOMOLOGUE OR ARE MISSING PIECES OR ADDITIONS ?

Ø  IF THERE IS A MISMATCH- NOT SIMILAR IN SIZE….. GO TO HOMOLOGUE SPECIFIC DIAGNOSIS

HOMOLOGUE SPECIFIC DIAGNOSIS; This is targeted at discovering what type of mismatch has occurred

Ø  IS IT A TRANSLOCATION CONDITION ( ADDITIONAL PIECES) ?

Ø  IS IT A DELETION CONDITION (MISSING PIECES) ?

EXAMPLES OF CHROMOSOMAL ABBERATIONS

TRISOMY 13, XX  (PATAU SYNDROME); Trisomy 13 (47,XX,+13)

TRISOMY 18 (EDWARD’S SYNDROME); Trisomy 18 (47, XY,+18)

TRISOMY 21 (DOWN’S SYNDROME); Trisomy 21 (47, XY, +21)

TRISOMY 16  X

Some occur on the sex chromosomes – disjunctions on sex chromosomes

KLINEFELTER SYNDROME ( 47,XXY) males

VARIATION OF KLINEFELTER’S SYNDROME  (49, XXXXY) males

JACOB’S SYNDROME (47, XYY) males

TRISONOMY X (47, XXX) females

TURNER’S SYNDROME (45, X0); This is a case of monosomy X.