What is a Karyotype?
The number and appearance of chromosomes (made up of DNA and supercoiled proteins) in an organism is known as a karyotype.
There are 2 kinds of cells, one is somatic cells and other is gametic cells. So, in the karyotype of a somatic cell of a normal human individual, there are 23 pairs of chromosomes, thus a total of 46, and there is only one pair of chromosomes in the karyotype of gametic cells.
For male karyotype, it has XY and for female karyotype it has XX.
What is Karyotyping?
Karyotyping is a process of pairing, arranging, and chromosomal organization to find out the pattern in chromosomal variations using a karyogram.
During the metaphase of the cell division cycle, a stained photograph of a nucleus having chromosomes is taken, which is known as a karyogram in which all the chromosomes are organized from larger to smaller in size and numbered with genetic chromosomes in the last.
It is the traditional and one of the most common techniques of cytogeny in practice. This technique of cytogeny is tedious and time taking and also less effective. It requires the use of other techniques like cell culture, staining, and banding which are the main elements in performing karyotyping.
Sample collection and tissue culture: Complete media such as RPMI-1640 is generally used to culture blood and the experiment is performed under aseptic conditions.
Arresting cell at metaphase: done by using colchicine for maximum visibility.
Separation of chromosomes: the hypotonic solution is used to separate the chromosomes.
Staining and banding of chromosomes: Staining is different for a different purpose.
- For numerical and structural chromosomal changes
- Trypsin this used to digest the protein present in chromosomes.
- Giemsa stains heterochromatin: dark blue
- Euchromatin: light color
- For the centromeric region of the chromosome.
- The banding pattern is the same as the G type.
- requires high-temperature treatment along with acid and alkali to open up the chromosome.
- fluorescent dye: Quinacrine.
- the process is similar to that of G-banding
- Quinacrine functions by binding to the AT-rich region of a chromosome or the heterochromatin region and emitting yellow fluorescence.
- Aka reverse banding: banding pattern is opposite from G banding.
- euchromatin: dark color, and heterochromatin: bright color.
Application of Karyotyping
A. To identify the difference in the number of the chromosomes
- main applications of karyotyping
- look for any variation from a normal individual’s karyotype
- G banding is used.
Some important genetic diseases that can be identified using karyotyping:
1. Down syndrome
- chromosome 21 has an extra chromosome(47,21+)
- It occurs in about 1 childbirth out of 800 live births.
- Symptoms: fold in either eye, a flat facial appearance, a round head, short height, furrowed tongues, mental retardation.
2. Patau syndrome
- Extra chromosome on chromosome 13 (47, 13+).
- Symptoms: mental retardation, growth failure, deafness, kidney cyst, double ureter, etc.
3. Edward syndrome
B. To identify the size variation in chromosomes: Deletion, insertion, duplication in size of chromosome.
1. Cri du chat syndrome
- deletion in the small arm of chromosome 5 (P arm): 46, 5P
- Symptoms: complications in gastrointestinal, cardiac muscles, mental retardation, large larynx, and glottis with a characteristic cry voice.
C. To identify variation in centromere location: using C banding pattern .identification of inversion and translocation in centromere.
D. To study satellite and telomere region in chromosome: telomere is a repetitive region at the end of the chromosome. Important for replication of DNA. Any change in this can be identified using T banding.
- Klug, W.S., Cummings, M.R., Spencer, C.A., et al. (2017) Essentials of Genetics. 7th Edition, Higher Education Press, Beijing, 446-464
- Bates SE. Classical cytogenetics: karyotyping techniques. Methods Mol Biol. 2011;767:177‐190.
- Schreck RR, Distèche C. Karyotyping. CurrProtoc Hum Genet. 2001; Appendix 4.