Chromosome analysis is the oldest genetic examination method. As part of this procedure, a karyogram (ordered representation of all chromosomes in a cell) is made. This allows changes in the number as well as in the structure of the chromosomes to be detected (numerical/structural chromosomal aberrations).
Humans have 46 chromosomes. The chromosome pairs 1-22 are the autosomes, the 23rd chromosome pair are the sex chromosomes (gonosomes; XY in man and XX in woman).
Chromosome analysis is still the method of choice for detecting a numerical chromosomal aberration: Ullrich-Turner syndrome (monosomy X), Klinefelter syndrome (karyotype: 47,XXY), trisomy 21, 13, 18 (Down, Pätau, Edwards syndrome).
Chromosomal analysis is used in prenatal diagnostics as well as postnatal diagnostics.
Indications (areas of application)
- Genetic diseases in the family.
- Relatives of individuals with structural chromosomal abnormalities.
- Age indication – if the expectant mother has exceeded 35 years of age.
- Suspected genetic diseases such as:
- Chromosomal syndromes such as Down syndrome, Turner syndrome, Cri-du-Chat syndrome, Prader-Willi syndrome.
- Newborns with congenital malformations (eg, hypospadias/developmental disorder of the urethra) or intersex genitals
- Infertility diagnostics – when the cause of infertility is unclear.
- Infertility diagnostics – condition after two or more spontaneous abortions (miscarriages) or stillbirths.
The procedure
Material needed in the unborn child (prenatal – before birth).
- Chorionic villi (in the 11th-14th week of pregnancy (SSW)).
- Amniotic fluid (from the 14th SSW).
- Fetal blood (from the 22nd SSW).
Material needed in children and adults
Preparation of the patient
- Not necessary
Disruptive factors
- None known
The laboratory method
To produce a karyogram, cells in culture are stimulated to divide. Then, the cell division process is stopped with the help of the spindle poison colchicine (poison of autumn crocus). The cell or its chromosomes are photographed and the individual chromosomes are excised from the photograph (using digital image techniques on a computer monitor). Then the chromosomes are stained with special dyes on the slide. Most often, GTG banding (G-bands by Trypsin using Giemsa) is used for staining. This allows changes in the number as well as in the structure of chromosomes to be detected (so-called numerical/structural chromosomal aberrations.
The resolving power of chromosomes by chromosome analysis is very limited. Depending on the position in the chromosome, deletions or duplications can only be detected up to a minimum of 5-10 mega base pairs. It should also be noted that smaller mosaics are not detected by conventional chromosome analysis.
Nowadays, higher resolution laboratory methods offer the possibility to detect the loss or gain of chromosomal material much better.
