Cytogenetics

Cytogenetics deals with chromosomal structures and their inheritance as applied to clinical medical genetics. Chromosomes are microscopic structures found in cells and malformations associated with them lead to numerous genetic diseases. Over the years, chromosomal analysis has improved in precision and resolution, and has led to improved diagnosis of various genetic diseases in all areas of medicine.

Genetic diseases begin with a mutation in chromosomal structure. The hundreds of types of genetic diseases include congenital abnormalities, reproductive wastage, and retarded mental growth. Statistics show that approximately 1 percent of all live births have congenital abnormalities (2 percent to women over 35) and they are present in 50 percent of all spontaneous abortions. Some common genetic disorders are Down syndrome, Turner's syndrome, cystic fibrosis, and Huntington's disease.

The study of chromosomes begins with the extraction of intact chromosomes from live body cells. The most common cells used for chromosomal studies [Page 468]are white blood cells (T lymphocytes) because they grow rapidly in cell cultures. These cells are arrested in metaphase, and treated with hypotonic solutions to release their chromosomes. The only disadvantage is that these cell cultures live only three to four days. Chromosomes may also be extracted from skin fibroblasts, bone marrow cells, lymphoblastoid cells, and fetal cells (from amniotic fluid and chorionic villus biopsy).

After cells are extracted, they have to be viewed so that chromosomes can be identified for abnormalities. The 48 pairs of chromosomes can be identified by using various staining techniques such as Q banding, R banding, C banding, G banding, and fluorescence in situ hybridization (FISH). G banding is the most used chromosomal staining method. Chromosomes are first treated with trypsin and then with Giemsa stain. All chromosomes can be individually identified using this technique. With Q banding, chromosomes are stained with quinacrine mustard or related compound, and examined by florescence microscopy, which is useful for detecting heteromorphisms—occasional chromosomal structural and staining variants. Before chemical staining using the R banding menthod, chromosomes are treated with heat, resulting in patterns that are easier to analyze. In C banding, regions of chromosomes containing heterochromatin are stained.

FISH is used to examine the presence or absence of a particular DNA sequence, and the number or organization of a chromosome or chromosomal region. With FISH, technicians use DNA probes to detect individual DNA sequences and copies of a particular chromosome. These probes can also be developed to scale entire lengths, and “paint” target chromosomes in metaphase and anaphase.