Cytogenetics is the study of chromosomes, specifically, diseases associated with abnormalities in chromosomal structure. Chromosomes are best studied in white blood cells (T lymphocytes) because these cells grow rapidly in cell culture. 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 3-4 days.
Study of these cell cultures help study various types of disease and other abnormalities.
Cells used for the study of chromosomes also include:
Bone marrow cells
Fetal cells (from amniotic fluid and chorionic villus biopsy)
Several methods have been devised to study chromosomes. For example, we have the G-banding method. Chromosomes are stained with Giemsa stain after penetrating chromosomes with trypsin. All 24 chromosomes are clearly identified.
According to these techniques, metaphase chromosomes are viewed as having alternating bands of black and white stripes. Furthermore, by chromosome number and band number, individual genes can be identified on individual chromosomes accurately and precisely. According to shape, there are three types of chromosomes according to the location of the centromere with respect to the center:
Metacentric: centromere is in the center
Submetacentric: centromere is off to the side and the two chromosome arms are of different lengths
Acrocentric: centromere is on the edge of the chromosome.
Another technique to view chromosomes is Fluorescence In Situ Hybridization (FISH). This technique has revolutionized cytogenetics because we can examine the presence or absence of a particular DNA sequence, the number or organization of a chromosome or chromosomal region. In FISH, we use DNA probes to detect individual DNA sequences, and copies of a particular chromosome. Additionally, these probes can be developed to scale entire lengths, and “paint” target chromosomes in metaphase and anaphase. Another application of FISH technique is to detect multiple probes attached to different chromosomes, for example, probes that have recognized telomers at ends of chromosomes.
There are two main types of chromosomal abnormalities: Aneuploidy and translocations.
Aneuploidy: Abnormal chromosomal number due to extra or missing chromosomes. Eg., Down Syndrome.
- Most common type
- 3 to 4 % of all clinically recognized pregnancies
- Further categorized into trisomy or monosomy
- Trisomy is usually caused by meiotic non-disjunction
- Nondisjunction is most common at meiosis I (left), but can occur during meiosis II, and even less frequently during mitosis
Balanced translocations: If, after the translocation, the cell ended up with a normal number of chromosomes. No symptoms are presented. More commonly found in couples that have had two or more spontaneous abortions. Main points:
- All material is present even if arranged differently.
- It is important to note that even though all material is present, it is unbalanced at the molecularly because it is arranged abnormally.
- This may not affect the individual in question, but subsequent generations.
- Inversions occur when a chromosome breaks at two points, the middle point inverts, and then rejoins the chromosome.
- Paracentric inversion occurs on one side of centromere
- Pericentric inversion occurs on both sides and includes the centromere
- Pericenric inversions can be identified only by FISH
- Inversions don’t usually result in abnormal phenotypes
- Reciprocal Translocations: Abnormal chromosome exchange between non-homologous pairs of chromosomes.
- Robertsonian Translocation involves two acrocentric chromosomes that fuse after loss of the short arms. This may result in a karyotype of 45 chromosomes.
Unbalanced translocations: There is additional or missing material. Main points:
- Deletions result in loss of chromosomal segment.
- This deletion results in people who have only one functioning chromosome from a pair of chromosome. This may result in haplo-insufficiency.
- Deletions also generated by abnormal segregation from a balanced translocation or inversion.
- Duplications originate by unequal crossing over or abnormal segregation.
- Less harmful.