Chromosomal abnormalities, alterations and aberrations are at the root of many inherited diseases and traits. Chromosomal abnormalities often give rise to birth defects and congenital conditions that may develop during an individual's lifetime. Examining the karyotype of chromosomes (karyotyping) in a sample of cells can allow detection of a chromosomal abnormality and counselling can then be offered to parents or families whose offspring are at risk of growing up with a genetic disorder.
Types of chromosomal abnormality
A chromosomal abnormality may be numerical or structural and examples are described below:
Numerical abnormalities
The normal human chromosome contains 23 pairs of chromosomes, giving a total of 46 chromosomes in each cell, called diploid cells. A normal sperm or egg cell contains only one half of these pairs and therefore 23 chromosomes. These cells are called haploid.
The euploid state describes when the number of chromosomes in each cell is some multiple of n, which may be 2n (46, diploid), 3n (69, triploid) 4n (92, tetraploid) and so on. When chromosomes are present in multiples beyond 4n, the term polyploid is used.
Aneuploidy refers to the presence of an extra chromosome or a missing chromosome and is the most common form of chromosomal abnormality. In the case of Down's syndrome or Trisomy 21, there is an additional copy of chromosome 21 and therefore 47 chromosomes. Turner's syndrome on the other hand arises from the absence of an X chromosome, meaning only 45 chromosomes are present.
Occasionally, aneuploid and regular diploid cells exist simultaneously and this is called mosaicism. The condition involves two or more different cell populations from a single fertilized egg. Mosaicism usually involves the sex chromosomes, although it can involve autosomal chromosomes.
In contrast to mosaicism, a condition called chimaerism occurs when different cell lines derived from more than one fertilized egg are involved.
Structural abnormalities
Structural abnormalities occur when the chromosomal morphology is altered due to an unusual location of the centromere and therefore abnormal lengths of the chromosome's short (p) and long arm (q).
- If the centromere is in the middle of the chromosome and the arms are about the same length, the chromosome is called metacentric. If the centromere is near to one end and the arms are unequal in length, the chromosome is called submetacentric. When the centromere is so close to one end that the short arm is very small, the chromosome is said to be acrocentric. Another abnormality is the presence of two centromeres, when a chromosomes is called dicentric, and when there is no centromere at all, it is referred to as acentric.
- In chromosomal analysis, chromosomes are stained to produce banding patterns that can be used to detect any alteration of structural arrangements within or between chromosomes. This may involve chromosomal breakage and rearrangement within the chromosome or with two or more other chromosomes. This can result in an unbalanced karyotype if chromosomal material is gained (insertion mutation) or lost (deletion mutation) during the process.
- When a chromosome breaks and unites with one or more other chromosome, this is called translocation. For example, the chromosomal translocation 9:22 means a part of the chromosome 9 was detached and then reattached to chromosome 22. This is called a Philadelphia chromosome and its presence increases the risk of developing chronic leukemias.
Examples of chromosomal disorders
Some of the most common chromosomal abnormalities include:
- Down's syndrome or trisomy 21
- Edward's syndrome or trisomy 18
- Patau syndrome or trisomy 13
- Cri du chat syndrome or 5p minus syndrome (partial deletion of short arm of chromosome 5)
- Wolf-Hirschhorn syndrome or deletion 4p syndrome
- Jacobsen syndrome or 11q deletion disorder
- Klinefelter's syndrome or presence of additional X chromosome in males
- Turner syndrome or presence of only a single X chromosome in females
- XYY syndrome and XXX syndrome
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