Chromosomal Abnormalities

Chromosomal abnormalities – sometimes also called cytogenetic disorders – are very common. Although we don´t see many affected people. How it this possible? Most fetus with some chromosomal abnormality usually do not survive. About 50% of first–trimester abortions is connected with some cytogenetic mistake. The incidence of chromosomal abnormalities is approximately 1 out of 200 of newborns. .

We recognize two types of chromosomal abnormalities: We are able to find the disorders due to karyotype testing. The cytogeneticists get the samples (blood, amnionic fluid), then stain the chromosomes and arrange them in the right order by their length. Then we can see some changes in pattern (banding). The most typical method is Giemsa stain, so it is usually called "G banding". This is important for structural abnormalities. Numeral abnormalities we can identify even easier.
 * 1) numeric;
 * 2) structural.

Other classification of chromosomal abnormalities depends on fact which type of chromosome is affected – autosomes (Down syndrome, DiGeorge syndrome) or sex chromosomes (Klinefelter syndrome, Turner syndrome).

Translocation

 * During translocation, a part of one chromosome is transferred to another chromosome.
 * It is very important whether the translocation is balanced or unbalanced. Balanced means that two chromosomes just exchange their parts but the number of chromosomes(46 chromosomes) as well as no loss of genetic material stays the same.
 * Fortunately – typical place of break is near the centromere, usually only small arms of the acrocentric chromosomes are lost. There are no crucially important genes coded by these chromosomal segments. So, a carrier of such Robertsonian translocation can normally survive this cytogenetic change.
 * Unfortunately problems occur during the fertilization with a gamete of a carrier of a balanced translocation. Carrier of the translocation may produce unbalanced zygotes, because the process of homologous chromosomes pairing during meiosis is interrupted. This is very important because unbalanced gametes lead to abnormalities in offspring. The reason is that the offspring receives altered chromosome from the carrier which may lack several important genes.
 * Therefore the only clinical symptom found in the carriers of balanced translocations may be the reproduction failure.

Types of translocation

 * 1) reciprocal – translocation between two chromosomes ("A segment" goes to "B chromosome" and "B segment" goes to "A chromosome")
 * 2) Robertsonian – translocation (or fusion) of two acrocentric chromosomes

Deletion

 * Deletion is characterized by the loss of a part of chromosome. Two breaks have to occur for deletion of the interstitial segment. For deletion of terminal segment (telomere) one break is enough. Segments which were deleted from the chromosome are not able to "live" on their own and the genes present in those segments are lost.
 * One special example of deletion exists. It is called "ring chromosome". It is a situation when chromosome lost both of its ends. The long and the small arms then connect together and chromosome became a ring shaped.

Isochromosomes

 * Isochromosomes are created by the incorrect division of centromere. Normally centromere divides vertically. In this case it divides horizontally.
 * The result is usually the loss of one arm. It means that newly created chromosome has just two long arms or two short arms which are normally connected by centromere.
 * It occurs relatively frequently in X chromosome.
 * It is a huge problem during the fertilization. Because fetus then becomes trisomic for one arm and monosomic for the second arm.

Inversion

 * For inversion are typical two breakages in the different part of the chromosome. The newly created segments then replace each other.
 * Inversion was discovered in 1921.
 * Although we still don´t know why inversion exists, we know that it is the most important mechanism of reorganizing of the genome.
 * We recognize 2 types of inversion:
 * 1) pericentric – causing deletions, insertions or abnormal centromeres;
 * 2) paracentric – more common type, it is less harmful for its carrier.
 * Inversion suppresses the recombination process.

How We Denote Parts of a Chromosome
When we want to describe the location of some structural abnormality we used special marks. We have to count the regions always from the centromere – for both arms. Example: 1q13 So the change is located on the long arm of chromosome 1 (see Fig. Chromosomal marking).
 * the first number is a number of chromosome – chromosome 1;
 * (p) and (q) are used for marking of arm – (p) for small arm and (q) for long arm – the long arm;
 * the third number is for the region of chromosome – region 1;
 * last number is for theband – band 3.

Related articles

 * Karyotype
 * Disorders of the Autosomes
 * Disorders of the Sex Chromosomes
 * The Sex Chromosomes and Sex Determination