|MoBio||Chromosome Abnormality||Chapter 8|
The cell contains three haploid sets of chromosomes (69,XXX or 69,XXY) caused by one of the following mechanisms:
The cell has only one copy of a specific chromosome. For example, individuals with Turner's syndrome have a single X chromosome and no Y.
The cell has an extra copy of a specific chromosome. Examples:
Both monosomy and trisomy arise from the same mechanism as explained in the following figure.
Chromosomal translocation is the interchange of parts between nonhomologous chromosomes. It is caused by breaks in more than one chromosome, followed by attachment of a chromosome fragment to a different chromosome. There are two main types: reciprocal translocation (Figure 8-E-2) and Robertsonian translocation.
Robertsonian translocation is caused by breaks at the centromeres of two nonhomologous acrocentric chromosomes (chromosomes with centromeres near their ends, e.g., chromosome 13, 14, 21, and 22). After the breaks, the long arms of two chromosomes join together to form a single chromosome. The short arms may join to form another chromosome, but it is usually lost within a few cell divisions. Therefore, the karyotype of a person with Robertsonian translocation usually exhibits only 45 chromosomes.
In addition to the trisomy 21 mentioned above, the Down's syndrome may also be caused by Robertsonian translocation, in which the long arm of chromosome 21 is attached to another chromosome, usually chromosome 14 or itself.
Inversion, Deletion and Ring Structure
Two breaks in a single chromosome can cause inversion, deletion or ring structure as shown in the following figure.