Chromosome Abnormality

Numerical abnormality

Triploidy

The cell contains three haploid sets of chromosomes (69,XXX or 69,XXY) caused by one of the following mechanisms:

1. Fertilization of a single egg by two sperm.
2. Fertilization between a normal egg and an abnormal diploid sperm.
3. Fertilization between an abnormal diploid egg and a normal sperm.

Monosomy

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.

Trisomy

The cell has an extra copy of a specific chromosome.  Examples:

•  Down's syndrome - with three copies of chromosome 21.
•  Klinefelter's syndrome - with three sex chromosomes (XXY).

Both monosomy and trisomy arise from the same mechanism as explained in the following figure.

Figure 8-E-1.  Origins of monosomy and trisomy.  
(a) During mitotic metaphase or meiotic metaphase II, one pair of sister chromatids does not align at the equatorial plane.  Consequently, both sister chromatids move to the same cell.  One daughter cell will show trisomy and another daughter cell will have monosomy.  
(b)  During meiotic metaphase I, two homologous pairs of sister chromatids do not align at the equatorial plane and subsequently move to the same cell (a phenomenon called nondisjunction).  The resulting daughter cells also have either monosomy or trisomy.

Structural abnormality

Chromosomal Translocation

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.

Figure 8-E-2. An example of reciprocal translocation. Breaks occur first in both chromosome 4 and chromosome 20. Then one fragment of chromosome 4 attaches to a fragment of chromosome 20 and another fragment of chromosome 4 attaches to another fragment of chromosome 20. Picture courtesy: National Human Genome Research Institute

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.  

Figure 8-E-3.  Chromosomal abnormality resulting from two breaks in a single chromosome. Inversion: The segment between two breakpoints is inverted before resealing the breaks. Deletion: The breaks reseal without including the segment between breakpoints.  Examples: cri-du-chat syndrome and William's syndrome. Ring chromosome: Two ends of the segment between breakpoints are joined to form a circular structure.