Numerical Abnormalities Of Chromosomes

The term "numerical abnormality" refers to gain or loss of chromosomes. As outlined above, all such abnormalities are presented in numerical order with the exception of the X and Y, which are always listed first. To designate an additional or a missing chromosome plus (+) and minus (-) signs

Fig. 5. (continued)

are placed before the specific chromosome number. Thus, -7,+18 would mean a missing chromosome 7 and an extra chromosome 18. Note that these abnormalities are presented in numerical order, regardless of whether they involve gain or loss of a chromosome. A + sign can also be used to denote additional copies of derivative chromosomes or accessory marker chromosomes, e.g., +der(6) or +mar (see below).

Numerical Abnormalities Involving the Sex Chromosomes

These can be constitutional (congenital) or acquired. ISCN 1995 provides special ways to distinguish between the two. As shown in the examples below, the + and—signs are not needed to designate constitutional sex chromosome aneuploidies.

Constitutional Sex Chromosome Aneuploidies

45,X Classical monosomy X or Turner syndrome

47,XXY Classical Klinefelter syndrome

47,XXX A female with three X chromosomes

48,XXYY Variant of Klinefelter syndrome with two X and two Y chromosomes

Acquired Sex Chromosome Aneuploidies

These involve chromosome changes seen in certain leukemias and solid tumors and are restricted to the affected tissues:

This describes a normal female with two X chromosomes but with the loss of one X chromosome in her tumor cells.

This is a normal female with two X chromosomes and gain of an extra X chromosome in her tumor cells.

This is a normal male with XY chromosomes and loss of the Y chromosome in his tumor cells.

This describes a male with acquired X and Y chromosomes in his tumor cells. 48,XXYc,+X

Here, we have a patient with Klinefelter syndrome who has an acquired X chromosome in his tumor cells. The letter "c" is placed next to XXY to show that the patient's sex chromosome complement is XXY and not XY or XXXY.

Fig. 5. (continued)

This is a Turner syndrome patient (45,X) with gain of an X chromosome in her tumor cells.

Numerical Abnormalities of the Autosomes

The situation here is similar to that involving the sex chromosomes, with the exception that (+) and (-) signs are used to designate constitutional abnormalities:

47,XY,+18 Male with trisomy 18

48,XX,+18+21 Female with both trisomy 18 and trisomy 21

45,XY,-21 Male with monosomy 21

46,XY,+21c,-21 Male trisomy 21 patient with loss of one chromosome 21 in his tumor cells 48,XX,+21c,+21 Female with trisomy 21 and gain of an additional chromosome 21 in her tumor cells

Mosaics and Chimeras

An individual with two or more cell types, differing in chromosome number or structure is either a mosaic or a chimera. If the two cell types originated from a single zygote, the individual is a mosaic (mos). If the cell types originated from two or more zygotes that subsequently fused, the individual is a chimera (chi). In designating mosaic or chimeric karyotypes, a slash (/) is used to separate the cell lines. The actual number of cells detected in each clone can be given within square brackets [ ]. The largest clone is recorded first, then the next largest, and so on. Whenever a normal cell line is present, it is always recorded last, irrespective of the number of normal cells detected.

Fig. 5. (continued)
Fig. 5. (continued)

Examples are as follows:

This is a Turner mosaic with two cell lines. Analysis of 20 cells showed that this individual has 4 cells that are 45,X and 16 cells that are 46,XX.

This represents a mosaic with three cell lines.

This is a mosaic with both trisomy 13 and normal cell lines.

Fig. 5. (continued)

In a chimera where the two cell lines are normal (46,XX and 46,XY) and both are present in equal proportions, either one of them can be listed first. If one cell line is larger than the other, the larger clone is listed first.

This describes a chimera with female and male cells in equal number.

This is a chimera with both female and male cell lines. The female cell line shows trisomy 21, whereas the male cell line is normal.

This represents a chimera with triploid and diploid cell lines. The triploid line is XXX, whereas the diploid line is XY.

Use of the abbreviations chi and mos is optional, as the presence of chimerism or mosaicism is usually evident from the karyotype.

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