Cytogenetics

Terms associated with chromosomal appearances and abnormalities are outlined below:

Term	Abbreviation	Description
Centromere	cen	The region that connects the two arms of the chromosome.
Deletion	del	Loss of all or part of one chromosomal arm, usually a part toward the end of an arm. An acentric chromosome (without a centromere) is typically lost completely after a few cell divisions. Examples: 46, XX, 4p- or 46, XX, del(13)(q12q14)
Duplication	dup	A segment of chromatin that is repeated on the same or another chromosome. Many are incompatible with life. Example: 46, XY, dup(1)(q11q22)
Fragile Site	fra	There is a site on one arm that has a demonstratable fixed break point. Example: 46, fra(X)(q27.3),Y
Insertion	ins	There must be two break points in one chromosome that allow a segment to insert between a break point in another chromosome. Example: 46, XX, ins(11)(p14q23q24)
Isochromosome	i	The arms of a chromosome are mirror images of each other. Monocentric isochromosomes have two long arms or two short arms. Dicentric isochromosomes have mirrored material between centromeres. Example: 46, X, i(Xq)
Inversion	inv	Two break points in a chromosome allow a 180 degree turn of a segment with fusion. The inversion is "paracentric" if it only involves one arm, while "pericentric" inversions involve the centromere and parts of p and q arms. Example: 46, XX, inv(8)(p23q22)
Marker	mar	A small extra piece of chromosomal material that travels with the other chromosomes. Example: 46, XY, +mar
Monosomy	-	Only one of a chromosome pair is present. Autosomal monosomies yield non-viable embryos. Only monosomy X is rarely liveborn. Example: 45, X or 45, XY, -14
Mosaic	/	Cells with an abnormal karyotype are present along with normal cells. Example: 45,X/46,XX
p arm	p	The short arm of the chromosome
q arm	q	The long arm of the chromosome
Ring	r	Breakage of a chromosome at two points is followed by repair with fusion to a circular chromosome. Example: 46, XX, r(1)(p36q44)
Translocation	t	There is an exchange of material between arms on different chromosomes. The translocation is "reciprocal" if there is an exchange of two chromosome segments on different (nonhomologous) chromosomes. It is "balanced" if no genetic material is lost. "Unbalanced" translocations involve more breakpoints that lead to loss of genetic material in the exchange. Example: 46, XY, t(9;22)
Triploidy	69,	Three haploid sets of chromosomes are present in a cell. Example: 69, XXY
Trisomy	+	An extra chromosome is present, making three copies. Extra genetic material reduces viability. Example: 47, XX, +18

Some of the more common abnormalities are diagrammed below:

Deletion

A portion of either the long or short arm of a chromosome may be lost, and this is termed a deletion. The severity of this lost genetic material depends upon the size of the deletion and the nature of the genetic material contained within it.

Inversion

A portion of a chromosome may occur when there are two interstitial break points, allowing a segment to turn around and then reunite.

Translocation

A "reciprocal" translocation occurs when there are break points in two chromosomes, with exchange of the fragments between these chromosomes, without loss of any portion. This is "balanced" because the total amount of genetic material stays the same.

A special type of translocation is called "centric fusion" or "Robertsonian translocation" when two acrocentric chromosomes (with very short "p" arms) have break points very close to the centromere. There is subsequent fusion of the long arms. The fused short arms are often lost, but contain so little genetic material that the consequences are not severe. The carrier of such a "Robertsonian" translocation will appear normal, though only 45 chromosomes are present, but fertilization with gametes from such a carrier will result in half of all zygotes being non-viable, in the case of Down syndrome where chromosomes 14 and 21 are involved.

Fertilization with a normal gamete and a "Robertsonian" gamete will potentially yield non-viable monosomy 14, trisomy 14, and monosomy 21. Potential viable fertilized eggs may be normal, be balanced carriers, or have trisomy 21.

Mosaicism

A mosaic is a person with a combination of two cell lines with different karyotypes (normal and abnormal). When karyotyping is performed, multiple cells are analyzed to rule out this possibility. An example would be a Turner's mosaic, with: 45,X/46,XX in cells karyotyped. Mosaicism, with a component of normal cells present, may mitigate some of the more severe chromosomal abnormalities, such as trisomies, and account for livebirths or longer survival.

Confined Placental Mosaicism

Sometimes the abnormal cell line may appear only in the placenta, while the fetus has a normal karyotype. This may account for some stillbirths where no fetal abnormality can be found, other than growth retardation. Such a placenta may be smaller than normal. Conversely, a normal placental karyotype may allow longer survival of a fetus with an abnormal karyotype.

Marker Chromosome

A small extra piece of genetic material (chromosomal "junk") may appear along with a normal set of chromosomes. This "marker" chromosome has significance because, if it arises de novo, then there is an increased risk for mental retardation for the fetus. However, if a parent has the marker as well, then it is familial and unlikely to be associated with mental retardation.