Terms associated with chromosomal appearances and abnormalities are outlined below:

TermAbbreviationDescription
CentromerecenThe region that connects the two arms of the chromosome.
Deletion delLoss 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)
DuplicationdupA segment of chromatin that is repeated on the same or another chromosome. Many are incompatible with life. Example: 46, XY, dup(1)(q11q22)
Fragile SitefraThere is a site on one arm that has a demonstratable fixed break point. Example: 46, fra(X)(q27.3),Y
InsertioninsThere 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)
IsochromosomeiThe 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)
InversioninvTwo 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)
MarkermarA 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 armpThe short arm of the chromosome
q armqThe long arm of the chromosome
RingrBreakage of a chromosome at two points is followed by repair with fusion to a circular chromosome. Example: 46, XX, r(1)(p36q44)
TranslocationtThere 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)
Triploidy69,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


Cells with a multiple of 23 chromosomes are euploid. Germ cells (spermatozoa, ova) are haploid with 23. A polyploid cell has a multiple of 23. Normal somatic cells are diploid with a complement of 46 chromosomes. Abnormal euploid numbers seen in humans include triploidy (69 chromosomes) and tetraploidy (92 chromosomes).

Aneuploidy refers to an abnormal number of chromosomes that is not a multiple of 23. There is either monosomy (one less chromosome of a pair), or trisomy (one extra chromosome). In general, either extra genetic material, or less, is detrimental. Most cases of aneuploidy result in fetal loss. In general, abnormalities of sex chromosomes are better tolerated than abnormalities of autosomes. Thus, aneuploidy can involve:

  • Autosomes (chromosome pairs 1 Ð 22)

  • Sex chromosomes (X or Y)

These abnormalities often occur because of nondisjunctional events in meiosis.

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.

Partial Aneuploidy

Rarely, a translocation of part of one chromosome to another in the parent will be passed on to the child as a partial trisomy (such as 6p+ or 16p+) which may not be as severe as a complete trisomy.

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.

Microdeletion

Microdeletions involve only small parts of chromosomes, but the missing segment may have important genes. An example of a microdeletion syndrome is the Prader-Willi syndrome characterized by decreased mental function, speech problems, and excessive eating.

Uniparental Disomy

Uniparental disomy is a rare phenomenon in which each of a pair of chromosomes came from one parent. This can occur when trisomy is followed by loss of one extra chromosome or when monosomy is followed by nondisjunction. Some cases of Prader-Willi syndrome result from uniparental disomy.