Pregnancy Glossary: A to Z Guide to Pregnancy Terminology

Chromosomal Abnormalities

Chromosomal abnormalities, alterations and aberrations are at the root of many inherited diseases and traits. Chromosomal abnormalities often give rise to birth defects and congenital conditions that may develop during an individual's lifetime. Examining the karyotype of chromosomes (karyotyping) in a sample of cells can allow detection of a chromosomal abnormality and counselling can then be offered to parents or families whose offspring are at risk of growing up with a genetic disorder.

Types of chromosomal abnormality

A chromosomal abnormality may be numerical or structural and examples are described below:

Numerical abnormalities

The normal human chromosome contains 23 pairs of chromosomes, giving a total of 46 chromosomes in each cell, called diploid cells. A normal sperm or egg cell contains only one half of these pairs and therefore 23 chromosomes. These cells are called haploid.

The euploid state describes when the number of chromosomes in each cell is some multiple of n, which may be 2n (46, diploid), 3n (69, triploid) 4n (92, tetraploid) and so on. When chromosomes are present in multiples beyond 4n, the term polyploid is used.

Aneuploidy refers to the presence of an extra chromosome or a missing chromosome and is the most common form of chromosomal abnormality. In the case of Down's syndrome or Trisomy 21, there is an additional copy of chromosome 21 and therefore 47 chromosomes. Turner's syndrome on the other hand arises from the absence of an X chromosome, meaning only 45 chromosomes are present.

Occasionally, aneuploid and regular diploid cells exist simultaneously and this is called mosaicism. The condition involves two or more different cell populations from a single fertilized egg. Mosaicism usually involves the sex chromosomes, although it can involve autosomal chromosomes.

In contrast to mosaicism, a condition called chimaerism occurs when different cell lines derived from more than one fertilized egg are involved.

Structural abnormalities

Structural abnormalities occur when the chromosomal morphology is altered due to an unusual location of the centromere and therefore abnormal lengths of the chromosome's short (p) and long arm (q).

If the centromere is in the middle of the chromosome and the arms are about the same length, the chromosome is called metacentric. If the centromere is near to one end and the arms are unequal in length, the chromosome is called submetacentric. When the centromere is so close to one end that the short arm is very small, the chromosome is said to be acrocentric. Another abnormality is the presence of two centromeres, when a chromosomes is called dicentric, and when there is no centromere at all, it is referred to as acentric.

In chromosomal analysis, chromosomes are stained to produce banding patterns that can be used to detect any alteration of structural arrangements within or between chromosomes. This may involve chromosomal breakage and rearrangement within the chromosome or with two or more other chromosomes. This can result in an unbalanced karyotype if chromosomal material is gained (insertion mutation) or lost (deletion mutation) during the process.

When a chromosome breaks and unites with one or more other chromosome, this is called translocation. For example, the chromosomal translocation 9:22 means a part of the chromosome 9 was detached and then reattached to chromosome 22. This is called a Philadelphia chromosome and its presence increases the risk of developing chronic leukemias.

Examples of chromosomal disorders

Some of the most common chromosomal abnormalities include:

Down's syndrome or trisomy 21

Edward's syndrome or trisomy 18

Patau syndrome or trisomy 13

Cri du chat syndrome or 5p minus syndrome (partial deletion of short arm of chromosome 5)

Wolf-Hirschhorn syndrome or deletion 4p syndrome

Jacobsen syndrome or 11q deletion disorder

Klinefelter's syndrome or presence of additional X chromosome in males

Turner syndrome or presence of only a single X chromosome in females

XYY syndrome and XXX syndrome

Further Reading

Symptoms & Types

There are many different symptoms -- and several different types -- of bipolar disorder. The primary symptoms are dramatic and unpredictable mood swings. The types of bipolar disorder range from mild to severe.

Warning Signs of Bipolar Disorder

Bipolar disorder can be sneaky. Infrequent episodes of mania can go undetected. Depression can overshadow other aspects of the illness. And substance abuse can further cloud the picture.

Recognizing Bipolar Disorder

Bipolar disorder can be difficult to diagnose, but these tips may help you recognize the symptoms.

The 2 Phases of Bipolar Disorder

The primary symptoms of bipolar disorder are periods of elevated or irritable mood accompanied by dramatic increases in energy, activity, and fast thinking.

What are Mania and Hypomania?

How to recognize the symptoms of mania and hypomania, part of the manic phases of bipolar disorder.

Depression Symptoms in Bipolar Disorder

The dramatic mood episodes of bipolar disorder do not follow a predictable pattern.

Overview of Bipolar Disorder Types

There are several types of bipolar disorder; all involve episodes of depression and mania to a degree.

Bipolar I Disorder

A person affected by bipolar I disorder has had at least one manic episode in their life. Most people with bipolar I disorder also suffer from episodes of depression.

Bipolar II Disorder

In bipolar II disorder, the "up" moods never reach full-blown mania. These less-intense elevated moods are called hypomanic episodes.

Rapid Cycling

In rapid cycling, a person with the disorder experiences four or more episodes of mania or depression in one year. This can occur at any point in the course of bipolar disorder.

Mixed Bipolar Episodes

A person with mixed bipolar episodes has symptoms of both mood "poles" -- mania and depression -- at the same time or in rapid sequence.

What Is Cyclothymia?

In cyclothymic disorder, moods swing between short periods of mild depression and mania.

Bipolar Spectrum

The bipolar spectrum refers to conditions that include not only bipolar disorder but also other types of mental conditions that involve depression or mood swings.

Bipolar Disorder Resource Center

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