chro·mo·some (‘krO-m&-“sOm, or ‘krO-m&-“zOm)
A chromosome is a microscopic, rod-shaped, threadlike linear strand of genetic material including DNA (deoxyribonucleic acid) and associated proteins, which functions in the transmission of hereditary information. Chromosomes are found in the nucleus of all animal and plant cells. Except in the case of aneuploidy, every individual in the same species has the same number of chromosomes, of which there are typically a total of 46 chromosomes (23 pairs) in humans. Of these, 22 pairs are known as “automsomes” (chromosomes which are not sex chromosomes) plus the X and/or Y chromomsomes, which are referred to as sex chromosomes since the makeup of this pair will determine the sex of the individual.
The scientific study of chromosomes is known as “cytogenetics.” There are a number of resources on the Internet for learning more about chromosomes, DNA and cytogenetics. If you’d like to learn more, one place to start is the Genetic Science Learning Center at the University of Utah, which provides a website geared to the general public, along with classroom activities and teacher guides, including an interactive tool that will all you to perform a karyotype. The index to this website sponsored by the University of Utah is located at http://gslc.genetics.utah.edu/units/basics/index.cfm.
In everyone’s body, each cell contains tiny twisted strings of molecules called chromosomes that contain genes, which in turn tell cells how they will grow and what they must do. Typically, humans have 23 pairs of chromosomes (46 chromsomes in the aggregate), and each parent commonly contributes one chromosome to each pair. These chromosomes are numbered and arranged when they are analyzed by labs in a test known as a karyotype. The 23rd pair, for example, are known as the sex chromosome pair. Except in certain instances of X and/or Y chromosome aneuploidy, a mother and father each contribute a single sex chromosome to the child. Thus, girls commonly have two X chromosomes (one from mother and one from father), while boys commonly have one X (from mother) and one Y (from father) chromosome.
Occasionally, a cell division anomaly known as “non-disjunction” may result in some (in the case of mosaicism or all cells having 44, 45, 47, 48 or 49 chromosomes. These chromosomal states in which one or more whole chromosomes are either missing, or are present in more than the typical number are referred to as an “aneuploidy” or plural, as “aneuploidies.”
Depending on context, the word “karyotype” may refer to (1) a kind of genetic test that is performed on cells taken from a blood sample or amniotic fluid (i.e., “Perhaps you should consider ordering a karyotype in order to confirm whether or not an aneuploidy is responsible for these symptoms”); or (2) the chromosomal characteristics of a cell or an individual (i.e., “We have confirmed through genetic testing that his karyotype is 47,XXY”).
A karyotype is created by staining the chromosomes with dye and photographing them through a microscope. This photograph, such as the one shown on the left, is then cut up and sorted so that the chromosomes are arranged into pairs according to number, size, shape and other characteristics. Although this test represents the only way to be certain about the presence of an aneuploidy, it is expensive (generally between $200 and $400) and some doctors are therefore reluctant to order it unless pressed by the patient. Additionally, there are a number of less costly tests from which a karyotype may be inferred (for example, in the case of X chromosome variations, a Buccal Smear test for Barr bodies, as well as the use of certain specialized DNA probes). Scientists are also working on a very low cost test (under $2 per test when performed on a large population) that could be used to screen all newborns at birth for X and/or Y chromosome variations. While these less costly tests have a somewhat higher error rate, if an atypical result is found, then a full karyotype can be ordered to confirm the diagnosis.
There are two types of cell division.
- Meiotic cell division is a special kind of cell division, which takes place during meiosis and that produces the reproductive cells, the egg and sperm. In human meiotic cell division, the chromosome content is typically reduced by half to 23 chromosomes (i.e., one copy of each autosome and either an X or a Y chromosome) that joins with the other half when an egg is successfully fertilized.
- Mitotic cell division is the other type of cell division, which occurs during mitosis and is the usual process involved in growth and development. In mitotic cell division, a single cell produces two daughter cells which are generally identical to one another, and to the original parent cell.
The term “non-disjunction” - which researchers say involves a sticky chromosome - refers to when a chromosome pair fails to separate correctly during meiosis or mitosis, such that one daughter cell has both and the other neither of the chromosomes of the parent cell, thereby resulting in aneuploidy.
In genetics, a mosaic or mosaicism refers to the presence of two or more chromosomal patterns in the cells of a person, resulting in two or more cell lines. Accordingly, mosaicism refers to a particular type of aneuploidy in which the cell division anomaly occurs sometime after the initial cell division. For example, when the cell division error (known as “non-disjunction”) occurs in froming either the sperm or the egg, or in the first cell division following fertilization, then all of the chromosomal patterns in the cells of that individual will generally repeat the same uncommon number of chromosomes. However, if the sperm contains only one chromosome for each pair and the egg also contains only one, and if the first cell division takes place in the typical fashion but non-disjunction occurs during the second cell division, the individual will present with a mosaic karyotype in which 50% of his or her cells have 46 chromosomes, and the other 50% have an atypical number of chromosomes. If non-disjunction occurs on the next cell division, then only 25% of the cells will have an uncommon number of chromosomes. Thus, depending on when and how the cell division anomaly takes place, mosaicism may range from very light to practically indiscernable using current diagnostic technologies.
According to the literature, about 8% of “Klinefelter syndrome” is mosaic. According to these studies, the principal types of mosaicism involving X chromosome aneuploidy in males are:
- 46,XY / 47,XXY = 7%
- 46,XX / 47,XXY = unknown
- 47,XXY / 48,XXXY = 0.2%
- 46,XY / 47,XXY / 48,XXXY = unknown
Although there can be exceptions, it is generally felt that the lighter the level of mosaicism, the less likely that the individual will present with the signs and symptoms of the classic condition in a non-mosaic subject. However, the sample size in any of the studies looking at this issue have been extremely small to date and thus this conclusion is questioned by some investigators pending a study with larger numbers of subjects.