Introduction To Chromosomes

Simply put, chromosomes are the structures that hold our genes. Genes are the individual instructions that tell our bodies how to develop and keep our bodies running healthy. In every cell of our body there are 20,000 to 25,000* genes that are located on 46 chromosomes. These 46 chromosomes occur as 23 pairs. We get one of each pair from our mother in the egg, and one of each pair from our father in the sperm. The first 22 pairs are labeled longest to shortest. The last pair are called the sex chromosomes labeled X or Y. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY). Therefore everyone should have 46 chromosomes in every cell of their body. If a chromosome or piece of a chromosome is missing or duplicated, there are missing or extra genes respectively. When a person has missing or extra information (genes) problems can develop for that individual's health and development. Each chromosomes has a p and q arm; p (petit) is the short arm and q (next letter in the alphabet) is the long arm. Some of the chromosomes like 13, 14, and 15 have very small p arms. When a karyotype is made (see below) the q arm is always put on the bottom and the p on the top. The arms are separated by a region known as the centromere (red in picture), which is a pinched area of the chromosome. The chromosomes need to be stained in order to see them with a microscope. When stained the chromosomes look like strings with light and dark 'bands'. Each chromosome arm is defined further by numbering the bands, the higher the number, the further that area is from the centromere.

A karyotype is an actual photograph of the chromosomes from one cell. The cells analyzed are usually white blood cells from a regular blood draw or from a prenatal speciman. After staining the chromosomes can be seen as banded strings under 1,000 x magnification. They are analyzed by specially trained cytogenetic technologists, Ph.D cytogeneticists, or medical geneticists. 'Cytogenetics' is a word for the study of chromosomes. After analysis under the microscope a picture (karyotype) is printed.

Normal Male Karyotype - a female would have two X's instead of an X and Y.

In a karyotype the chromosomes can appear bent or twisted. This is normal and is simply reflecting how they are sitting on the slide. Chromosomes are flexible structures made up of DNA. The coding order of that DNA makes up the genes. Chromosomes are analyzed during a time in the cell cycle when they are compact. During other times in the cell cycle the chromosomes unwind into long strands of DNA. At that time we would not be able to see them under the microscope. If you were to pull out all the chromosomes into long strands of DNA there would be over 7 feet of DNA in each cell! That's about 80 billion miles of DNA in the average human adult!

Sometimes when chromosomes are analyzed a 'High Resolution Analysis' is performed. This means the chromosomes are examined when they are a little longer than a standard analysis. Since they are longer more bands can be seen. This is usually done when a small deletion or duplication is thought to be present. There are different types of staining that make the chromosomes look differently. The stain which is used depends on what type of abnormality cytogeneticists think they might be seeing. This helps to help clarify the results.

In 1960 the first meeting to propose a standard system of naming the chromosomes took place. Since that time this method of describing chromosomes and chromosome abnormalities has been revised and added to several times. It has produced an International Standard of Cytogenetic Nomenclature. This allows one lab to 'write out' the chromosome findings. Any other lab will know what they have found without looking at the karyotype.

Here are some examples:

There are literally millions of types of abnormalities. If your child has a chromosome abnormality the above nomenclature describes exactly what it is. Ask your genetic counselor, physician, or health care professional to describe the chromosome abnormality found. Below are a few of the codes used in the standard nomenclature.

add = Addition material of unknown origin
del = Deletion
de novo = A chromosome abnormality which has not been inherited
der = Derivative Chromosome dic Dicentric
dup = Duplication
fra = Fragile Site
idic = Isodicentric chromosome
ins = Insertion
inv = Inversion
i or iso = Isochromosome
mar = Marker chromosome
mat = Maternal origin
Minus sign (-) = Loss
mos = Mosaic
p = Short arm of chromosome
pat = Paternal origin
Plus sign(+) = Gain
q = Long arm of chromosome
r = Ring chromosome
rcp = Reciprocal
rea = Rearrangement
rec = Recombinant chromosome
rob = Robertsonian translocation
t = translocation
tel = Telomere (end of chromosome arm)
ter = Terminal end of chromosome
upd = Uniparental disomy
? = Uncertain

It is important to note that most chromosome abnormalities occur as a accident in the egg or sperm. Therefore every cell in the body would have the abnormality. Some abnormalities can happen after conception and individuals can have a mosaicism (some cells with the abnormality and some without). Chromosome abnormalities can be inherited from a parent, like a translocation, or be 'de novo' (new in that individual).

A chromosomes deletion is when a part of a chromosome(s) has been deleted. A deletion can occur on any chromosome, at any band, and can be any size (large or small). What a deletion causes depends on how big a piece is missing and what genes are missing in the section (i.e. where the deletion is). Under chromosome analysis the section that is missing can usually be determined. However it is difficult to compare one child with a particular deletion to another with the 'same' deletion.

Remember that looking at the chromosomes is the big picture, like looking at an encyclopedia set from about 10 feet away. We are usually able to detect the deletion. Some are too small to see and other technologies can be used, but it is impossible to say at exactly what spot the deletion started and ended. So one individual might have a few more genes deleted than another individual with the 'same' deletion.

In the above example the area in the blue brackets is not present (deleted) in its pair designated by the red arrow. The other 22 pairs of chromosomes were normal (not shown). The nomenclature for this deletion would be:

Some deletions occur more frequently and are associated with a particular syndrome such as 46,XX,5p-, also called cri-du-chat syndrome.

A duplication is just that, a duplication of a section of a chromosome. A duplication is sometimes referred to as a 'partial trisomy'. Trisomy refers to three. Therefore if a duplication exists, that individual has three copies of that area instead of two. This means there are extra instructions (genes) present that can cause an increased risk for birth defects or developmental problems.

In the picture, red arrows point to identical bands on each chromosome. The blue arrow points to a duplication of the band at the red arrow. You can see that the chromosome on the right is longer. The nomenclature for this abnormality would be:

A ring chromosome can happen in two ways. One is demonstrated in the picture; the end of the p and q arm breaks off and then stick to each other. The blue parts of each are lost thus resulting in loss of information. Second, the ends of the p and q arm stick together (fusion), usually without loss of material. However the ring can cause problems when the cell divides and can cause problems for the individual.

It is also possible to have a ring and be apparently healthy with no delays in development. As with all chromosome abnormalities it depends on what is actually found, the size of the ring, how much material was lost, which chromosomes are involved etc.

Translocations can be a little tricky. Above is an example of a balanced translocation. The long arms of chromosome 7 and 21 have broken off and switched places. So you can see a normal 7 and 21, and a translocated 7 and 21. This individual has all the material needed, just switched around (translocated), so they should have no health problems, because it is 'balanced'. However there can be a problem when this person has children.

Remember that when the egg or sperm is made, each parent gives one of each chromosome pair. What would happen if this person gave the normal seven and the 21p with 7q attached? Look below:

There is an extra copy of 7q. If you count them you will find three copies of 7q instead of two. And there is only one copy of 21q. Therefore this is 'unbalanced', there is extra and missing information that can lead to birth defects, cognitive abnormalities, and an increased risk for miscarriage. For many unbalanced rearrangements it is not possible to predict what abnormalities to expect.

An inversion consists of two breaks in one chromosome. The area between the breaks is inverted (turned around), and then reinserted and the breaks then unite to the rest of the chromosome. If the inverted area includes the centromere it is called a pericentric inversion. If it does not, it is called a paracentric inversion.

Notice that in a pericentric inversion one break is in the short arm and one in the long arm. Therefore an example of a cytogenetic nomenclature might read 46,XY,inv(3)(p23q27). A paracenteric inversion does not include the centromere and an example might be 46,XY,inv(1)(p12p31).

When a parent has an inversion there is an increased risk for offspring with an incorrect amount of genetic material. This can lead to babies with birth defects and/or abnormal development or an increased risk for miscarriage. The possible pregnancy outcomes for an individual with an inversion is rather complicated and depends on how big the inversion is, where it is, and what type of inversion is present, paracentric or pericentric. There are many inversions that occur in the general population that are called normal variants. Including Inv(9) and Inv(2). These inversions are not related to an increased risk of birth defects and/or developmental difficulties.