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Genetics and Chromosome Analysis:

Straightforward answers to
important questions

Chromosome analysis provides a "birds eye view" of an individual's genetic information.  Missing, extra, or rearranged chromosome material may be responsible for such diverse problems as birth defects, mental retardation, autism, infertility, and repeated miscarriage.  Chromosome abnormalities are also found in certain cancerous cells.

What are chromosomes?
Chromosomes are microscopic structures containing all of an individual's genetic information.  They are composed of DNA and various proteins.  The DNA contains the actual genetic code of an individual, and the proteins protect the DNA and allow the DNA to duplicate properly when the cell divides.

Each cell in our body, except for red blood cells, has a nucleus in which chromosomes are contained.  Every cell contains a set of chromosomes which encode all the genetic information of an individual.  Through a process known as gene expression, only selected portions of the DNA are expressed.  Thus, only the DNA necessary for a brain cell is used in brain cells, although all of the genetic material needed for a liver cell or any other type of cell is contained within that brain cell nucleus.

Most individuals have 23 pairs of chromosomes; one of each pair coming from the mother and the other from the father.  The non-sex or autosomal chromosome pairs are numbered 1 through 22, and there are two sex chromosomes, designated X and Y.  Individuals with two X chromosomes are female, and individuals with an X and a Y chromosome are male.

When a cell divides, each of the 46 chromosomes is duplicated, and each of the two new cells contains an exact duplicate of the genetic information in the original cell.  An egg or sperm cell divides one more time, so that only one chromosome from each pair remains.  When a sperm fertilizes an egg, the fertilized egg should have 23 chromosomes from each parent, making a total of 46 chromosomes.

What are examples of chromosome abnormalities?

    Extra or missing chromosome material:

One of the most common chromosome abnormalities is Down syndrome, which is normally caused by an extra chromosome 21.  Down syndrome occurs in about 1 in 800 live born babies.  Although Down syndrome is more frequent in older women, 80% of Down syndrome occurs at a maternal age less than 35.  Other serious chromosome abnormalities related to maternal age are trisomy 18 and trisomy 13, which are due to an extra chromosome 18 and an extra chromosome 13, respectively.  Features of all three of these chromosome abnormalities, like most chromosome abnormalities involving extra chromosome material, include birth defects, growth problems, mental retardation, or a combination of these.  The presence of an extra X or Y chromosome tends to be more subtle.

Since as many as 70% of miscarriages are due to chromosome abnormalities, performing chromosome analysis on the miscarried products of conception can often determine why a miscarriage occurred and whether there is a risk that this may happen again in the future.  The most common chromosome abnormalities found in early miscarriages include an extra chromosome 16 or other large chromosome, a missing X chromosome, or three sets of chromosomes (triploidy) instead of the normal pair.

Deletions or duplications of part of a chromosome, rather than the entire chromosome is another type of chromosome abnormality.  Clinical features may be much more severe than in Down syndrome, or much less severe, depending on the particular chromosome region which is duplicated or deleted.

Complex chromosome abnormalities consisting of extra chromosomes, missing chromosomes, chromosome rearrangements, or combinations of these abnormalities are common in cancerous cells.  Chromosome analysis on these cells can help in making a correct diagnosis and deciding on appropriate treatment.

  Rearranged chromosome material:
In some individuals, portions of one chromosome are exchanged with another chromosome in what is known as a balanced translocation.  Since, in spite of the rearrangement, there is the correct amount of genetic material, this usually has no health implications.  However, male or female carriers of balanced chromosome translocations may have infertility, recurrent pregnancy loss, or live born children with problems due to missing and/or extra genetic material (an unbalanced translocation).

Chromosomes are invisible unless the cell is in the process of cell division.  Therefore, in order to do chromosome analysis, living cells must be available.  These can either be white cells from a blood sample, cells from an amniocentesis or chorionic villus sample, or products of conception.

How accurate is chromosome analysis?

Chromosome analysis is highly accurate  Very subtle chromosome abnormalities may not be detected by routine chromosome analysis, but may be identified by more detailed testing using a DNA-based technique known as chromosome microarray analysis .  It is important to understand that normal results of chromosome analysis or microarray analysis do not guarantee that there are no genetic problems.  A geneticist can discuss these issues more thoroughly, on an individual basis, after obtaining a detailed medical and family history, reviewing relevant medical information, and performing a physical examination.
What if a chromosome result is abnormal?
Accurate interpretation is as important as an accurate result.  Clinical geneticists and cytogeneticists have experience in dealing with chromosome abnormalities and can explain the clinical implications of a chromosome result.  Genetic counseling, with either a physician geneticist or a genetic counselor, will often be recommended when a chromosome abnormality is discovered.  Genetic counseling includes explaining what results of genetic and chromosome testing mean for an individual, his or her family, and future family members.  Clinical geneticists can help other physicians manage medical care of children and adults with chromosome abnormalities.
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