Chromosome Abnormalities

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Chromosome Review

Before we find out the results of the chromosome analysis, let's review normal chromosomes, and structural and numerical chromosome abnormalities.

Recall that a fertilized human egg cell normally has a total of 46 chromosomes, 23 from the mother and 23 from the father. When a child is born with multiple congenital anomalies, there is a significant chance that the baby either has an abnormal number of chromosomes or has an abnormal structural arrangement of chromosomes.

Normal Karyotypes

Often the first step in analyzing chromosomes is to request a karyotype of the patient. If quick results are important, a FISH metaphase analysis may be used with probes for chromosomes 13, 18, 21, X, and Y (the most common aneupliodies in liveborns). Used in this fashion, FISH analysis provides results about aneupliody in 1-2 days with 98% accuracy (karotype results can take a few weeks). Additionally, different FISH probes are available to test various chromosomal locations associated with specific conditions. Depending on a patient's clinical features, FISH probes may be used to rule out specific conditions caused by small chromosomal deletions or duplications. If nothing is found using these two techniques, it is important to remember that this does not rule out the possibility of a mutation. In the case of unexplained mental retardation, a comparative genomic hybridization (CGH) is often requested to search for microdeletions or microduplications throughout the genome. If a patient's last genetic evaluation was over 10 years ago and nothing was found, it may be important to retest the patient using the latest techniques. It is also important to remember that the uncertainty of not knowing the cause of the problem is often psychologically difficult for the patient's family to handle.

Normal Female Karyotype (46, XX):

Normal Male Karyotype (46, XY):