Chromosomes and Chromosome Abnormalities

What are chromosomes?

Chromosomes are thin threads of genes (DNA) present in almost every cell of our body. Genes carry the information that tells the body how to grow, function and determines all our physical characteristics. A person normally has 23 chromosome pairs for a total of 46 chromosomes. We inherit 23 chromosomes from our mother and 23 chromosomes from our father. The autosome chromosomes are labeled 1 through 22 from the largest to the smallest. There is one pair of sex chromosomes, which determines if the individual is a male (X and Y) or female (2 X chromosomes). Chromosomes can be seen using a special microscope. Below is a picture of chromosomes.

The chromosomes can be evaluated to make sure the correct number of chromosomes is present and that they appear structurally normal (no missing or extra pieces).

Typically, the laboratory will arrange the chromosomes in the following manner:

What are Chromosome Abnormalities?

Chromosome abnormalities are genetic disorders in which there is either an incorrect number of chromosomes or an incorrect amount of genetic information (an extra piece or missing piece of a chromosome).

About 1 out of every 200 babies born will have a chromosomal abnormality. Trisomy 21 (Down syndrome) is the most common chromosome abnormality in babies. It is caused by of an extra copy of the # 21 chromosome (one of the smallest chromosomes) being present in every cell. There are many different types of chromosome abnormalities. Many individuals with a chromosome abnormality will have mental retardation, learning disabilities, and birth defects. The type of chromosome abnormality will determine the severity of mental retardation and the types of birth defects that may be present.

What causes chromosome abnormalities?

Chromosome abnormalities usually occur in the egg or sperm before conception. The chances of an egg having an abnormal number of chromosomes increase with age.