Chromosomes: agents of history and destiny

There are thought to be about 40,000 genes in a kind of genetic soup in the nucleus of each human cell. Genes control everything, and they all interact. Some genes serve to inhibit, suppress, or repress the actions of other genes, and other try to identify and repair genetic and metabolic errors. There are factors that activate or hide genes, and there are genes that change the speed or liklihood of specific chemical interactions.   Making a healthy little person is an amazingly complex process!

Before cells can divide, genes come together into 23 couples - chromosome pairs. The DNA sequences comprising genes always go to the same location of a particular chromosome, which for convenience are numbered from biggest (1 - most genetic material) to to smallest (22) plus the sex chromosomes (XX female, XY male). Eggs (ova) from Mom and sperm from Dad are special cells that contain a single set of 23 chromosomes. Conception completes the chromosome set and cell replication and differentiation begins: chromosome pairs separate and move to opposite sides of the nucleus, the entire cell divides into two new cells. iThe process repeats. As moe and more cells accumulate, genes in each region turn on or off, which is how individual organs and tissues form, eventhough each cell has the same genetic chromosome set.

If a pair of chromosomes do not separate cleanly then, after division, one cell will have too many chromosomes, and the other will have too few. When the problem happens during the first few cell divisions, the essentially all of the cells of the embryo will be abnormal. When the problem happens after many cell divisions, then only a portion of the cells of the mbryo will be abnormal. This is referred to as a "mosaic", and the clinical severity of the problem may be less. Examples of these commonly occurring problems of chromosome number are Down's Syndrome (extra chromosome or 'trisomy' #21) and Turner's Syndrome (sinngle or 'monosomy' X). 'Karyotyping' is the name for the microscopic technique that identifies each chromosome in a cell. By looking at the band pattern of chromosomes, additional problems can be identified in which there has been a or rearrangement of gene sequences.

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