Our bodies are made up of millions of cells. Each cell contains thousands of genes. Genes are like a set of instructions, controlling our growth and how our bodies work. Genes are responsible for many of our characteristics such as our eye colour, blood type or height. Any alteration in a gene can stop a gene working properly.
Genes are carried on thread like structures called chromosomes. Chromosomes are arranged in 23 pairs, according to size. We normally inherit one of each of the pairs from our mother and one from our father. Twenty two of these are the same in both sexes. The twenty third pair are the sex chromosomes. Women have two X chromosomes (XX) and men have an X and a Y chromosome (XY).
The gene that causes FXS lies near the tip of the X chromosome. Under a microscope, the tip looks like it has broken off but not quite separated. This is how the disorder got its name.
At the beginning of the normal fragile X gene a small part of the genetic code is repeated several times. In people with FXS the repeated part of the gene is larger than normal (they have more repeats). This is called an expansion, because part of the gene has grown bigger or expanded. If it grows too big it interferes with the normal instructions and causes fragile X.
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