Genes are the basic units of heredity, which are found on structures called chromosomes within the heart of the cell. Most genes instruct cells to make particular proteins. Each child has 46 chromosomes, 23 inherited from each parent, and each of the 23 pairs of chromosomes contains a complete set of genes. Every child has two sets of genes, one from each parent. if a part of a gene is flawed, it can cause a biological malfunction that can cause disease.

In order for a child to have CF, each normal parent must carry one abnormal CF gene and one normal CF gene. The parent with an abnormal CF gene does not have the disease, because the normal CF gene is strong enough ("dominant") to suppress the action of the abnormal CF gene. But if both parents have one abnormal CF gene each, their child has a 25 percent chance of inheriting two copies of the abnormal CF genes—one from each parent. A child born to two parents who both have the disease (very unlikely) would have a 100 percent risk of developing CF.

The CF gene was identified in 1989. The biochemical abnormality in CF is caused by a mutation in a gene that produces a protein responsible for the movement of chloride ions through cell membranes. The protein is called CFTR—cystic fibrosis transmembrane regulator. However, this protein accounts for only 70 to 80 percent of all CF cases. This means that other mutations (there are at least 400) seem to be responsible for the remaining CF cases.

Differences in disease patterns are probably caused by the combined effects of a particular mutation and other unknown factors in the CF patient and the environment.

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