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Name: Bevan C.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: August 2002


Question:
Why are disease genes recessive?



Replies:
Actually, disease genes are not always recessive. For instance, Huntington's disease is dominant. But most gene mutations are recessive because there are 2 copies of every gene (1 from mom and 1 from dad), and it is pretty unusual that one mutation can wipe out the function of both copies. For the vast majority of genes, one working copy is enough to do the job.

Two exceptions to that rule:

1) Enzymes at a rate limiting step of a biochemical reaction can be very sensitive to small changes in the amount of enzyme present. (In biology small means less than about a 10-fold change.)
2) "Dominant-negative" mutations make messed up proteins that can interfere with the function of the other normal proteins coming from the other copy of the gene. For instance, in a gene that has 2 regions ("domains"), a mutation might lop off a regulation domain that limits when and where a protein works. The remaining catalytic domain (the function end of the protein) is then left without an on/off switch and works like crazy even when it's not supposed to.

Christine M. Ticknor, Ph.D.
Case Western Reserve University
Ireland Cancer Center


They are not always. The genes for Marfan's syndrome and Huntington's Chorea, for instance, are dominant. Those conditions, however, do not usually kill before people's childbearing years. Genes causing diseases that kill before their holders have children will not be passed along to succeeding generations.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PGResearch Foundation, Darien, Illinois


Some disease genes are recessive to their normal alleles, and some disease genes are dominant over their normal alleles. In that case the normal alleles are recessive. So what determines if a particular gene will be dominant or recessive? It depends on the function of the gene in the cell.

If the gene is producing an enzyme (a functional protein) and the faulty gene means that the cell produces less enzyme, is the cell, or the person, able to get by with less? Perhaps so. E.g., carriers of Tay Sachs disease have less of the enzyme Hex A but it is enough for their needs so they do not have the disease, and so we say the disease gene is recessive.

However if the gene produces a faulty component (faulty instead of less), it could affect the cell and cause a disease even if the gene is present in a single dose. In that case the disease gene is dominant. An example is a connective tissue disorder in which every collagen molecule (a structural protein) in the body includes a faulty component made by the one disease gene.

Sarina M. Kopinsky, MSc, CGC, HED


They are not always recessive. Think about this. You get two copies of each gene: one from mom and one from dad. Genes code for proteins. Proteins do many things in the body, they form muscles, carry oxygen, are hormones, and are enzymes. It depends on the defect in the gene as to the effect it has on the body. If you inherit a normal copy a gene from one parent and a mutated form from the other, it depends on what the protein does as to the effect it will have. If half the amount is enough to get you by, as in the case of the gene that causes cystic fibrosis, you will be fine. In this case, you need two mutated copies of the gene to get the disease. In the case of sickle cell trait, if you have one normal copy of the hemoglobin gene and one mutated copy, you do not have sickle cell anemia and in are actually protected from the disease malaria.

You only get sickle cell anemia if you have two mutated copies. However, in the case of Huntington disease, if you have one normal copy of the gene, and one mutated copy, the mutated copy interferes with the normal copy and so even having one copy gives you the disease; therefore this trait is dominant. In the case of achondroplasia, a form of dwarfism, the ability to make cartilage is affected. Two copies of the defective gene results in miscarriage because there is no ability to make cartilage. One copy gives you dwarfism because you only have half the amount you need. Therefore, it is dominant. To be of normal height, you need two normal copies of the gene. (Height is a multifactorial trait-there are many genes that contribute to height, this is only one). So to answer your question-not all disease genes are recessive.

vanhoeck


Amelie,

you raise a very good point. The first bacteria on Earth do appear to have been anaerobes. If you study one of the major pathways that bacteria use to break down sugars, you will find that it does not require oxygen at all. (Look up "glycolysis" when you get to a biology or biochemistry course that covers this). Later on, bacteria and other organisms developed the ability to use the oxygen that built up in the atmosphere (from photosynthesis) to break down foodstuffs more efficiently. Nevertheless, the original primitive pathway of glycolysis remains as a reminder of our history, in both bacteria AND humans.

I believe that scientists are not so concerned with finding planets with oxygen, as they are finding planets with WATER.

Paul Mahoney, PhD



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