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What Causes some genes to be ressesive and other genes to be dominant?

Think about the fact that genes code for directions for making proteins. There are many different kinds of proteins in our body-enzymes for regulating metabolism, structural proteins for building our bodies, hormones for regulating processes etc. If the gene for the protein is structural, then it is important to have the right kind and the right amount. If the gene is defective (usually recessive genes are defective, but not always) the right protein will not be made and the structure will either be defective and won't work at all or there won't be enough to maintain the structure. Sometimes you need both genes to be working to get enough of the structure. So a homozygous person will have the strongest structure. A heterozygote would have one gene that is working a may produce enough of the protein to maintain the structure, but maybe not. So in some cases, just having one copy of the dominant (working) gene is enough. If it is a trait for something like eye color, this is not going to cause a defect, just a difference. In this case, being heterozygous or homozgyous for the dominant trait produces the same color of eye. In the case of sickle cell anemia, the recessive gene changes the protein structure of the shape of the red blood cell. If you have one good copy and one bad copy of the gene, some of your cells will be normal and some will be sickle cells. One good copy of the gene gives you enough normal red blood cells to stay healthy. But if you don't have a normal copy, all of your cells have the capability to sickle under certain conditions and this can be fatal. So think of dominant and recessive genes in terms of what they produce and what that protein is supposed to do and then think of what would happen if the dominant gene was able to overtake the effect of the recessive gene.

Dominant or recessive depends on the function of the gene. The term is mostly used for genetic diseases. Look at it this way. If a gene codes for a protein, hormone or factor that is needed in whatever amount, the lack of a portion of that protein would not be disastrous. So if one of the two genes present in a diploid cell would be damaged (mutated), but the other one is intact, the cell or organism would still be pretty healthy. However when both gene copies are damaged, the cell would suffer from disease. That is a recessive gene. When a gene product and it's amount is essential, and a mutant form would cause damage to the cell or the organism, even one copy of a mutated gene would cause damage: the healthy second copy of that gene could not prevent this. That gene is causing a disease that is inherited in a dominant form.

Dr. T. Wassenaar

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