Genes and Dominance
Name: Patricia P.
Date: May 2003
Will a gene that expresses complete dominance always show in offspring? I can not find any
information using the search engine that really deals with complete dominance and feel like recessive
traits can always show up? What is the difference between dominance and complete dominance?
The term dominance means that with the typically two different forms of a gene, one will be expressed and
the other form will not. The simplest example is when a plant or animal produces a pigment it is a certain
color and when it does not produce the pigment it is white. These would be deferent forms of the gene...or
alleles. Many times, the protein the gene codes for is more or less effective in its enzymatic activity and
not an all or none deal where one form works and the other does not. In reality, the genes and their
expressions are far more complex. Genes that are dominant will sometimes not be expressed...called
nonpenetrant. For example, in "dominant hearing loss, under some circumstances the dominant gene will be
fully expressed and in other cases it will be nonpenetrant (not expressed). Moreover most phenotypes
(physical appearances) are also polygenic...more than one gene involved. BY FAR most diseases are
polygenic. I suspect that in most cases where phenotypes seem unpredictable there are more genes involved
than presently recognized. Eye color is a well known example. To say that eye color is based on a single
dominant and recessive gene is outright wrong. Otherwise how would all the colors of blue, green, hazel
brown, dark brown and nearly black occur...more genes are involved.
A dominant trait is one where if the gene is present, its effect is seen. That is the simple answer.
Remember that genes code for proteins and proteins have many functions. They can be hormones, enzymes,
structural components etc. The way the gene is expressed depends on what the protein does. Complete
dominance is when even in a heterozygote enough protein is produced to give the full phenotype. So the
recessive is "hidden". In some cases however, because a heterozygote only gives half the protein, it is
only partially expressed. For example, in some snapdragons, homozygous red gives red flowers. But a
heterozygote has pink flowers because one allele is not enough to get red. There is also something called
penetrance and is usually a percentage. This is the percent of people who even though they inherit a
dominant form of the gene, actually express the phenotype. Sometimes, people who have the gene, do not
seem to show it. This is called incomplete penetrance.
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Update: June 2012