One Gene, One Protein
One of the rules of molecular genetics is that one gene usually specifies one protein. This protein may be an enzyme, or it may be a regulatory protein that promotes or inhibits the expression of another gene (and hence the synthesis of another protein).
A mutation is any chemical change in a gene sequence. It might be a change in one nucleotide of a base pair. This type of mutation is rare, and almost always either neutral in effect or outright deleterious.
A mutation might be a rearrangement of a long multi-gene segment of a chromosome. In meiosis, the process where germ cells are reduced to pollen cells or ovules, often results in swapping of homologous segments between members of a pair of chromosomes.
A mutation can also be the deletion or the insertion of a segment of DNA in a chromosome.
If a given color is to appear in a flower, its pigment, with a particular chemical structure, must be synthesized from sugars and amino acids, in the cells of that flower.
The chemical reactions that lead to the synthesis of any natural substance, including the pigments of daylily flowers, are promoted by specific proteins called enzymes. For every enzyme, there is a DNA gene that specifies the enzyme's structure.
There are usually a series of chemical reactions that must happen together in the same cell for a particular pigment to be formed. Each reaction in the series will be catalyzed by its own enzyme. Each enzyme is coded by its own gene.
A mutation in a gene can cause the enzyme coded by that gene to be defective. A defective enzyme will usually not catalyze its chemical reaction efficiently -- often, not at all. So most mutations inactivate the enzymes produced from the mutant genes. It is very rare that Nature generates a new enzyme, catalyzing a new chemical reaction.
When we see a complicated pattern of colors in a flower, we are seeing a different level of control. We are seeing differential expression of genes, according to physical location of the cells in the flower structure.
The mutations that lead to new genes are rare. The rearrangement of genes, wherein an entire gene section of a DNA chain is moved from one region of the DNA to another, or even to another chromosome entirely, can occur more often. When the DNA binding site of a regulatory gene moves to a new position, it can then control a new gene. These sorts of changes are leading to new shapes of flowers (bigger, rounder, ruffled) and to new color combinations and patterns (white flowers, eyes and picotee edges, braided edges, etc.).
In tetraploid daylilies, there are twice as many opportunities for such rearrangements to occur as in diploids. When you duplicate genes, as in converting a diploid to a tetraploid by doubling the chromosomes, you can retain the old functions by preserving one set of genes in the original form. But you can take advantage of the redundancy to selecting for changes in the extra sets of genes. Redundancy allows you to have your cake and eat it too! We are beginning to see the usefulness of the extra gene sets in the tetraploid daylilies.
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Revision of 5 January 2004
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