Sp

No ( pigment

No ( pigment

White flowers (r)

of many proteins. However, as will be discussed in detail later, certain proteins control the expression of genes rather than acting as enzymes. Some of these regulatory proteins control just one or a few genes whereas others control large numbers of genes. Thus a defective regulatory protein may affect the levels of many other proteins. Modern analysis has shown that some types of dwarfism are due to defects in a single regulatory protein that controls many genes affecting growth. If the concept of "one gene—one enzyme" is broadened to "one gene—one protein," it still applies in most cases. [There are of course exceptions. Perhaps the most important is that in higher organisms multiple related proteins may sometimes be made from the same gene by alternative patterns of splicing at the RNA level, as discussed in Chapter 12.]

Genetics has been culturally influenced by idealized notions of a perfect "natural" or "original" state. Mutations tend to be viewed as defects relative to this.

Mutants Result from Alterations in Genes

Consider a simple pathway in which red pigment is made from its precursor in a single step. When everything is working properly, the flowers shown in Figure 1.02 will be red and will match thousands of other red flowers growing in the wild. If the gene for flower color is altered so as to prevent the gene from functioning properly, one may find a plant with white flowers. Such genetic alterations are known as mutations. The white version of the flower color gene is defective and is a mutant allele. The properly functioning red version of this gene is referred to as the wild-type allele (Fig. 1.03). As the name implies, the wild-type is supposedly the original version as found in the wild, before domestication and/or mutation altered the beauties of nature. In fact, there are frequent genetic variants in wild populations and it is not always obvious which version of a gene should be regarded as the true wild-type. Generally, the wild-type is taken as the form that is common and shows adaptation to the environment.

Geneticists often refer to the red allele as "R" and the white allele as "r" (not "W"). Although this may seem a strange way to designate the color white, the idea is mutation An alteration in the genetic information carried by a gene regulatory protein A protein that regulates the expression of a gene or the activity of another protein wild-type The original or "natural" version of a gene or organism

Phenotypes and Genotypes 5

FIGURE 1.04 Three Step Biochemical Pathway

In this scenario, genes A, B, and C are all needed to make the red pigment required to produce a red flower. If any precursor is missing due to a defective gene, the pigment will not be made and the flower will be white.

Gene A

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