Qualitative vs. Quantitative Traits Help
Qualitative vs. Quantitative Traits
The classical Mendelian traits encountered in the previous chapters have been qualitative in nature; i.e., traits that are easily classified into distinct phenotypic categories. These discrete phenotypes are under the genetic control of only one or a very few genes with little or no environmental modification to obscure the gene effects. In contrast, the variability exhibited by many traits fails to fit into discrete phenotypic classes (discontinuous variability), but instead forms a spectrum of phenotypes that blend imperceptively (continuous variability). Economically important traits such as body weight gains, in livestock mature plant heights, egg or milk production, and yield of grain per acre are quantitative or complex traits with continuous variability. The basic differences between qualitative and quantitative traits involve the number of genes contributing to the phenotypic variability and the degree to which the phenotype can be modified by environmental factors. Quantitative traits are governed by many genes (perhaps 10–100 or more), each contributing a small amount to the phenotype are their individual effects cannot be detected by Mendelian methods. For this reason, quantitative traits are also referred to as polygenic traits. Stretches of DNA with closely linked genes responsible for phenotypes associated with quantitative traits are called quantitative trait loci or QTLs.
Furthermore, each gene usually has effects on more than one trait. The idea that each character is controlled by a single gene (the one-gene-one-trait hypothesis) has often been falsely attributed to Mendel. But even he recognized that a single factor (or gene) might have manifold effects on more than one trait. For example, he observed that purple flowers are correlated with brown seeds and a dark spot on the axils of leaves; similarly, white flowers are correlated with light-colored seeds and no axillary spots on the leaves. In Drosophila, many loci (e.g., genes named dumpy, cut, vestigial, apterous) contribute to a complex character such as wing development. Each of these genes also has pleiotropic effects on other traits. For example, the gene for vestigial wings also effects the halteres (balancers), bristles, egg production in females, and longevity.
Some genes encode products such as enzymes that participate in multistep biochemical pathways or proteins that regulate the activity of one or more other genes in metabolic, regulatory, or developmental pathways. Because of the complex interactions within these pathways, a gene product acting at any one step might have phenotypic effects on other characters. For a given gene, some of its pleiotropic effects may be relatively strong for certain traits, whereas its effects on other traits may be so weak that they are difficult or impossible to identify by Mendelian techniques. It is the totality of these pleiotropic effects of numerous loci that constitutes the genetic base of a quantitative character. In addition to this genetic component, the phenotypic variability of a quantitative trait in a population usually has an environmental component. Statistics can help determine the magnitude of the genetic and environmental components of the total phenotypic variability of each quantitative trait in a population. Only some of the more easily understood phenomena of this branch of genetics will be presented in this chapter. Table 8-1 summarizes some of the major differences between quantitative and qualitative genetics.
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