We have seen that the process of identifying the specific hereditary components of a biological system is called genetic dissection. In the same way that an anatomist probes biological structure and function with a scalpel, the geneticist probes biological structure and function armed with genetic variants, usually abnormal ones. If the geneticist is interested in a biological process X, he or she embarks on a search for genetic variants that affect X. Each variant identifies a separate component of X. In much the same way that a novice auto mechanic can learn a lot about how an internal combustion engine works by pulling out a spark-plug lead, for example, the geneticist "tinkers" with a living system. This approach is tremendously effective in charting the unknown -- the invisibility and magnitude of which are often not appreciated by those who have not attempted some kind of research -- and represents a truly powerful tool.
Geneticists use and analyze hereditary variants from the molecular level, probing cellular and organismal processes, all the way up to the population level, where the variants reveal evolutionary processes. The spectrum of variants that has been obtained and studied effectively by geneticists is staggering; as we have seen there are variants affecting shape, number, biochemical function, and so on. In fact, the general finding has been that genetic variants can be obtained for virtually any biological structure or process of interest to an investigator.
Genetic analysis, as we have seen, must start with parental differences. Without variants, no genetic analysis is possible. Where do these variants -- these raw materials for genetic analysis -- come from? This is a question that can be answered in full only in later chapters. Briefly, most of the variants like the ones used by Mendel (and by ancient and modern breeders of plants and animals) arise spontaneously in nature or in the breeders' populations without the deliberate action of geneticists.
Let us emphasize again that variants can range from rare to common. Some rare variants are abnormal. Undoubtedly in a natural setting many of them would be weeded out by natural selection, but they can be kept alive by nurture so that the alleles responsible can be studied. On the other hand, for many genes there are two or more common alleles in a population, resulting in genetic polymorphism -- the coexistence of genetically determined variant phenotypes in a population. Although the reason for the existence of polymorphism is usually not easy to discover, for the geneticist they are a useful source of variant alleles for study.
We have seen that the genetic analysis of variants can identify a particular gene that is important for a biological process. This central aspect of modern genetics is called genetic dissection. Mendel was the first genetic surgeon. Using genetic analysis, he was able to identify and distinguish among the several components of the hereditary process in a way as convincing as if he had microdissected those components. The fact that the genes he was using were for pea shape, pea color, and so on, was largely irrelevant. Those genes were being used simply as genetic markers, which enabled Mendel to trace the hereditary processes of segregation and assortment. A genetic marker is a variant allele that is used to label a biological structure or process throughout the course of an experiment. It is almost as though Mendel were able to "paint" two alleles two different colors and send them through a cross to see how they behaved! Genetic markers are now routinely used in genetics and in all of biology to study all sorts of processes that the marker genes themselves do not directly affect. Markers are often morphological, as in the pea example. But molecular variants (DNA and proteins) are increasingly used as markers too.
Probably without realizing it, Mendel had also invented another aspect of genetic dissection in which the precise genes used were important. In this type of analysis, genetic variants are used in such a way that by studying the variant gene function, we can make inferences about the "normal" operation of the gene and the process it controls. Every time a gene is identified by Mendelian analysis, it identifies a component of a biological process.
Once a gene has been identified as affecting, say, petal color in peas, we call it a major gene, but what does this really mean? It is major in that it is obviously having a profound effect on the color of the petals. But can we conclude that it is the single most important step in the determination of petal color? The answer is no, and the reason may be seen in an analogy. If we were trying to discover how a car engine works, we might pull out various parts and observe the effect on the running of the engine. If a battery cable were disconnected, the engine would stop; this might lead us erroneously to conclude that this cable is the most important part of the running of the engine. Other parts are equally necessary, and their removal could also stop or seriously cripple the engine. In a similar way it can be shown [...] that several genes can be identified, all of which have a major and similar effect on petal coloration.
Mutations are very useful. In the same way that we can learn how the engine of a car works by tinkering with its parts one at a time to see what effect each has, we can see how a cell works by altering its parts one at a time by inducing mutations. The mutation analysis is a further aspect of the process we have called genetic dissection of living systems. As we have seen, the first stage of any mutational dissection is the mutant hunt. The effectiveness of the hunt can be improved dramatically by using mutagens.
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