2 human height is an example of a trait that is

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2 Human height is an example of a trait that is affected by many genes—it is a polygenic trait.Traits that are determined by a single gene with two alleles— monogenic traits such as seed texture in peas or flower color in the four- o’clock plant—will occur as two or three distinct phenotypes. If as few as three differ- ent genes work together to determine a single trait, each represented by only two alleles, the distribution of phenotypes within a population becomes continuous. Other examples include weight,human skin color,coat color in many mammals,and even some traits that do form discrete phenotypes, such as the number of whiskers on the face of a mouse. Mendel’s Work Generated New Fields of Inquiry At the turn of the 20th century,when Mendel’s laws were gaining new adherents in the sci- entific community, it was not entirely clear how Darwin’s theory and Mendel’s laws com- plemented one another. Many who read Mendel’s work believed that genetic traits must occur as discrete, all-or-none phenotypes, such as the purple versus white flower color in peas, or yellow versus agouti coat color in mice. The way in which polygenic traits can create a continuum of phenotypes was unknown.Darwin’s champions pointed to the many traits that formed a continuum of phenotypes and insisted that although such gradual differences between individuals could not be explained by Mendelian genetics,they were
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86 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? necessary for natural selection. Many believed that the two theories were incompatible; contentious, even vitriolic disagreements developed. In the 1930s, the controversy was finally put to rest. A new field of biology emerged, called population genetics , which employed mathematics and statistics to prove that the variety required for natural selection could arise from Mendelian genetics. Instead of focusing on just one or a few genes occurring in individuals,the population geneticists used mathematical models to study the movements of many genes through entire populations over time.Population genetics is an exciting field that has continued to grow and provide answers to evolutionary questions ever since.We will take up the topic of population genetics again in Chapter 8. Recall that, to Mendel, the hereditary “factors” were nothing more than theoretical entities. His theories did not require the ability to visualize genes moving from parent to gamete then to offspring. But the question remained, what exactly are these mysterious hereditary factors? And how are they copied, passed on, and expressed in their carriers? The answers to these questions have a fascinating history,leading up to the development of modern molecular biology.While Mendel was cultivating peas, others in Europe and elsewhere were learning that all living things are composed of fundamental units called cells.The hereditary factors, as we will learn in Chapter 4, were found in cells.
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