Plants are heterozygous for flower color and exhibit

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plants are heterozygous for flower color and exhibit incomplete dominance. F 2 F 1 F 1 3. All genetic variety can ultimately be traced to mutation—the sudden appearance of new alleles in the genes of a population or species. Most mutations arise from errors that occur when the genetic information is copied. 4. Most mutations are deleterious or neutral. Because organisms are finely tuned com- binations of traits,errors in the information encoding those traits are likely to do more harm than good.The occasional mutation that improves the functioning of an organism can be advantageous to its owner and may be passed on to future generations. 3-3 Do Mendel’s Laws Always Apply? Mendel’s results were reported in 1865 as a series of lectures to the Brünn Society for the Study of Natural Science, and they were published in German in 1866 in a volume of proceedings of that society.Although the volume was widely distributed to libraries throughout Europe, few of Mendel’s contemporaries recognized the significance of his work. The quantitative approach that Mendel used, counting the number of offspring showing each trait and calculating ratios, is a hallmark of modern science, but it was vir- tually unknown and certainly unappreciated in the latter part of the 19th century.Then in the early part of the 20th century, three botanists—Carl Correns in Germany, Hugo de Vries in the Netherlands, and Erich von Tschermak in Austria—who were also con- ducting experiments in plant hybridization, all rediscovered Mendel’s original paper and realized its significance. Unfortunately, Mendel died in 1884, too soon to see his work assume its rightful place in the history of great discoveries. Mendel’s Laws Are Extended by Experimental Evidence In 1899,Carl Correns was performing controlled crosses by using a flowering plant called the four-o’clock.The blooms of the four-o’clock occur in three colors:White,red,and pink (Figure 3-13). When Correns crossed true-breeding red-flowered four-o’clocks with true-breeding white-flowered plants, the hybrid offspring all had pink flowers—a shade intermediate between those of the parents. If Correns had ended his experiments there, he, too, might have drawn the same erroneous conclusion that many of his predeces- sors, including Darwin, had drawn. He might have been fooled into thinking that the traits of offspring were blended averages of parental traits. But Correns went on to cross the pink-flowered plants of the with other pink plants and found that the plants exhibited all three traits: Some were white, others were pink, and still others were red. F 2 F 1 F 1
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3-3 Do Mendel’s Laws Always Apply? 83 R 1 R 1 R 1 R 1 R 2 R 2 R 1 R 2 R 1 R 2 R 2 R 2 Male Alleles Female Alleles Figure 3-14 Punnett square illustrating possible offspring of a cross between a red- flowered and a white-flowered four- o’clock plant.The ratio of phenotypes is the same as the ratio of genotypes, namely white/pink/red. 1 : 2 : 1 Incomplete Dominance: Observe the results when no one allele is dominant.
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