Genetic recombination or the production of new

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genetic recombination , or the production of new combinations of genes not found in either par- ent.We will see other ways in which genetic recombination can be achieved in Chapter 5. In fact, not all of the genes in living organisms have different forms, or alleles. It has been estimated that, on average, about 30% of human genes are actually heterozygous in a way that influences phenotypes and thus are a source of variety. If we assume that humans have about 30,000 genes, then 30% of 30,000 would be 9,000 heterozygous genes.To calculate the number of possible human phenotypes this could produce, you would raise 2 to the power of 9,000. Chances are you will exceed the capacity of your calculator. Even if 70% of the genes in humans are homozygous, and thus not a source = 11 * 10 300 the number of possible different phenotypes = 2 1,000 = 1,267,650,000,000,000,000,000,000,000,000 = 1.26765 * 10 30 the number of possible different phenotypes = 2 100 2 2 , 2 1 = 2 n = number of possible phenotypes = 2 n
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80 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? of genetic recombination, there is still sufficient variety to produce a nearly infinite number of different phenotypes—variety upon which natural selection can operate. It is no wonder that, aside from identical twins who begin life with all the same alleles, no two people present the same phenotype. In our discussion of heredity thus far, we have considered only two different forms, or alleles, for each gene. Mendel’s peas were either smooth or wrinkled; the gene for one chain of the human hemoglobin protein was described as either normal or the sick- le form.But whereas any individual can carry only two alleles for a given gene,there may be many different alleles for a trait in a population. Different combinations of these various alleles may produce more than two phenotypes for the trait. For example, there are three different alleles that determine blood type in human populations, namely A , B , and O .An individual can have only two of the three possible alleles, although a pop- ulation of humans will have all three alleles represented. The possibilities for variety are endless. Now imagine a certain combination of traits that bestows on its owner some com- petitive advantage. Imagine a desert plant, say a cactus, with an allele that creates a waxy coating on its surfaces.That trait alone would prevent water loss due to evaporation and help the plant to survive in the hot, arid environment of the desert. But living plants also need a mechanism for taking up carbon dioxide gas for photosynthesis. A waxy cuticle might be a barrier to carbon dioxide uptake. If that same cactus had the alleles that permitted it to take up carbon dioxide only at unwaxed entry points,the combination of a waxy cuticle and special carbon dioxide uptake sites would be a powerful advantage. That cactus would fare much better than one with only a waxy cuticle or one with specific gas exchange sites.Natural selection operates not on single traits,but on whole organisms, which are combinations of many, many traits.
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