6 mendels law of independent assortment applies when

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6. Mendel’s law of independent assortment applies when two or more genes are considered simultaneously.It states that the alleles of one gene are passed to offspring independently of the alleles for other genes.The Punnett square illustrates this law, as well. 3-2 Why Aren’t Members of the Same Species Identical? Like begets like.When pea plants are crossed, the resulting offspring are never roses or geraniums: They are always peas. When dogs are bred, the result is puppies, and when racehorses are bred, inevitably they bear foals. Each species is a particular combination Exploration
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3-2 Why Aren’t Members of the Same Species Identical? 79 of genetic traits which characterize that species and make it different from all the oth- ers. But to anyone who has bred racehorses for competition or dogs for show, it is abun- dantly clear that there are differences among members of the same species. These differences among individuals in natural populations are at the very core of Darwinian evolution.Where do these differences come from? What is the source of variety among natural populations? Independent Assortment Is an Important Source of Variety Almost every organism, every phenotype, is the result of perhaps thousands or tens of thousands of genes working together.When we considered just two of those genes in our dihybrid cross, body color and wing shape in the fruit fly, and ignored all the other genes of fruit flies, we saw that a single breeding pair of flies, both heterozygous for those two genes, could produce offspring with as many as four different phenotypes. A mathe- matical rule that expresses the number of possible different phenotypes that can result from a cross between heterozygotes for any number of traits is where the number of traits, or genes, considered. The 2 represents the two differ- ent forms of each trait; there are two alleles possible. For a monohybrid cross, there can be possible phenotypes (for example,round or wrinkled seed shape;ebony or pale striped body color; broad or vestigial wings). For a dihybrid cross, n is equal to 2 be- cause we are considering two different genes simultaneously, each occurring in two forms.The number of possible phenotypes is or 4.This is exactly the number of phe- notypes we saw with regard to body color and wing shape in fruit flies. Now consider the number of possible phenotypes that could result if we considered many traits simultaneously, say, 100 traits or 1,000 traits, each of which could occur in two different forms. It would be a daunting task to list all of the combinations, but we can easily calculate how many different ones are possible. For 100 traits, For 1,000 traits, or the number 11 followed by 300 zeros. That is more phenotypes than there are stars in the heavens or grains of sand on all the beaches of the world.The astonishingly large number of new combinations of alleles that can occur each time a breeding pair produces offspring is an important source of variety in populations—exactly the kind of variety that is required for evolution by natural selection. This is one example of
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