Remarkably the two different traits appeared to be

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two traits at a time, he found that all of his original conclusions applied to both traits. Remarkably, the two different traits appeared to be operating utterly independently of one another as they passed from parents to offspring. In his words,“the relation of each pair of different characters in hybrid union is independent of the other differences in the two original parental stocks.” This important observation is called Mendel’s law of independent assortment . It states that the alleles of one gene are passed to offspring independently of the alleles of other genes. There are exceptions to the law of independent assortment, but the exceptions do not invalidate Mendel’s general conclusions. We shall see in Chapter 5 that the Mendelian factors, the genes, are carried on structures in cells called chromosomes . Each chromosome can carry many, many genes.When the genes for two different traits are found on the same chromosome, they have a tendency to travel together. But that does not minimize the importance of independent assortment. Genes that occur on dif- ferent chromosomes do segregate independently, and even those that share a carrier chromosome may exhibit some degree of independence. It is well worth a closer look at independent assortment. Whereas Mendel preferred to work with the accommodating garden pea,we can just as easily describe independent assortment by using another species that has played a starring role in the history of genetics:The fruit fly, Drosophila melanogaster (Figure 3-8). Fruit flies are tiny,are easy to keep,and have short life cycles,all characteristics that make them good candidates for studying the transmission of genetic traits.Females lay hundreds of eggs in a lifetime.When many offspring are produced from a single cross, it is easy to calculate the ratios of different phenotypes that result from that cross. Hundreds of different traits have been experimentally bred into the many strains of flies that inhabit genetics laboratories throughout the world. Let’s look at two of them simultaneously: Body color and wing size. The Dihybrid Cross Most fruit flies that live in the wild have broad, straight wings and pale-colored bodies with dark transverse stripes.These traits are dominant to their alternatives, vestigial (shriveled) wings and ebony body color (Figure 3-9). A trait that is usually found in organisms in their natural, or wild, state is called the wild type .Thus, broad wings and pale, striped bodies are the wild-type forms of the traits for wing size and body color. Vestigial wings and ebony body color are recessive forms of the traits, Figure 3-8 Female and male fruit flies of the genus Drosophila . Note that the male has a mutation that causes white eyes.The red color of the female’s eyes is the wild type. Wild type (V E ) Ebony (V ee) Vestigial wings (vvE ) Figure 3-9 Wild-type and mutant fruit flies.A wild-type fly has at least one V allele for wing shape and at least one E allele for body color.An ebony fly must have two e alleles for body color. Likewise, a fly with vestigial wings must have two v alleles for wing shape.The dashes
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