The dashes found in the genotype designations

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alleles for wing shape.The dashes found in the genotype designations indicate that either allele will result in the same phenotype.
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Dihybrid Cross: Cross two traits simultaneously. 74 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? called “mutant” forms. 1 Fruit fly genes are usually named for the mutant form; upper- case letters designate the dominant alleles, and lowercase letters designate the reces- sive alleles. Starting with parents that are homozygous for both traits, let’s engineer a cross between a broad-winged (VV) , striped (EE) parent (wild type for both traits) and a vestigial-winged , ebony (ee) parent (mutant for both traits). Now our shorthand for the genotypes must include two sets of alleles, one for two different traits: VV EE (broad-winged, striped) (vestigial-winged, ebony) Compare this with the first parental crosses that Mendel made between parents showing different forms of a single trait.All the of this cross will exhibit the dominant forms of both traits:Broad wings and pale,striped bodies.Their genotype is heterozygous for both traits: (broad-winged/striped) Now engineer a cross between the members of the Because the flies are hybrids (heterozygous) for both traits and we are following the fate of two traits simul- taneously, this experimental cross is called a dihybrid cross : Each new fly receives one allele for wing shape and one allele for body color from each parent, but recall that the law of segregation states that the actual allele that an off- spring receives for each trait is random. It is just as likely that a fly will inherit a V allele as it is that it will inherit a allele for wing size. The same goes for the E and e alleles for body color. The law of independent assortment also applies. Hence, these alleles can be inher- ited by the offspring in any combination, as long as each new fly receives two alleles for wing shape and two alleles for body color.The possible genotypes that can result from this cross can be examined by using a Punnett square, as shown in Figure 3-10. These four alleles can combine to give a total of 16 genotypes, not all of them different, but each with equal likelihood. Of the 16 combinations, 9 are different. How many different phenotypes do these 16 genotypes represent? Recall that alleles designated with an uppercase letter are dominant to those designated with a lowercase letter. Fig- ure 3-11 shows a Punnett square filled in with phenotypes instead of genotypes.From the 9 different genotypes, a total of 4 different phenotypes can occur, and they occur in a very specific ratio : or 9 wild type for both traits to 3 wild-type wing size with mu- tant body color,to 3 mutant wing size with wild-type body color,to 1 mutant for both traits. The ratio is characteristic of the offspring of a dihybrid cross in which both traits follow Mendel’s laws of segregation and independent assortment. Mendel did many dihybrid crosses in which he chose several different pairs from the seven traits he
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