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Name_______________________________ Date________ Period____ Dihybrid Cross Activity In a gamete with either chromosome from any other pair of homologous chromosomes the genes that are located on non-homologous also assort independently as you can see in the following diagram: Chromosome 1 allele 1 Chromosome 2 allele 2 Because of independent assortment, a plant that is heterozygous for two traits (Genotype AaBb) will produce equal numbers of 4 types of gametes- AB, Ab, aB, ab. In the example that follows, we will predict the results of a cross between 2 plants that are heterozygous for both pod color and pod shape. Sample Problem What are the phonotypic ratios of the offspring resulting from a cross between 2 pea plants that are heterozygous for pod color and pod shape? What are the phenotypes of the parents in the cross? Steps 1 choose letters to represent the genes in the cross. We will use G for Green and g for yellow color. And N for smooth and n for constricted pods. Step 2 Write the Genotypes of the parents Since the parents are heterozygous for both traits, their genotype must be GgNn. The cross then can be written as: GgNn X GgNn Step 3 Determine the possible gametes that the parents can produce. Use the FOIL method to determine this: Front Outside Inside Last Each Parent produces 4 types of gametes- GN, Gn, gN and gn. Step 4 Enter the possible gametes at the top and side of the Punnett square. GN Gn GN gN gn Gn gN gn Step 5 Complete the punnett Square by writing the alleles from the gametes in the appropriate boxes GN GN GGNN Gn GGNn gN GgNN gn GgNn Gn GGNn GGnn GgNn Ggnn gN GgNN GgNn ggNN ggNn gn GgNn Ggnn ggNn ggnn Step 6 Determine the phenotypes of the offspring. In this example 9/16 have green smooth pods (G_N_) 3/16 have green constricted pods (G_nn) 3/16 have yellow smooth pods (ggN_) and 1/16 has yellow constricted pods (ggnn) Step 7 Using the results from the step 5 and step 6 determine the answer to the problem. Parent Genotypes are GgNn and GgNn Phenotypic ratios are 9 green smooth: 3 green constricted: 3 yellow smooth: 1 yellow constricted (G_N_) (G_nn) (ggN_) (ggnn) Practice Problems In mice, the ability to run normally is the dominant trait ( R ) The recessive trait causes mice to run in circles only. This is called Waltzing Mice ( r ). Black hair (B) in mice is dominant over brown hair (b) 1. Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous black mouse. Parental Genotypes______X_______ Offspring Phenotypic Ratio: 2. Cross a homozygous running, homozygous black mouse with a heterozygous running, brown mouse. Parental Genotypes______X_______ Offspring Phenotypic Ratio: 3. Cross a waltzing brown mouse with a waltzing brown mouse Parental Genotypes______X_______ Offspring Phenotypic Ratio: 4. Cross a homozygous running, heterozygous black mouse with a waltzing brown mouse Parental Genotypes______X_______ Offspring Phenotypic Ratio: 5. Cross a heterozygous running, brown mouse with a heterozygous running, homozygous black mouse Parental Genotypes______X_______ Offspring Phenotypic Ratio:
