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Instructions
The Dihybrid Cross
Figure out the phenotypic ratio of the parents. Dominant traits dominate over recessive traits. For example, in flowers, call P = purple and p = white since white is recessive to purple. L = long and l = short since short is recessive to long. For this example, mate two purple flowered plants with long stems that are heterozygotes. The purple flowered, long stemmed plant phenotypic ratios would be PpLl for both plants.
Determine the possible gametes from each parent. Each gamete only contains one allele from each parent. In the example, each parent could produce the following gametes: PL, Pl, pL and pl. If the parent is homozygous dominant for each allele, it is only able to produce PL, and if it is homozygous recessive (a white flowered, short stemmed plant), it can only produce pl. A plant heterozygous for color but homozygous for long stems produces PL and pL.
Draw a punnett square, which is four squares across by four squares down. There should be 16 squares in total.
Place each gamete from one parent along the top edge of the square, and each gamete from the other parent along the side edge of the square. If the possible alleles repeat, as in a homozygous cross, repeat the gametes in each of the boxes. For example: PL, PL, PL, PL if the plant is homozygous dominant for both traits. In this example, you have PL, Pl, pL and pl along the top, and the same along the side since each parent is heterozygous for both traits.
Fill in the table. In the example you get one PPLL, two PPLls, two PpLLs and four PpLls. All nine of these would have purple flowers and long stems, since purple and long are both dominant. You get one PPll and two Pplls. These three are purple flowered with short stems. You also get one ppLL and two ppLls. These three are white flowered with long stems. Finally you get one ppll, a white-flowered short-stemmed plant. Thus, the predicted ratio of a dihybrid cross between two heterozygotes is 9:3:3:1.