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Dihybrid Crosses **Crossing 2 different traits at the same time

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**1. Determine dominant and recessive traits**

Carefully read the problem!

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B = black b = white H = Hairy h = no hair 2. Assign letters for the trait

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**3. Determine genotype for parents**

*Parents must have 4 alleles (aka 4 letters) Heterozygous black, hairless male = Bbhh White heterozygous hairy female = bbHh

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**4. Draw the punnett square with 16 boxes**

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**5. Put parent gamete combinations on box**

Bbhh gametes: Bh, Bh, bh, bh bbHh gametes: bH, bh, bH, bh Bh Bh bh bh bH bh BbHh bbHh Bbhh bbhh

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**6. Determine the Genotype and Phenotype Ratio Genotype:**

4 BbHh : 4 bbHh : 4 Bbhh : 4 bbhh Phenotype: 4 Black and Hair : 4 White and Hair : 4 Black and Hairless : 4 White and Hairless

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Sample Problem

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Sample Problem Genotype Ratio: ___ RRYY : ___ RRYy : ___ RrYY : ___ RRyy : ___ RrYy : ___ rrYY : ___ Rryy : ___ rrYy : ___ rryy Phenotype Ratio : ___ Round and Yellow : ___ Round and Green : ___ Wrinkled and Yellow : ___ Wrinkled and Green

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**Sample Problem Genotype Ratio: 1 RRYY : 2 RRYy : 2 RrYY : 1 RRyy :**

4 RrYy : 1 rrYY : 2 Rryy : 2 rrYy : 1 rryy Phenotype Ratio : 9 Round and Yellow : 3 Round and Green : 3 Wrinkled and Yellow : 1 Wrinkled and Green

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**Practice Problem Determine the genotype of the following bunnies:**

Heterozygous Black-Hairy:_________ White hairless: ____________

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**Practice Problem Determine the gamete combinations**

BbHh: _______________________________ bbhh: _______________________________

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Practice Problem Complete the punnett square:

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Practice Problem Complete the punnett square: BH Bh bH bh bh

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**Practice Problem BH Bh bH bh bh BbHh Bbhh bbHh bbhh**

Complete the punnett square: BH Bh bH bh bh BbHh Bbhh bbHh bbhh

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**Practice Problem Give the genotype ratio: Give the phenotype ratio:**

4 BbHh : 4 Bbhh : 4 bbHh : 4 bbhh 4 Black and Hair : 4 Black and Hairless : 4 White and Hair: 4 White and Hairless

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**Extra tips When determining the probability of events**

If the problem states “and” then multiply the chances for each Chance of heads and heads = ½ x ½ = ¼ or 25% If the problem states “or” then add the chances for each Chance of heads or tails = ½ + ½ = 1 or 100%

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