Dihybrid Crosses

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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
• bH
• bh

BbHh BbHh bbHh bbHh
Bbhh Bbhh bbhh bbhh
BbHh BbHh bbHh bbHh
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 bh bh bh




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Practice Problem

• Complete the punnett square

BH Bh bH bh
bh bh bh bh
BbHh Bbhh bbHh bbhh
BbHh Bbhh bbHh bbhh
BbHh Bbhh bbHh bbhh
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