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

2. 1. Determine dominant and recessive traits
Carefully read the problem!

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

4. 3. Determine genotype for parents
*Parents must have 4 alleles (aka 4 letters)
Heterozygous black, hairless male = Bbhh
White heterozygous hairy female = bbHh

5. 4. Draw the punnett square with 16 boxes

6. 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

7. 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

8. Sample Problem

9. 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

10. 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

11. Practice Problem Determine the genotype of the following bunnies:
Heterozygous Black-Hairy:_________
White hairless: ____________

12. Practice Problem Determine the gamete combinations
BbHh: _______________________________
bbhh: _______________________________

13. Practice Problem
Complete the punnett square:

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

15. Practice Problem BH Bh bH bh bh BbHh Bbhh bbHh bbhh
Complete the punnett square:
BH Bh bH bh bh
BbHh
Bbhh
bbHh
bbhh

16. 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

17. 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%