Dihybrid Crosses

**Crossing 2 different traits at the same time

1. Determine dominant and recessive traits

Carefully read the problem!

2. Assign letters for the trait

B = blackb = whiteH = Hairyh = no hair

3. Determine genotype for parents

*Parents must have 4 alleles (aka 4 letters)

Heterozygous black, hairless male = Bbhh

White heterozygous hairy female = bbHh

4. Draw the punnett square with 16 boxes

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

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

Sample Problem

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

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

Practice Problem

Determine the genotype of the following bunnies:

Heterozygous Black-Hairy:_________ White hairless: ____________

Practice Problem

Determine the gamete combinations

BbHh: _______________________________ bbhh: _______________________________

Practice ProblemComplete the punnett square:

Practice ProblemComplete the punnett square:

BH Bh bH bh

bh

bh

bh

bh

Practice ProblemComplete the punnett square:

BbHh Bbhh bbHh bbhh

BbHh Bbhh bbHh bbhh

BbHh Bbhh bbHh bbhh

BbHh Bbhh bbHh bbhh

BH Bh bH bh

bh

bh

bh

bh

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

Extra tipsWhen 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%