Genetics Notes 3
Dihybrid Cross
Dihybrid Crosses
• A dihybrid cross is still a cross between two
parents, but this time we look at 2 traits.
• A dihybrid cross may look confusing, but once
again, there are specific rules.
• If you follow them, you will always get it right!
Steps for Doing a Dihybrid Cross
1. Read the question completely.
2. Figure out each parent’s genotype.
3. FOIL each parent’s genotypes
SEPARATELY to find the gametes.
4. Put the gametes on a Punnett Square.
5. Fill in the Punnett Square to find possible
offspring.
6. Figure out the genotypic ratio & the
phenotypic ratio.
Dihybrid Cross Example
1. Read question completely.
Cross an individual that is homozygous
dominant for widow’s peak and homozygous
recessive for blue eyes with an individual that is
heterozygous for both traits.
Traits in This Cross
• Trait 1 is widows peak. Use W or w.
Dominant: Widow’s peak
Recessive: No widow’s peak
• Trait 2 is eye color. Use B or b.
Dominant: Brown Eyes
Recessive: Blue Eyes
Dihybrid Example Continued
2. Figure out the genotype of each parent.
Parent 1: Homozygous dominant for widow’s
peak: WW and homozygous recessive for eye
color: bb.
Parent 2: Heterozygous for both traits: Ww and
Bb.
Genotypes in Dihybrids
• Once you find the genotypes for each parent,
you can put them together.
Parent 1: WWbb
Parent 2: WwBb
The cross we are doing is: WWbb x WwBb
Dihybrid Example Continued
3. FOIL each parent’s genotypes separately.
FOIL: (First, Outer, Inner, Last)
Parent 1: WWbb
F: Wb O: Wb I: Wb L:Wb
Parent 2: WwBb
F: WB O: Wb I: wB
L wb
Dihybrid Example Continued
4. Put the gametes on a Punnett Square
Wb
Wb
Wb
Wb
WB
Wb
wB
wb
Dihybrid Example Continued
5. Fill in the Punnett Square.
Wb
WB
Wb
wB
wb
Wb
Wb
Wb
Probability in Dihybrid Crosses
6. Figure out the genotypic ratio and the
phenotypic ratio.
The genotypic ratio is 4:4:4:4
WWBb: 4 WWbb: 4 WwBb: 4 Wwbb: 4
The phenotypic ratio is 8:8
Widows Peak+ Brown Eyes= 8
Widows Peak+ Blue Eyes= 8
Do One on Your Own
• Cross two individuals who are both
heterozygous for widows peak and eye color.