1. Polygenic Traits & Dihybrid Crosses

2. N11-Polygenic Traits and Dihybrid Crosses
Polygenic Traits– when more than one gene is involved in a single trait
Most Traits are Polygenic
Eye color –
Gene 1: B- Brown eyes, b- blue eyes
Gene 2: G – Green eyes, g – blue eyes
Phenotype
Genotypes
Brown
BB gg; Bb gg;
Hazel **
BB GG; BB Gg; Bb GG; Bb Gg
Green
bb GG; bb Gg
Blue
bb gg
** Brown is also dominant (incompletely) over green because the brown pigment is darker than the green pigment. Therefore Hazel Genotypes may show up as Brown eyes or Hazel eyes

3. N11-Polygenic Traits and Dihybrid Crosses
How to Use a Punnet Square – Dihybrid Cross (Two Traits)
Determine the code for the alleles of each gene
Determine each of the four possible combinations of alleles for each parent – FOIL their Genotypes
FOIL = First pair, Outside pair, Inside pair, Last pair
Eye Color examples
Bb Gg
F- BG, O- Bg, I- bG, L- bg
Bb GG
F- BG, O- BG, I- bG, L- bG
bb Gg
F- bG, O- bg, I- bG, L- bg
Use the square to combine the parent’s allele to determine the probability that the offspring will have brown, blue, green, or hazel eyes

4. N11 Continued 4. Dihybrid Cross Example – B- Brown Eyes G – Green Eyes
b – Blue Eyes g – Blue Eyes
Mom’s Genotype – BbGg : FOIL Mom –
Dad’s Genotype – bbgg : FOIL Dad –
BG, Bg, bG, bg
bg, bg, bg, bg BG Bg bG bg

5. 4. Dihybrid Cross Example – B- Brown Eyes G – Green Eyes
When filling in the cross, write the alleles alphabetically then capital before lower case
4. Dihybrid Cross Example –
B- Brown Eyes G – Green Eyes
b – Blue Eyes g – Blue Eyes
Mom’s Genotype – BbGg : FOIL Mom – BG, Bg, bG, bg
Dad’s Genotype – bbgg : FOIL Dad – bg, bg, bg, bg BG Bg bG bg
BbGg
bbgg bg
Bbgg
bbGg
BbGg bg
Bbgg
bbGg
bbgg bg
BbGg
Bbgg
bbGg
bbgg
Bbgg
bbGg
bbgg bg
BbGg

6. 4. Dihybrid Cross Example – B- Brown Eyes G – Green Eyes
b – Blue Eyes g – Blue Eyes
Mom’s Genotype – BbGg
Dad’s Genotype – bbgg
Offspring Phenotype Probability:
Brown Eyes –
Green Eyes –
Hazel Eyes –
Blue Eyes –
25% BG Bg bG bg
BbGg
bbgg
bbGg
Bbgg

7. 5. Dihybrid Cross Practice – B- Brown Eyes G – Green Eyes
b – Blue Eyes g – Blue Eyes
Mom’s Genotype – BBGg
Dad’s Genotype – Bbgg
Offspring Phenotype Probability:
Brown Eyes –
Green Eyes –
Hazel Eyes –
Blue Eyes –
50% 0% BG BG Bg Bg BG BG Bg Bg
BBGg
BBGg
BBgg Bg Bg
BBgg
BBgg
BBgg
BBGg
BBGg
BBGg Bg
BBGg Bg
BBgg
BBgg
BBGg
BBGg
BBgg
BBgg bg bg
BbGg
BbGg
Bbgg
Bbgg
BbGg
BbGg
Bbgg
Bbgg
Bbgg
BbGg
Bbgg bg
BbGg
Bbgg
BbGg
Bbgg bg
BbGg