1. Dihybrid Crosses

2. When is it used?
A dihybrid cross is exploring the possibilities of TWO traits. For example, what will the offspring look like when a yellow, wrinkled pea plant is crossed with a green, smooth pea plant. You are looking at two traits- pea color and pea texture.

3. Steps to Set up a Dihybrid Punnett Square
Write out allelic combination
USE THE F.O.I.L. method to set up Punnett Square
F=first allele combination
O= outer allele combination
I= inner allele combination
L= last allele combination
Conduct the test cross
Evaluate the genotypes and phenotypes

4. Practice Example
In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d)..  If a squash plant true-breeding for white, disk-shaped fruit  is crossed with a plant true-breeding for yellow, sphere-shaped fruit,  what will the phenotypic and genotypic ratios be for:
a. the F1 generation?     b. the F2 generation?

5. 1. Write down the cross in terms of the parental (P1) genotypes and phenotypes:
WWDD (white, disk-shaped fruit) X wwdd (yellow, sphere-shaped fruit) F= WD F=wd O= WD O=wd I=WD I= wd L=WD L= wd

6. F2 generation will have two heterozygous parents crossed
All F1 progeny will be heterozygous for both characters (WwDd)  and will have white, disk-shaped fruit . WD wd
WwDd
F2 generation will have two heterozygous parents crossed

7. Determine the F2 gametes and write out all possible allele combinations
WwDd   X     WwDd
F=WD F= WD
O=Wd O=Wd
I=wD I=wD
L=wd L= wd

8. Conduct the test cross

9. Evaluate your results
Genotypic ratios:  1/16 will be homozygous dominant for both traits (WWDD) 2/16 will be homozygous dominant for color and heterozygous for shape (WWDd) 2/16 will be heterozygous for color and homozygous dominant for shape (WwDD) 1/16 will be homozygous dominant for color and homozygous recessive for shape (WWdd) 4/16 will be heterozygous for both traits  (WwDd) 2/16 will be heteozygous for color and homozygous recessive for shape (Wwdd) 1/16 will be homozygous recessive for color and homozygous dominant for shape (wwDD) 2/16 will be homozygous recessive for color and heterozygous for shape (wwDd) 1/16 will be homozygous recessive for both traits (wwdd)

10. Phenotypic ratios: 9/16 will have white, disk-shaped fruit 3/16 will have white, sphere-shaped fruit 3/16 will have yellow, disk-shaped fruit 1/16 will have yellow, sphere-shaped fruit
This is a 9:3:3:1 phenotypic ratio.

11. Practice this in your notes
President Obama and Michelle Obama want to have another child and want to figure out their chances of having certain traits in their offspring. They wonder about the following traits.
E= round eyes D= dark skin T= tall W= widows peak
e= oval eyes d= light skin t= short w= no widows peak

12. Problem 1
Barack Obama
1-Homozygous Dominant eyes w/ heterozygous for height
Michelle Obama
1-Heterozygous for eyes w/ homozygous for height