Dihybrid Cross Problems

Example Problem

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?

Solution

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)

 

2.  Determine the P₁ gametes, place them in a Punnett Square and fill in the resulting genotypes:

WWDD       X        wwdd

3. Determine the genotypic and phenotypic ratios for the F₁ generation:

All F₁ progeny will be heterozygous for both characters (WwDd)  and will have white, disk-shaped fruit .

 

4. Write down the cross between F₁ progeny:

WwDd (white, disk-shaped fruit)     X     WwDd (white, disk-shaped fruit)

5.  Determine the F₁ gametes, place them in a Punnett Square and fill in the resulting genotypes:

WwDd   X     WwDd

6. Determine the genotypic and phenotypic ratios for the F2 generation:

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)

This is a 1:2:2:1:4:2:1:2:1 genotypic ratio!  Yuck!

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.