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

 wd WD 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

 WD Wd wD wd WD WWDD WWDd WwDD WwDd Wd WWDd WWdd WwDd Wwdd wD WwDD WwDd wwDD wwDd wd WwDd Wwdd wwDd 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.

D. Sillman   dys100@psu.edu
08/11/2001