Monohybrid Cross Problems

Example Problem
In summer squash, white fruit color (W) is dominant over yellow fruit color (w).  If a squash plant homozygous for white is crossed with a plant homozygous for yellow, what will the phenotypic and genotypic ratios be for:

a. the F1 generation?     b. the F2 generation?
c. What will the phenotypic and genotypic ratios of the offspring be if you perform a testcross with the F1 generation?

Solution
1. Write down the cross in terms of the parental (P1) genotypes and phenotypes:

WW (true-breeding white fruit)  X ww (true-breeding yellow fruit)

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

WW       X        ww

 w W Ww

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

All F1 progeny will be heterozygous (Ww)  and will therefore have white fruit color.

4. Write down the cross involving the F1 progeny:

Ww (white fruit)     X     Ww (white fruit)

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

Ww           X             Ww

 W w W WW Ww w Ww ww

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

Genotypic ratios:  1/4 will be homozygous dominant (WW), 1/2 will be heterozygous (Ww) and 1/4 will be homozygous recessive (ww).   This is a 1:2:1 genotypic ratio.

Phenotypic ratios: 3/4 will have white fruit color and 1/4 will have yellow fruit color.  This is a 3:1 phenotypic ratio.

7.  Perform the testcross using the F1 generation:

Recall that a testcross is a cross between an individual (usually of unknown genotype, dominant phenotype) and a homozygous recessive individual, often used to determine the genotype of the first individual.

Ww (F1)            X            ww (homozygous recessive)

 w W Ww w ww

Genotypic ratios: 1/2 heterozygous (Ww) and 1/2 homozygous recessive(ww).  This is a 0:1:1 genotypic ratio.

Phenotypic ratios: 1/2 will have white fruit color and 1/2 will have yellow fruit color.  This is a 1:1 phenotypic ratio.

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