Monohybrid Cross Problems
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?
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
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
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.