Name: KEY
Incomplete Dominance Punnett Square Practice
Most genetic traits have a stronger, dominant allele and a weaker, recessive allele. In an individual with
a heterozygous genotype, the dominant allele shows up in the offspring and the recessive allele gets
covered up and doesn’t show; we call this complete dominance.
However, some alleles don’t completely dominant others. In fact, some heterozygous genotypes allow
both alleles to partially show by blending together how they are expressed; this is called incomplete
dominance. Other heterozygous genotypes allow both alleles to be completely expressed at the same
time like spots or stripes; this is called codominance. Examples of each are listed below.
1. Write what each type would be if they were heterozygous.
a. Complete dominance: if a red (RR) and a white flower (rr) were bred, resulting in 100%
heterozygous offspring (Rr), what phenotype would be seen according to the rules of
COMPLETE dominance?
Heterozygous phenotype: Red flower
b. Incomplete dominance: if a red (RR) and white flower (rr) were bred, resulting in 100%
heterozygous offspring (Rr), what phenotype would be seen according to the rules of
INCOMPLETE dominance?
Heterozygous phenotype: Pink flower
c. Codominance: if a red (RR) and white flower (WW) were bred, resulting in 100%
heterozygous offspring (RW), what phenotype would be seen according to the rules of
CODOMINANCE?
Heterozygous phenotype: both red and white mixed flowers, like stripes or spots.
Use a Punnett Square to predict the offspring of the following matches.
2. Snapdragons are incompletely dominant for color. They will have red, pink, or white flowers.
The red flowers are homozygous dominant, the white flowers are homozygous recessive, and
the pink flowers are heterozygous. Write the genotypes for each phenotype using the letters
“R” and “r”.
Red snapdragon: RR Pink Snapdragon: Rr White Snapdragon: rr .
a. Pink x Pink
Genotypic Ratios:
RR-1/4
Rr-2/4 or 1/2
rr-1/4
Phenotypic Ratios:
Red-1/4
Pink-2/4 or ½
White-1/4
b. Red x White
Genotypic Ratios:
Rr-4/4
Phenotypic Ratios:
Pink-4/4
c. Pink x White
Genotypic Ratios:
Rr-2/4 or 1/2
rr-2/4 or 1/2
Phenotypic Ratios:
Pink-2/4 or ½
White-2/4 or ½
