Genetics page 1 Incomplete Dominance / Codominance
INCOMPLETE DOMINANCE Name:
and CODOMINANCE
Reading: CMR p.168-169
Incomplete Dominance
Previously you have studied genes in which one allele (one copy of a gene) was dominant and the
other recessive. For instance, a heterozygous individual whose genotype is Hh, where “H” = right
handedness and “h” = left handedness, would be right handed, since the right handed allele is
dominant.
There are some gene pairs in which neither allele is dominant, and when both alleles are present in
the chromosomes both traits are expressed in the phenotype. The resulting heterozygote
phenotype is typically in between the two different homozygote phenotypes. This pattern of
inheritance is called incomplete dominance. Examples of this include petal coloration in some flower
species, curlyness of human hair, and human hypercholesterolemia (see text for info on some of these).
In the first example, snap dragons or carnations that are homozygous for a red petal allele are red,
flowers which are homozygous for a white petal allele are white, and heterozygous flowers appear pink
due to the lack of dominance between the red and white color alleles.
In this case it is common to use two different capital letters of the alphabet to represent the red and
white alleles, such that: R = Red color
W= White color
In this example, red carnations would be RR, white carnations would be WW, and pink carnations
would be RW. This is summarized below.
Genotype Resulting Phenotype
RR Red flowers
WW White flowers
RW Pink flowers (usually, what phenotype would you expect for this genotype? _______)
Sample Problem #1
a. A red carnation is crossed with a white carnation and the resulting offspring are all pink (indicating
incomplete dominance). What would be the result of a cross between a pink and a white
carnation?
Pink carnation genotype: RW White carnation genotype: WW
R W Fill in the Punnett square and give the phenotype
percentages of this cross:
W
______ % are _________________ color
W ______ % are _________________ color
b. Could Red offspring be made from a cross of a White carnation with a Pink one? ____________
(over for answers)
if this were a dom/rec relationship
