Incomplete Dominance &
Codominance
Review of Mendel’s Principles



Genes are passed parents  offspring;
get one allele from each parent
During Meiosis, the alleles for a gene
segregate from each other.
During Meiosis, genes independently
assort with each other.
Exceptions to Mendel’s principles

Sometimes, there is no dominant or
recessive gene, or the trait is controlled
by many alleles or genes.
Incomplete Dominance

In Incomplete Dominance, every genotype has its own
phenotype. (One allele not completely dominant over the
other.) Third phenotype that is a blending of the parental
traits. (2 alleles produce 3 phenotypes.)
 Result: Heterozygous phenotype somewhere in
between homozygous phenotype.
1. Incomplete Dominance
Examples:
 Trait: Flower Color
Expressions: Red x
White  Pink
RR= Red; RW= pink; WW=
white

straight hair, wavy, curly
Incomplete Dominance
Codominance
In codominance, neither allele are dominant;
both are expressed. A cross between
organisms with two different phenotypes
produces offspring with has both phenotypes
of the parental traits shown.
2. Codominance

Both alleles contribute to the phenotype.

Example: In come chickens
Black Chicken x White  Speckled Chicken
YOU tell me which type of
dominance…
Codominance!
Type of
Dominance?
Incomplete
Dominance!
Type of Dominance?
Incomplete Dominance!
Test Cross (Back Cross)…
Used to determine whether an individual is
homozygous dominant or heterozygous.
RULES:



Always cross unknown genotype with a homozygous
recessive.
Observe (count) large numbers of offspring to insure
accuracy in determining the unknown genotype.
Then…
 if ANY offspring show the recessive trait, the
unknown genotype is heterozygous
 if ALL the offspring have the dominant trait, the
unknown genotype is homozygous dominant