Complete dominance is only one possible relationship between alleles. Two important alternatives are incomplete dominance and codominance.

Incomplete dominance

In incomplete dominance, the heterozygote has a phenotype intermediate between the two homozygotes for the measured trait.

For example:

RR → red flowers
Rr → pink flowers
rr → white flowers

For the cross

$$Rr\times Rr,$$

the genotype ratio is still

$$1RR:2Rr:1rr,$$

but now all three genotypes have distinguishable phenotypes. The phenotype ratio is therefore also

$$1:2:1,$$

not $3:1$.

The law of segregation has not changed. Only the mapping from genotype to phenotype differs.

Codominance

In codominance, both allele-associated products or features are detectably expressed in the heterozygote rather than blending into one intermediate state.

The human ABO blood-group locus provides a useful example. Three common alleles occur in the population:

$$I^A,\qquad I^B,\qquad i.$$

$I^A$ and $I^B$ are codominant with each other: genotype

$$I^AI^B$$

produces the AB phenotype, in which both A-associated and B-associated molecular markers are present on red blood cells.

Both $I^A$ and $I^B$ are dominant over $i$ in the usual ABO phenotype classification:

IᴬIᴬ or Iᴬi → type A
IᴮIᴮ or Iᴮi → type B
IᴬIᴮ         → type AB
ii           → type O

A population can contain more than two alleles

A diploid individual usually carries at most two allele copies at one autosomal locus, but a population can contain many alternative alleles. ABO illustrates this distinction with three common alleles in the population even though each diploid person carries two copies.

Segregation remains the same

Incomplete dominance, codominance and multiple alleles modify phenotype interpretation; they do not abolish chromosome segregation.

A heterozygote still sends one allele copy into each gamete. What changes is what phenotype is produced when two particular alleles meet again in the offspring.

The correct order of reasoning is therefore:

alleles segregate → offspring genotype forms → allele interaction determines phenotype

rather than assuming that every genetic cross should produce a dominant/recessive $3:1$ phenotype ratio.