Phenotype

One of the most important aspects of Dominant Blue Eyes (DBE) genetics is that the presence of a genetic variant does not always predict the visible phenotype precisely.

Current evidence demonstrates variable expressivity among DBE cats. Incomplete penetrance of the blue-eye phenotype has also been documented for several PAX3-associated variants. Genetically confirmed cats carrying DBE-CEL, DBE-ALT or DBE-RE without displaying blue eyes have been reported. No non-expressing DBE-AGO cat was identified in the family examined to date.¹ ² ⁴

In breeder terminology, genetically positive cats that do not display the blue-eye phenotype are often described as latent. This is a descriptive term rather than a separate genetic category. These cats still carry the relevant variant and can transmit it to their offspring.

The factors responsible for variable DBE expression have not yet been established. Genetic background and developmental variation are plausible contributors, but their influence on eye colour and white spotting has not been demonstrated directly in DBE cats.

Reported Phenotypes

Cats carrying a DBE-associated variant may present with a range of visible phenotypes, including:

Two blue eyes — Both irises appear blue.

Odd eyes (heterochromia iridum) — One iris appears blue while the other is normally coloured.

Sectoral heterochromia — Part of a single iris appears blue while the remainder is normally coloured.

Latent phenotype — A genetically positive cat that does not display the blue-eye phenotype. Such cats still carry the relevant variant and can transmit it to their offspring.¹ ² ⁴

Minimal white spotting — Small or subtle areas of white, which may sometimes be difficult to detect.¹ ² ³ ⁴

Extensive white spotting — Some DBE cats may also show larger areas of white. In certain genetically characterised cats, extensive white spotting was explained by the co-inheritance of a separate white-spotting allele of the KIT gene. Extensive depigmentation has also been reported in homozygous or compound PAX3 genotypes. The extent of white spotting therefore cannot always be attributed to a single DBE variant based on appearance alone.² ⁴

Why Does This Happen?

The PAX3 gene functions within a complex biological network involved in neural crest and melanocyte development.

PAX3 acts as an upstream regulator of MITF, a gene essential for the differentiation of melanoblasts into melanocytes. Melanocytes contribute to pigmentation of the coat, skin and iris and are also required for normal inner-ear function.² ³

This regulatory network involves additional genes such as SOX10, KIT and EDNRB, which are known to influence pigmentation and, in some contexts, hearing phenotypes in mammals.

However, the specific genetic or developmental factors responsible for variable eye colour and white spotting in DBE cats have not yet been identified. The involvement of modifier genes therefore remains a plausible hypothesis rather than an experimentally demonstrated mechanism.