Part Four: Partial Penetrance & Variable Expression

Why some recessive genes “show a little” even when they shouldn’t

Most breeders learn the simple rule:

Dominant genes show with one copy

Recessive genes only show with two copies

But real-life genetics isn’t always that tidy. Some recessive genes don’t stay completely hidden in carriers. Instead, they can influence the phenotype just a little, even when only one copy is present.

This is called: Partial penetrance — the gene doesn’t fully express, but expresses sometimes or incompletely or Variable expression — the trait shows in different degrees, not all-or-nothing

In other words:

A cat may not show the full recessive trait — but it may show a hint of it.

The Longhair Gene & British Shorthairs

A perfect real-world example

The longhair gene (l) is recessive:

• L- = Shorthair

• ll = Longhair

Traditionally, we say carriers (Ll) look completely shorthair — but in some breeds, especially the British Shorthair, that’s not entirely true.

What breeders have observed:

British Shorthairs that carry longhair often have:

• A softer, silkier coat

• Less dense “crisp” texture

• Slightly longer guard hairs

• A less plush feel

They aren’t longhaired — but they don’t always look like classic “true” shorthairs either.

This is a classic case of variable expression: The recessive longhair allele is influencing the coat a little, even though the cat is still phenotypically shorthair.

British Long Hair Makdissy Prince Harry

Simple Illustration

Genotypes and Coat Appearance

Genotype	Genetic Status	Coat Look
LL	Shorthair, not a carrier	Dense, crisp, plush coat
Ll	Shorthair, carrier of longhair	Shorthair, but often softer, finer, less texture
ll	Longhair	Full long coat

So even though Ll cats are genetically shorthair, they sometimes show a “softened” phenotype. This is NOT full expression of the longhair gene. It’s partial influence.

Why Does This Happen?

Two reasons:

1) Incomplete Recessivity

Some recessive genes aren’t completely silent in the heterozygous form. They can subtly affect the trait.

2) Modifier Genes

Coat length and texture aren’t controlled by a single gene. Carriers may interact with other coat genes, amplifying the effect.

So the longhair allele doesn’t fully express — but it can “nudge” the phenotype.

Why This Matters in Breeding

For breeds where coat texture is a key standard — like the British Shorthair — this means:

• Too many longhair carriers in a line may soften coat quality

• Selecting only for colour (especially recessives) can make texture drift

• Cats with the best texture may often be LL non-carriers

This doesn’t mean carriers shouldn’t be used — but it does mean coat texture should stay a selection priority, not an afterthought.

Key Takeaways

• Some recessive genes don’t behave strictly “on/off”

• Longhair carriers in some breeds show subtle coat changes

• This is called partial penetrance or variable expression

• Carriers are still valuable — but phenotype should guide selection

• Texture traits may require balancing carriers and non-carriers in breeding plans

Final Thought

Genetics loves exceptions. The longhair British Shorthair is a perfect reminder that what a cat carries can sometimes whisper through its coat, even when the recessive gene isn’t fully expressed.

Beautiful breeding happens when we consider both the genotypeand the phenotype

And recognise that sometimes, the truth lies in-between.