Before anything else, I want to start by saying this is a sensitive topic, and what I’m about to share is simply my personal perspective.

This isn’t meant to criticize anyone or any breeding practice — just a chance to talk honestly about something that matters to many of us in the reptile and animal-breeding world.

Understanding Inline and Outline Breeding

If you’ve done any breeding — whether with geckos or any other animals — you’ve probably heard of inline breeding and outline breeding.

We often see animals sold under names like “pure line” or “original line.”There’s nothing wrong with that — in fact, I personally love pure lines too.

In almost every field of animal breeding — not just reptiles, but also cattle, chickens, pigs, and even laboratory mice — inline breeding is a process that happens and, in some cases, is even necessary.

For example, when we want to strengthen a new morph or stabilize a specific trait, inline breeding is sometimes the only path forward.

The Genetic Foundation

Inline breeding, by its nature, reduces genetic diversity — the variety of possible genotypes within a population.

A genotype is simply the genetic combination that makes up an individual. Each gene comes in pairs. If one is dominant and one is recessive (Aa), genetic diversity is high. If both are the same (AA or aa), diversity is low.

As inbreeding continues, the number of heterozygous pairs (Aa) decreases, and the number of homozygous pairs (AA or aa) increases.

When homozygosity rises, certain genotypes and phenotypes become more consistent and visible, which is why we say the “line is fixed.”This is the reason offspring begin to look more and more like their parents — and in many breeding programs, that’s often seen as success.

And yes, in a way, it is.

The Hidden Problem

However, this is where problems begin to appear.

Many recessive genes don’t cause any issues when they are paired as Aa — but once inbreeding leads to aa combinations, those hidden defects start to show up.

In other words, as heterozygosity decreases, we see a higher frequency of genetic disorders, deformities, and reduced vitality.

Comparing Inline and Outline Breeding

This isn’t just theoretical — human history gives us many examples.

Lessons from History

In royal families, hemophilia was known as “the royal disease.”Because European royalty often intermarried to preserve their bloodlines, this recessive disorder spread widely.

In ancient Egypt, the pharaoh Tutankhamun was the product of generations of royal inbreeding — and his bloodline ended with him.

The Habsburg dynasty of Spain is another classic case, where generations of inbreeding led to the well-known “Habsburg jaw” and severe health problems.

And of course, the same risks apply to animals.

Examples in Animal Breeding

Tigers, for instance, became victims of excessive inbreeding when people tried to produce white or golden tigers for zoos. The result? Many were born cross-eyed, deformed, or with kidney defects.

Guppies offer another example. When breeders wanted to fix certain colors or tail patterns, they repeatedly inbred siblings or even backcrossed offspring to parents. Once the traits became stable, they were called “fixed strains.”But these guppies also became fragile — they had weaker immune systems, shorter lifespans, and less adaptability compared to “mutt guppies.”While mixed guppies might live 2–4 years, fixed-line guppies often live only around six months to a year.

Even in insects, like the king stag beetle (Dorcus Hopei), breeders have practiced years of inline breeding to produce massive jaws — but eventually ended up with deformities such as elytra (wing covers) that wouldn’t close properly.

Why We Still Do It

And yet, we still do it — fully aware of the risks. Because when we want to understand, strengthen, or prove a genetic trait, inline breeding is sometimes unavoidable.

Without it, many of the morphs and lines we know today would not exist. What we see on MorphMarket — all those morph names, patterns, and colors — many were first revealed through controlled inline breeding.

Of course, today some labs can sequence individual genomes and identify morph-defining mutations scientifically, without any inbreeding at all —but for most breeders, that level of research is still out of reach.

The Role of Genome Complexity

Another thing to consider is how genome complexity affects the outcome of inbreeding. Simply put: the more genes an organism has, the higher the risk.

• Single-celled organisms (like bacteria or yeast) have a few thousand genes.

• Multicellular animals (like reptiles, mammals, or humans) have tens of thousands. That means more genetic interactions — and more ways for something to go wrong.

  
  

Genome Size and Genetic Risk

So, how complex are the animals we breed?

• Most gecko species have a genome size of about 1.5–2.2 Gbp, roughly half the size of the human genome. Their estimated gene count is around 18,000–22,000.

• Guppies, on the other hand, have a genome of only 0.7–0.8 Gbp — less than half that of geckos.

So yes, you can imagine what that means: Geckos, with roughly twice as many genes, have a more complex genome —and therefore, inbreeding problems are more likely to appear.

Finding Balance

So what should we do as breeders?

In my opinion, inline breeding cannot be completely avoided,but unnecessary inline breeding should be minimized.

Our first priority should always be producing healthy, strong animals.

If you must line breed, choose wisely:

• Sibling or parent-child pairings (tight inline) → rapid homozygosity, high risk.

• Cousins or half-siblings (mild inline) → slower, more stable, moderate risk.

If possible, prefer the second option — or even better, work toward outline breeding whenever you can.

The Value of Outline Breeding

Outline breeding takes more time and effort. It’s slower, more complicated, and sometimes less predictable —but over multiple generations, it builds healthier, stronger animals with more genetic stability.

And when breeders invest that time and effort to produce a healthy morph line —that’s something truly worth supporting and celebrating.

Final Thoughts

Inline breeding has its place in the development of new traits. But every breeder should remember that behind every pattern and color we love, there is a living creature that depends on our responsibility.

So let’s strive to balance genetic discovery with animal welfare, and continue building this community on knowledge, respect, and care.

Happy gecko keeping, everyone. 🦎