This is the first inheritance post in the Genetics 101 series. If you want to start at the beginning to learn more about the terms used in this article and why they are important, start here. If you want to learn more about breeding with harder genes, including dominant and recessive genes, look here.
Incomplete dominant genes are one of the easiest genes to work with in livestock, which is good because they are also one of the most common. They are found in plumage genes (think blue/black/splash in chickens), fur and skin genes (roan in cattle), and overall general defects and disqualifications (like scurs in sheep). Very few general color mutations are completely recessive or completely dominant. This article will be focused on autosomal (normal) incomplete dominant - sexlinked genes will have their own post that builds on this information. You may also see references to co-dominant traits. Codominance is slightly different than incomplete dominant, but works in the exact same way. This article applies to both of them, but incomplete dominance is used more often so that's what it will be referred to in this article to keep things simple.
What is incomplete dominant?
Just as the name suggests, it means the gene is neither completely dominant nor completely recessive. Let's back up just a bit. Each animal has two spots on each locus, so there are three different ways for the gene to show up: wildtype (which is no genes at all on either spot), heterozygous (one spot filled up by the gene, and one spot that is regular or filled up with something else), and homozygous (both spots filled up by the gene).
Incomplete dominant genes show up differently in all three forms. The wildtype, heterozygous, and homozygous forms all look different enough that you can see the difference without any testing. The wildtype form will look completely normal, without any genes. The homozygous form (two copies) will be the full color or pattern or general expression of the gene. The heterozygous form (one copy) will be an in-between version of the two. For colors, this is often a mixed color. For patterns, you may see some of the pattern showing up, but not all of it. For other structural things, it follows the same pattern.
Being able to see whether you have zero, one, or two copies of a gene is a huge asset when breeding, because it allows you to sort out the animals with certain traits right away and get an idea of what you will get out of a pairing right away, without any test breeding.
Breeding in Incomplete Dominant Genes:
There are many reasons why you would want to breed in an incomplete dominant gene. Maybe you're looking for a new color pattern, or structural trait that you would like to see in your breed or variety. It doesn't have to be hard. In fact, with proper breeding techniques, it is possible to go from a single split pair to a full gene in less than four generations, every time. Most of the difficulty comes in two things: your starting stock, and any other genes involved.
There are five total combinations and crossings that you can do with incomplete dominant genes. Each will have different outcomes. The process you work through to get there can change, but the same outcomes will always be the same.
Let's start with the goal: hopefully here it is to get the homozygous (two copy) form. If you are breeding towards any sort of standard, almost all of them will be geared towards the homozygous form, like fee in Coturnix quail, or beards and muffs in chickens. There are exceptions to this, of course. Not all genes are desirable in homozygous form, like blue in chickens (which is heterozygous), but most are. So in general, you will want to keep most of your homozygous and heterozygous birds, and eliminate wildtype.
If you look at the chart above, you can see the goal is to get to the top of the chart, with a color that breeds true. The hardest way to get there is to start at the bottom and work your way up - that will take at least three generations if you plan it right. The easiest way is to start near the top already, with a homozygous and heterozygous breeding pen. Then it only takes one generation to get the final result: all homozygous birds.
Let's track an example, using Dotted White in Coturnix quail. You can follow along with whatever gene you are working with. Our goal is to get to English White (which is the homozygous form), and we know that Tuxedo is our heterozygous form. For clarity, we will start with our hardest pairing - Tuxedo (heterozygous) and Wildtype, and work our way up from the bottom.
First Generation: Wildtype (we'll name her Barb) and Tuxedo (We'll name him Tux).
(Wildtype x Heterozygous).
Outcomes: Half of the chicks will be tuxedo, half will be wildtype. We don't need any of the wildtype chicks, so those will be sold or used for a different breeding project, or raised for butcher. We call those culls. We can also sell Barb at this point, as long as we get enough chicks from her.
Of those tuxedo chicks, we need at least one female. We'll name this gal Oreo.
Second Crossing: We need to move up the ladder. So we'll cross Oreo with Tux or Tux Jr.
(Tuxedo x Tuxedo, or heterozygous x heterozygous).
Outcomes: half of the chicks will be tuxedo again, but we really don't need more tuxedo. So these will be culls. A quarter of the chicks will be wildtype, and we don't need those either. Our last quarter will be the awaited English White! Those we will save. We'll say we hatched a dozen chicks, and got three English White: Billy (male), Bob (male), and Briar (female).
Third Crossing: Here we have some options. We can keep Briar and Billy, and have our final English White crossing. But maybe there's problems with Billy and Bob. Maybe they have crooked beaks or don't grow as fast as we want, or got mean. Or maybe we want to avoid full sibling crosses. Whatever the reason, we can always add one more generation if needed. We'll use Tux or Tux Jr. one more time and cross him with Briar.
(Tuxedo x English White, or heterozygous x homozygous).
Outcomes: Half of the chicks are tuxedo, and half are English White. This gives us much better odds of hatching out some nice English White to cross over Briar, or even breed back to Billy or Bob if they turned nice again.
Fourth Crossing: English White x English White. (Homozygous x homozygous).
Outcome: All English White chicks! We did it. It took a few generations, but working through the chart, we were able to get to all English White.
Obviously, this is a single example. It is possible to do this several different ways and combinations, and if you start somewhere else on the chart, you could achieve the same outcome with significantly less time and space. But this shows how you can plan and set up your breeding pens to breed in a gene.
Breeding Out Incomplete Dominant Genes:
Breeding out an incomplete dominant gene works pretty much exactly the opposite of breeding the gene in. It can get a bit tricky to begin, because you'll need to start with an animal that isn't homozygous. This may mean you need to bring in a new bloodline, if you've been breeding homozygous genes in for a while. Just be careful, because outcrossing to a new bloodline, variety, or breed may bring in other genes and mutations that can make it harder to see and get to the end goal here.
This time, we are starting from the top of the chart and working our way to the bottom. Like breeding in the gene, the closer we start to the bottom, the easier it will be to get to the goal. In general, you'll want to keep any wildtype, and cull down to that. I won't go through a long example here, because you can start with F3 on the previous example and work backwards to get to F1. There isn't too many twists or turns here. Just follow the chart.
Maintaining Incomplete Dominant Genes:
In general, once you've established a line with two copies of the gene (homozygous), it will always breed true. This makes it easy to keep the color or trait going while starting to select hard for the non-Mendelian traits, or things that are a bit harder to breed for, like temperament or size or stature, and other useful aspects. You can continue linebreeding (father x daughter and mother x son) until you are happy with the birds you are getting out, then switch to spiral breeding (more on that coming soon) to keep that consistency while preventing inbreeding depressions. All of this will keep working with that full-blooded, homozygous line. However, continuously breeding for homozygous traits aren't always the best thing.
In some cases, like with the Blue/Black/Splash gene in chickens, breeding with only splash (homozygous, two copies of BBS) for generations and generations will cause the bird to become a wishy-washy, poorly colored bird regardless of how much you select for good markings. In this situation, it is best to outcross every so often, bringing back the blue (heterozygous for BBS) every three generations or so. This will cut the total number of splash birds in half (as half will be blue), but increase your overall color quality. It is important to be careful what blue birds you are bringing in though. Outcrossing with a different breeder's line can sometimes bring in hidden recessive genes or other troublesome things you'll have to deal with later. Many breeders avoid this by keeping a small flock of blue birds alongside the splash. Similar things can be done with Dotted White in quail (English White and Tuxedo) to maintain a line of consistently pretty and well-marked Tuxedo (heterozygous form) alongside the English White to prevent the birds from losing the wildtype marking on the back of their heads.
There are also circumstances where establishing a homozygous line isn't practical or may not be the end goal. For example, the frizzle gene in chickens causes a condition called frazzle in homozygous form. The feathers on frazzle birds will be so brittle that they often break off and bleed, and the bird often has pain from even normal living conditions. It is not ethical to breed for frazzle in chickens, even though frizzle (the heterozygous form, with only one copy) is highly desired and doesn't have any health defects. There's an even more extreme example called Lethal Yellow in Coturnix quail. Just as the name suggests, the gene is lethal in homozygous form. Chicks with two copies of Lethal Yellow will die in the shell after only a few days of development, making it impossible to breed with the gene or maintain the line the way we normally would. In all of these cases, the end goal changes: we want to establish the heterozygous line, instead of the homozygous one. Other examples are Andalusian and Silver in Coturnix quail, and Tufted in chickens.
In this situation, we want to avoid homozygous at all costs. The best way to keep a line going here is to breed heterozygous birds to wildtype. You can cross a frizzle to a non-frizzle, and get half of the chicks to be frizzle without risking any health issues. You can also do this with the tufted gene, Lethal Yellow, and any gene where you want to avoid the homozygous form. The fastest way to start this type of flock is to have a heterozygous rooster (so one frizzle rooster, or one tufted rooster, or one Andalusian quail), over multiple wildtype hens. Regardless of how many wildtype hens you have, because they all have the same father, you'll get half the chicks to have that trait. Once you get the set-up established - say F2 or F3, you'll often want to switch to a wildtype cockbird over heterozygous hens. Changing the above scenario: wildtype cock over Andalusian hens, or smooth feathered rooster over frizzle hens. This allows you more control over individual traits (spread the wealth across the pen), and more ability to see how consistent the trait is coming through across birds. Either way is fine - you will end up with half of the chicks in heterozygous form, but both serve different purposes in the breeding pen.
The end message here is to know your genes, and know them well. Even once you've established your optimal color or pattern, knowing how the gene interacts with both other colors and mutations, as well as its own, really can change how you set up your breeding pen and the path you want to take to achieve your goals.
Examples of Incomplete Dominant Genes:
There are a lot of examples of incomplete dominant genes across the poultry world and beyond, though some are hard to see. That is where the real-life work differs from the paper. It takes practice and familiarity to be able to separate out what is true wildtype, and what is heterozygous and homozygous from looks alone.
Here are just a few examples of the "wildtype", heterozygous, and homozygous forms of different incomplete dominant genes so you can see how easy (and difficult) it can be in real life to spot them.
Gene | Species | Wildtype | Heterozygous | Homozygous | Notes: |
Andalusian | Quail | Pharaoh | Andalusian | Dead | Lethal Gene |
Blue | Quail | Any Base | Blue | Cream-colored | N/A |
Blue | Chickens | Black | Blue | Splash | N/A |
Crest | Chickens | No Crest | Partial Crest | Full Crest | N/A |
Rumpless | Chickens | Full Tail | Rumpless | Fused Vertebrae | Partial Lethal |
Dotted White | Quail | Any Base | Tuxedo | English White | N/A |
Fawn | Quail | Pharaoh | Italian | Manchurian | N/A |
EB | Quail | Pharaoh | Rosetta | Tibetan | N/A |
About the Author:
Breanna Patz, more affectionately known as Bre, is the current ACBA president and member of both the APA and ABA. She was recognized on Coturnix Corner's Recommended List in both 2023 and 2024. She is a current WI District Livestock and Poultry Judge and has been working on industrial poultry research since 2024. Bre currently resides in Seymour, WI where she spends her days planning the next projects for Pips 'n Chicks, caring for and showing her animals, playing piano for the local churches, and working part-time for County Rescue.
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