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The North American Curly Horse Genetics

Genealogy 1
Following are some general genealogy points regarding Noth American Curly horse breeding. There are current, ongoing research projects attempting to determine the passing of the Curly gene.
I do not profess to be a genealogy expert, but I do have lots of references for you to investigate and some research on your own.
The only thing we know for certain, is that we do not know what gene passes the curly traits for certain!

When you breed your heterozygous curly stallions to a straight mare you are basically flipping a coin, is dad going to pass a C (curly) or a c (straight) gene, because mom has only c (straight) to pass. Baby needs at least 1 C to be curly. So just like tossing a coin, just because you got heads one time does not guarantee you tails the next. Each breeding is an independent roll of the genetic dice. The only thing you can bank on is your odds.

Example 1

Curly Dad - CC (this is homozygous)
can only pass C

Mom straight - cc
can only pass a c

Since the C is dominant, you will get a curly foal.

Example 2

Curly Dad - Cc (this is heterozygous)
can pass either C or c

Mom straight - cc
can only pass a c

Baby can end up with the combination of Cc (curly) or cc(straight). 50/50 odds

Example 3

Curly Dad - Cc (this is heterozygous)
can pass either C or c

Curly Mom - Cc (assuming heterozygous if she has ever given birth to a straight) can also pass either a C or c

Baby- this is where it gets tricky because you have to account for any combination so this gets you:

Dad      Mom
C           C      (curly)
C            c      (curly)
c             C      (curly)
c             c      (straight)

3/1 odds in favor of curly

So you can see the big ho-ha with having a homozygous (CC) stallion or mare because no matter what you breed that horse to they only have a C that they can pass so therefore ALL offspring are guaranteed to get at least one curly gene and will be curly.

If you get a true straight from any crossing then you know that both the stallion and mare must carry at least one c gene. If the parents were both curlies then you know that they are both heterozygous (Cc).

Visually the way it works, just look at the F1 generation and substitute C for S and c for s

To learn more on genetics vocabulary go to

To see a current discussion of Curly genetics go to


How the heck do you pronounce "homozygous"?!
Hoe - mo - z-eye- gus
Homozygous means "the same" - All straights are homozygous, they have two no genes (cc). It is thought that some curlies can be homozygous for the yes genes (CC) which would mean that they would always send a yes gene to their babies and since you only need one yes gene (C) to see curls then they would always produce curls no matter what they are breed to.
Right now there is no test to determine if a horse is homozygous for curls other than to look at their breeding record. A dead giveaway is any straight offspring then they could not be homozygous (CC) because for that baby to be straight then both genes must say no (cc) and one of those no had to come form each parent. If only curlies babies have resulted then they might be homozygous, and the more curlies babies they have the more likely it is.

What is all this talk about recessive genes in other breeds? I.E. we hear it about Morgans and work horses. Do they actually carry a curly gene? Or do we know.
It is believed that there are type ways that a curly can occur, dominant and recessive. The dominant type are the ones that you are most familiar with in your breeding. The recessive type are believed to be the cause of a curly out of nowhere in another breed such as Morgans, Arabs, mini's, etc. This will make more sense if you jump to the next part first.......If you weren't already confused.... there appears to be another gene spot (loci) on the chromosomes that can also cause the curly coat but it is a "recessive trait" symbolized by "s" for yes to curls. As a recessive trait it would require that BOTH genes say yes (ss) for the coat to be curly. So if one say no (S) then no curls. What happens in other breed is you have some horse running around with a few of these yes genes (s) but you won't know because the no gene (S) makes them have a straight coat. If two of these Ss horses breed then there is a possibility that they BOTH might pass the yes gene to their offspring (1 in 4 chance). If that happens the baby from two straight parents ends up with the two recessive yes's it needs to look curly (ss) and Voila... here is a curly coated baby from non curly parents.
This type of inheritance is much more illusive and is more speculative than the dominant curly, mostly because there are not that many to study.

So does this mean that all horses have two genes for coat type? One from each of their parents?
YES!!! For every trait there are two genes. You and most every animal gets 1/2 a set of genes from mom and 1/2 set from dad to create a full set and therefore a human, or horse. When the sperm and egg are produced each parent has passed only one copy of the two genes they have for any given trait to the baby. What the baby looks like (phenotype) depends on how the two inherited genes (genotype) interact. The genes come in different flavors (alleles). The curly coat in our horses is believed to be a dominant trait which means that in the set of genes controlling if the horse has it or not only one gene has to say yes- symbolized as "C". But remember they have two genes there. If the other one says no to curls - symbolized as "c" then you have a Heterozygous horse or (more than one allele) - Cc. So what does this mean to the baby, well remember back to the idea of dominant. The gene say yes "C" dominates over the gene saying no "c" so you see a curly baby. If that baby goes on to breed it will send only one of those two genes to its off spring - C or c- but which one? That is where the % come in. Half the time C (yes) and half c (no). But you also have to account for who this horse is bred to. If it is breed to a straight who would have no gene saying yes (CC) then this horse would always send a no gene (c) to the offspring and it would be up to the heterozygous curly (Cc) to supply the yes gene (C). If the Heterozygous curly only has a 50/50 chance of sending that yes gene then that would mean that you only have a 50/50 chance of seeing curl on that next baby.

Is it possible to have a normal straight haired horse that is homozygous for the straight hair gene?
Again this goes back to what is the THEORY of how the inheritance is happening, nothing is proven yet... that said, to be a truly straight curly (not a minimally expressed curly but straight as a board no curly characteristics what so ever) they must be homozygous (cc) because if they had a yes gene (Cc) they would show some curl because the Curly gene that we are concerned with appears to be dominant.

Is it possible to have a homozygous curly, that, if bred to a homozygous straight, cannot help but produce a 50/50 chance of getting a curly. If you have a homozygous curly stallion (CC) they can ONLY pass a yes gene (C) so.... no matter what they are breed to they would always produce curly offspring as this gene is dominant.

Are you saying that the curly gene will always be dominant, given a chance, over a straight gene.
Yup, if that yes gene is passed (C) it will win the arm wrestling match.
If you have a heterozygous curly station (Cc) (Yes/no) then you would get the following chances depending on the genotype of the mare:
Heterozygous curly mare (Cc)- 75% chance of curls
Homozygous curly mare (CC) - 100% chance of curls (because of the mare)
Homozygous straight mare (cc)- 50% chance of curls

There are many other ways that genes work to influence what we see in a organism. The notion that the Curls that we see and all the other cool traits that seem to come along with it appear to be controlled by such a simple process is very exciting, but again it remains to proven. Right now the evidence that the genetic big wiggs (of which I ain't) supports this theory and that is what we are looking at is trying to get enough info to hopefully have numbers to back up the theory.
The above information was obtained from Emily, a student in genetics and is all based on theory assumed at the present time.