The Overo Paint Horse

Overo Details

Overo Characteristics

1.  Generally white will not cross the back between the withers and the dock of the tail. 

2.  An overo may have no white markings on their legs to the expreemly high four white socks.

3.  Overo horses generally have white markings on thier faces.  Usually there is a blaze or more, this would also include an apron or bonnet face markings.

4.  The horse tail is generally one color.

5.  The overo markings are generally scattered and irregular.

6.  Overos can be either predominantly dark or white.

Overo Genetics

Overo genetics are not as well understood as tobiano genetics.  There may be as many as 10 genes producing the overo color.  It is possible to produce an overo cropout by breeding two solid horses that are not registered with the APHA, yet they are registered with the AQHA or the Jockey Club (thoroughbred) registries and thus can still be registered with the APHA.  I have read several books which state that the gene is ressessive but the studies which have been released recently say that the gene is dominant, yet still more complex than the tobiano gene.

There are also genetic problems such as leathal white which is fatal to the horses which recieve an overo gene from each parent.   A lethal white foal is essentially a homozygous overo.  It is also possible for a breeding stock (solid) which was produced by an overo parent and that is bred to an overo to produce a lethal white foal, which would die within 48 hours.  Lethal white foals are almost 100% white, generally. 

If the overo gene is truly a dominant gene then the following are true, except we must consider the genetic condition called leathal white.  The capital "O" will represent the dominant overo gene.  A small ø would represent a non-overo horse.  Non-overo horses could be either solid or tobiano colored horses.

1.  OO would be a homozygous for the overo gene but this foal will typically die within 48 hours of birth.

2.  Oø or øO is an overo colored horse which is heterozygous for the overo gene.

3.  øø is a non-overo horse.   There have been some solid horses by an overo which have produced overo color when bred to other solid horses.  This phenomina is not explained if the gene is truely dominant.  Another phenomina which is difficult to explain would be the cropout overos by two solid horses not registered with the APHA.  

Example 1

Table 1 is representing a breeding between a heterozygous overo stallion (Oø) and a non-overo mare (øø).  The chances that an offspring will be colored overo or non-overo is represented by the yellow boxes.  There is a 50% chance that the offspring will be colored overo and a 50% chance that the offspring will be a non-overo horse.

Example 2

Table 2 represents a cross between two heterozygous overo horses which are bred together, there is a 25% chance of getting a leathal white foal (OO), there is a 50% chance of getting an overo colored foal (Oø  or øO) and there is also a 25% chance of getting a non-overo foal (øø ).

If you have any questions, comments or suggestions you can e-mail me at breeze@turbonet.com.