[Cc Cc] Conformation: This is a double gene pair that determines how well-built your horse is. A horse with low conformation genes is considered to have heavy conformational faults. Conformation determines the RISK of a horse, though it is not a guarantee one way or the other. Horses with good conformation sometimes develop high risk, and horses with bad conformation sometimes have virtually no risk at all. This also affects your horse's maximum potential. Excellent and Outstanding conformation will give potential a boost while Fair and Poor will lower it.
[Dd Dd] Development: This is a double gene pair that determines how fast a horse matures. A horse with higher development will physically mature faster, making it safer to race them at a young age. Horses who develop very slowly will not be suitable to racing at young ages, because their bodies are not physically ready to handle that much work.
[Ii Ii] Intelligence: This is a double gene pair that determines how quickly a horse learns. This influences how fast a horse gains stats through training. A horse with low intelligence will have a much harder time gaining stats than a horse with higher intelligence. Ultimately, this can also impact at what age a horse will be ready to run (at their best).
[Hh Hh] Health: This is a double gene pair that determines how long a horse can handle racing. Racing puts some wear and tear on a horse's body, but some can withstand that more than others. Horses with higher health will likely be able to have long racing careers, racing several years. Horses with lower health may only be able to have short careers before their soundness starts to decline. Health corresponds directly to the longevity bar on your horse's page.
[Tt Tt] Temperament: This is a double gene pair that determines the attitude of a horse. It loosely influences consistency and to a lesser extent courage. Horses who have a better (more docile) temperament will be more consistent in races, while horses with poor attitudes will tend to have a lot of ups and downs.