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by Nicole Kinsey

SCID is a simple recessive gene. This means that for a horse to be affected, it must get the gene from *both* parents. As long as one parent does not have the gene, the foal will absolutely not be affected.

Clear x Clear = 100% Clear
Clear x Carrier = 50% Clear, 50% Carrier
Carrier x Carrier = 25% Clear, 50% Carrier, 25% Affected

As you can see, only the cross of two carriers holds the danger of a SCID affected foal.

For what it's worth, this is the same inheritance pattern for any recessive gene (lethal or not).

For instance, the "lethal white" gene found in several pinto patterned breeds is inherited like SCID. The only difference is that the carriers of lethal white are almost always marked with one of the prettiest pinto patterns: frame overo. All frame overos are lethal white carriers. A small percentage of lethal white carriers will not have the frame overo pattern visible, but they are still considered to be frame overos and can pass the pattern to their offspring. Here's the inheritance of lethal white:

Solid x Solid = 100% Solid
Solid x Frame Overo = 50% Solid, 50% Frame Overo
Frame Overo x Frame Overo = 25% Solid, 50% Frame Overo, 25% Lethal White

I've used the term "Solid" to refer to those horses that are not frame overo (they could still be some other pinto pattern, so not actually solid).

Hey, and while I'm at it, it's also the inheritance pattern of something like "not-grey." I use this example since so many people like to avoid grey (personally I love greys!). The gene to get not-grey is a recessive (this is the same as saying the gene for grey is a dominant). I will use heterozygous greys (ie. grey "carriers" or horses that have only one copy of the grey gene) to show this same inheritance model:

Not-Grey x Not-Grey = 100% Not-Grey
Not-Grey x Heterozygous Grey = 50% Not-Grey, 50% Heterozygous Grey
Heterozygous Grey x Heterozygous Grey = 25% Not-Grey, 50% Heterozygous Grey, 25% Homozygous Grey

~Nicole