How Can Katie be F3 and 46% Serval?

It is a complicated explanation on how theoretically a Savannah can be three generations away from a Serval but of an equivalent Serval % to a first generation Savannah. Here's how the theory goes:

If you cross a domestic cat to a Serval, the progeny is designated F1 and theoretically is 50% Serval (TWiG %: Theoretical Wild Gene %),

If you cross that 50% F1 back to a Serval, the progeny is still F1 as they are still one generation away from the Serval, but now the TWiG is 75%.

Do this cross-back to the Serval again and the progeny is 87.5% F1.

Next cross-back would get 93.75% F1 and then again would give 96.875%

This extremely high % F1 is what Katie's grandfather is registered as.

You will see explained elsewhere that Savannah males, for the first four generations away from the Serval, are infertile, and fertility is generally reached in the F5 male. This is possibly due to the fact that a standard F5 SV male would be 3% Serval and 97% Domestic. Well, in reverse, you might expect that a 97% Serval and 3% Domestic Savannah might also be fertile.

So Katie's breeder crossed this 96.875% F1 male with an 87.5% F1 female to produce Katie's dad who is 92%. Thus Katie is F3 but 46%.

This means that Katie's kittens will be F4 with a TWiG of 23%… equivalent to a normal F2 kitten. This makes them very unusual F4 kittens!