As Savannahs are produced by crossbreeding servals and domestic cats, each generation of Savannahs is marked with a filial number. For example, the cats produced directly from a serval × domestic cat cross are termed F1, and they are 50% serval.
F1 generation Savannahs are very difficult to produce, due to the significant difference in gestation periods between the serval and a domestic cat (75 days for a serval and 65 days for a domestic cat) and incompatibilities between the two species’ sex chromosomes. Pregnancies are often absorbed or aborted, or kittens are born prematurely. Also, servals can be very picky in choosing mates, and often will not mate with a domestic cat.
Savannah backcrosses, called the BC1 generation, can be as high as 75% serval. Such 75% cats are the offspring of a 50% F1 female bred back to a serval. Cases of 87.5% BC2 Savannah cats are known, but fertility is questionable at those serval percentages. More common than a 75% BC1 is a 62.5% BC1, which is the product of an F2A (25% serval) female bred back to a serval. The F2 generation, which has a serval grandparent and is the offspring of the F1 generation female, ranges from 25% to 37.5% serval. The F3 generation has a serval great grandparent, and is at least 12.5% serval.
The F4 generation is the first generation that can be a “stud book tradition” (SBT) cat and is considered “purebred”. A Savannah cross may also be referred to by breeders as “SV × SV” (where SV is the TICA code for the Savannah breed). Savannah generation filial numbers also have a letter designator that refers to the generation of SV-to-SV breeding. The designation A means one parent is a Savannah and the other is an outcross. B is used when both parents are Savannahs, with one of them being an A. The C designation is used when both parents are Savannahs and one of them is a B. Therefore, A × (any SV) = B; B × (B,C,SBT) = C; C × (C, SBT) = SBT, SBT × SBT = SBT. F1 generation Savannahs are always A, since the father is a nondomestic outcross (the serval father). The F2 generation can be A or B. The F3 generation can be A, B or C. SBT cats arise in the F4 generation.
Being hybrids, Savannahs typically exhibit some characteristics of hybrid inviability. Because the male Savannah is the heterogametic sex, they are most commonly affected, in accordance with Haldane’s rule. Male Savannahs are typically larger in size and sterile until the F5 generation or so, although the females are fertile from the F1 generation. As of 2011, breeders were noticing a resurgence in sterility in males at the F5 and F6 generations. Presumably, this is due to the higher serval percentage in C and SBT cats. The problem may also be compounded by the secondary nondomestic genes coming from the Asian leopard cat in the Bengal outcrosses that were used heavily in the foundation of the breed.
Females of the F1–F3 generations are usually held back for breeding, with only the males being offered as pets. The reverse occurs in the F5–F7 generations, but to a lesser degree, with the males being held as breeding cats and females primarily offered as pets.