Given the relatively small gene pool for Turkish Vans, inbreeding is an issue of high importance to our breeders.  But why is it such a problem, how do we measure it, and what can we do to minimize it?

By discussing this topic we will inevitably rattle a few cages, since it’s a very sensitive subject with widely differing views being held by interested parties.  However, we’ll try to keep to the facts and avoid the moral issue.

To some extent all pedigree cats are inbred, since we only select a mating pair from that group of cats purporting to be of the same breed.  Further to that, the choice is reduced by geographical limitations, personal relations, and timescales, even to the extreme where Hobson’s Choice is all that remains.  Working within these constraints, a breeder will be looking to enhance the breed, improve the quality of their cats (and by quality we should mean physical & mental health as well as conformance to the breed standard), and produce loving kittens that will be adored by all!  This takes time, and sometimes requires decisions to be made regarding the acceptable level of inbreeding.

By mating a cat to one of its relatives we can try to enhance certain features which are a trait of that line.  Usually this means outwardly visible signs like coat pattern, bone structure, ear positioning, or physical size, since these are the only things we are able to easily measure.  What can’t be seen are the internal traits, like immunodeficiency, dysplasia, or cryptorchidism.  By the time we see the effects of these problems it’s already too late.

 So breeders have to follow a path somewhere between inbreeding and outcrossing (which, of course, would have serious implications for the breed), something which we might call line-breeding.

Rather than waiting until the signs of inbreeding become obvious (for example fertility problems & higher mortality rates), breeders can gauge the level of inbreeding by calculating an index known as Wright’s Coefficient.  The basis of this is as follows.  Inbreeding problems are caused by a lack genetic variety, or more specifically, the probability that a cat will acquire two identical copies of the same gene.  Without launching into a full lesson on genetics, let me explain that when an offspring is conceived it receives two sets of chromosomes, one from each parent, and it retains these two sets of chromosomes for life.  Each chromosome contains a large number genes, and it is these that determine how that offspring will develop.  The large variety of different combinations explains why no two people are alike, except of course for identical twins, and this is nature’s method of progression and evolution.  There are however certain combinations of genes that together cause problems, and inbreeding reinforces and magnifies these adverse effects since they have a stronger presence.

The problem arises when two offspring with common ancestors both have a common descendent, perhaps following a mating between cats that share a common sire, for example.  For the gene contributed by the sire, the probability that identical copies of the same gene will be passed on to both offspring is 50%, since there is only a choice of two.  Similarly, the probability that the offspring from a mating between these two siblings will receive identical copies of the gene from each parent is also 50%, making the overall probability of a kitten having two identical genes 25%, i.e. 50% of 50%.  So in this example the inbreeding coefficient is 25%.

In fact it’s not quite as simple as this, since we’ve assumed that the first generation were unique, in that they themselves had an inbreeding coefficient of zero.  If this is not the case, then this has to be factored into the equation for the siblings and their offspring, so things can very quickly get very complicated.  For this reason we rely on pedigree programs to generate inbreeding coefficients for us, although the results are only as good as the data we provide.  One consequence of this is that to accurately compute the inbreeding of a cat, you also need to know the inbreeding of its ancestors, and to know that, you need to know that of their ancestors, and so on!  This is therefore an iterative process, and can be quite intensive.  The problem can be reduced by limiting how far back down the ancestral tree we look, knowing that the further back the cat is, the less affect it will have on the inbreeding figure.

A warning note:  Since we like to trace our Turkish Vans back to the cats that came from Turkey, often on a pedigree we’ll see many entries like “Unregistered, from Turkey”, or just “Unknown”.  If we have several of these, then a pedigree program will consider them all to be the same cat, and will skew the inbreeding coefficient of their descendents accordingly.  This can be prevented by leaving these entries blank, or by limiting the number of generations that are used for the inbreeding calculation.

One interesting point to note is that it isn’t necessarily a bad thing to mate two cats that each have a moderately high inbreeding coefficient.  Remember that what we are looking for is the probability that an offspring can receive identical versions of a gene from both parents, but if both parents have completely independent ancestral trees then there is no way that the offspring can get identical genes from both sides, so the inbreeding is 0%.  Of course this is rather hypothetical, and the converse is also true, but it serves to remind of what we are looking for, i.e. little or no ancestors appearing in the pedigrees of the two cats to be mated.

Different breeders will have different opinions about what is acceptable and what is not.  Also, accidents happen, but this does not mean that the offspring will be deformed, sickly, or otherwise unacceptable.  Remember it’s all about probabilities, and the reinforcing of defects over several generations.  Just how much you are willing to accept in a cat is an individual matter, you have to weigh up the odds!

Steve Lloyd