larg little/small little.

Question
hi doctor.cats have always been my love and passion and i just love and enjoy having persian cats in my life.currently i have four of them and after experiencing so many events about them i know many things about  cats ,but there is a question which by far i couldn't find any answer about that.simply why some females have larg litters and others have small one.i have inspected on so many cats including my own cats and my friends cats but there isn't any certain rule in this case I'm afraid.some experienced breeders told me it depends on the size of the queen,some other said it depends on her age,but my experiences have proved vice versa.some very tiny queens surprisingly give birth to 6 to 8 kittens each time and some big females only 2 or 3!there isn't any certain rule it seems.i've kept thinking about this because it really matters for me,some times the tom cat and the queen are very beautiful so naturally i want a big litter and some times i can't stand so many kittens and i prefer a small litter.can you help me please?

Answer
Ho Soheil
Now this question requires you to understand a number of things and I will try to explain them in a way I hope that you can understand.
First there is something called breeding vigour
Second is inbreeding depression
Third is SNP protein influence

First Breeding Vigour
Usually this happens on the first mating of two non related cats. There could be a large litter produced as it is a mating with vigour on both the tom and the queen. The second mating of either cat to any other cat usually results in smaller litters.

Inbreeding Depression
If cats are related to one another by parents or grand parents then this can affect the size and health of a litter. Inbreeding usually produces a small litter and some may not survive.

SNP Protein Influence
It has been suggested that a particular SNP Protein can influence the number in a litter. What happens is that a gene combines with another gene by either A C or T binders. If the gene binds by a C cell there may be greater numbers in the litter that if it binds using a T cell binder.

A cell binders do not influence litter sizes as they normally do not affect the x y chromosomes

Also some breeds do produce small litter numbers no one knows precisely why it just happens

Sorry it is a bit scientific but you must truly understand that litter size is very unpredictable. The only thing you can bet on is colour.

Hope this answers your question

Ray