I have a toy dog, age 4 month old, whose testicles have not descended.
Does anyone know if this is hereditary?

It's still early days, and there's a chance they may yet descend, so don't give up hope. There certainly seems to be a hereditary element to this problem - have any of his relatives had the same condition?

If it is inherited it comes from the bitch line via the X chromosome

My vet advises leaving puppy dogs bits alone & if they haven't both come down by 12 months then go back to your vet

If he's a show puppy(or bought for showing)your breeder should have ensured that both were down before selling him They can go up & down & the more owners check the more likely the puppy is to retain one or both if he has not inherited the condition

>>If it is inherited it comes from the bitch line via the X chromosome

Nothing to do with the x chromosome, that would be sex-linked inheritance as in haemophilia.

Cryptorchidism may give the impression of sex-linkage as it is limited to one sex but this is because it is physically impossible to express in a female, not because of its inheritance.

It is in fact a sex-limited trait located on the autosomes and some sources believe it to be a simple recessive while others say polygenic.  Either way, it will be passed on by males and females.

Julie

The difficulty is finding a breeder who will admit it is in their lines :(

Fagan has a retained one and also his brother, and although I completely trust his breeder when she says she has never had it in a litter before but when you get the main breeder of his line saying the same thing for some reason I just dont believe it.  They tend to cover up this sort of thing which IMO is very wrong :(