Will Female Dog Mate If Not On Heat?

No she cannot mate when not in season.

A dog will not mate if she is not in heat. A dog will come into heat two time a year.

We have a spaniel dog of 7 and a spaniel bitch at 11 months old, no signs of being in season.  We have a jack russell bitch in season and standing; we have kept her from the dog for the last fortnight but last night the dog served the spaniel bitch; could this have happened because of the jack russell being in the height of season?  The dog has not shown any interest in the spaniel bitch until last night and has shown no interest in her since.

No. It is physically impossible. Their biology would not allow it.

Yes a dog can mate when not in season, it's a rare event but it has happened. I have known 2 de-sexed dogs to engage in the act.
Also I have observed my Bull Arab bitch stand for my male husky. I found it interesting as she was the one to initiate the whole thing.
Her the body language was more subtle than a bitch in heat. The other thing was, she wouldn't tolerate any other male mounting her out of season, still there may be some reason behind this I don't know? He may have had a pheromone she really liked why it happened, I will never know.

Hope this answered your question

Just some general advice. 

if you have a Female that seems to attract males out of the normal seasonal periods it's possible she has a uterus infection or Pyometra and should be taken to the vet immediately.
Should you be concerned about unwanted litters but don't want to de-sex your dog ask your vet about tubal ligation or for the boys K9 Vasectomy. I'd recommend doing lots of research before making decisions.

Well my dog has been out of heat for a month and a half...but today she allowed my male to mount her.

They can not reproduce or become pregnant if not in season. She may have an infection that is more difficult to detect that the male is picking up on and confusing or it may be a dominance thing. It is also important to know your girl's cycle if they aren't spayed in case they go through what is known as a 'silent' heat. They would still be in season but missing many of the obvious signs like bleeding everywhere. This is most common during the first season as they are immature but if it's a usual thing they should usually be spayed as it indicates there is something off. Often a minor issue like a hormonal imbalance that can be corrected by removing the reproductive organs but sometimes it may be something a little more serious like thyroid problem or other issue.

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