My new JRT puppy has black spots on her belly, and when she is wet, I notice she has them under her coat as well. Is this normal? Will they start to show through the white parts of her coat?
Yes, this is perfectly normal. The majority of Jack Russell Terriers have these skin spots. The spots will change in shade and size as your puppy matures. Below are some examples. The first picture was taken when this pup was 14 weeks old. The second photo was taken at age 2.5. As you can see, her spots have changed considerably; they still continue to do so.
Your puppy may or may not develop "ticking." Ticking refers to small spots -- usually just a few hairs -- of color in the white coat. Ticking can be tan or black. Some dogs have both colors of ticking. It can be just a few spots or many, many spots. It changes with age as well. All remain at least 50% white. The dog pictured in the 3rd photo is heavily ticked. The first photo is again at 14 weeks. She had no ticking at this age. The 4th photo was taken at nearly 2 years of age. The ticking started to appear in this particular dog's coat when she was 7-8 months, and grows darker and thicker in the winter.