The answer to this question is straightforward and has been answered before. It needs to be re-addressed because people still ask the question.
The picture below answers it.
It has been decided by experts that almost 10,000 years ago a North African wildcat decided to befriend a farmer, perhaps in an area which we now called Syria. Both farmer and cat benefited from the relationship. That was the beginning of the domestication of the cat as we know it. It happened in a place called the Fertile Crescent which is roughly the area where Syria is.
A grave was dug up in Cyprus and in that grave there was the skeleton of a man and his cat companion. The grave was dated at about 9500 years of age.
The various sub-species of African wildcat are in effect tabby cats and the forerunner of the current tabby domestic cat as the photographs indicate. There are three types of tabby cat; the striped tabby or mackerel tabby, the classic or blotched tabby and the spotted tabby. The variations are due to an evolution of the original tabby coat. You can read about tabby cats in straightforward language by clicking on the link below.
The reason why, therefore, that some tabby cats have spots on their bellies and stripes on their backs is because they have inherited the genetics which create that coat type from their wild cat ancestor the North African or Near Eastern wildcat (2 names for the same sub-species of wildcat).
P.S. Spots on bellies is typical of the tabby cat coat. Stripped tabbies are also called mackerel tabbies.
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