For example, if I breed a Winter Sawsbuck with Ditto in XY or ORAS, will the hatched Deerling be Winter, or will it be the default Spring form?
When bred with another Sawsbuck, the resulting offspring will have the form of the mother, and if bred with a Ditto the Sawsbuck's form will be passed down. So if you bred a female Sawsbuck in its Winter Form, and a male Sawsbuck in its Summer Form, then the baby Deerling will always be in its Winter form, the form of its mother. If you breed a male Winter Form Sawsbuck with a Ditto, then the Deerling will hatch as its Winter Form.
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