Answer

In Jainism, the karma most directly associated with being born as a boy or girl is Veda mohaniya karma — especially puruṣa-veda (male), strī-veda (female), and napuṃsaka-veda (neuter). Jain texts describe the rise of these veda tendencies as resulting in the corresponding sex. ( jainworld.jainworld.com)

Explanation

If you mean the sex/gender of the next birth, that is explained under mohaniya karma. The nāma karma shapes the body and its features more generally, while veda mohaniya is the specific karmic cause for male, female, or neutral birth. ( jsmw.org)

Takeaway

So the short answer is: Veda mohaniya karma decides whether one is born as boy or girl, and nāma karma determines the body overall. ( jainworld.jainworld.com)