When breeding your own monsters, the Plus of an offspring is equal to the higher plus of the two parents + 1. When breeding with another player or NPC, the plus of your egg is equal to the plus of your own monster + 1.

In either case, the sum of the parents' levels increases the Plus additionally as follows:

Lv    Bonus
-----------
20-39   0
40-59   1
60-75   2
76-99   3
100+    4

A few specific breeding combinations result in an additional +2 on the offspring.

Each statistic score of a newly-hatched, level 1 monster, including personality characteristics, is equal to one-half the mean of its parents' scores. (or 1/4th the sum of the parents' scores)