The International Nepenthes Grex Registry

The INGR lists the female parent first in all grex derivations. In addition to providing a consistent method of representing grex parentage, this convention implicitly acknowledges the belief, widely held among Nepenthes growers, that the characteristics of the female parent are usually dominant in hybrid offspring. This idea is known as the theory of maternal dominance.

A discussion of the evidence for and against this theory can be found at the link below.

Discussion of the theory of maternal dominance in Nepenthes hybrids

However, for the sake of internal and historical consistency, the INGR will continue to use the convention of parental order outlined above in perpetuity, even if the theory of maternal dominance should eventually lose favor.