#then that would make spock a tigon and lorian a liger

[odekiisu]

Vulcan Eyebrow Pointiness is governed by gene(s) in an imprinted region of the genome.

Imprinted genes are expressed differently depending on which parent they're inherited from; either the maternal or paternal copy of the gene is suppressed. On Earth, this can be seen in some human genetic disorders such as Prader-Willi/Angelmann syndrome (both caused by a large deletion on chromosome 15; which one presents is dependent on which parent the mutation is inherited from).

Another case where genomic imprinting causes noticeable differences in phenotype are lion/tiger hybrids. They have the same complement of genes, being 50% lion and 50% tiger, but ligers (male lion/female tiger) are absolutely heckin massive, being larger than both parent species and the largest feline in the world, and are lighter in base colour (more the colour of a lion than a tiger).

Fig. 1: Liger

Tigons (male tiger/female lion) on the other hand are about the size of a tiger and more tigery orange in colouration. This is because of growth-suppressing genes that are active when inherited from a female lion, but not when inherited from a male one, and corresponding growth-promoting genes that are only active when inherited from a male. Female tigers do not have the required growth-suppressing genes to counteract the latter, so ligers get huge, but tigons don't.

Fig. 2: Tigon. Note that both are stripey.

In the case of Vulcans, the maternal copy of at least one gene that governs eyebrow pointiness is suppressed, whereas the paternal one is active. Behold:

Fig. 3: Spock, half-Vulcan with a Vulcan father, has pointy ears and pointy eyebrows (obvious usage of eyebrow liner notwithstanding).

Fig. 4: Lorian, half-Vulcan with a Vulcan mother, still has pointy ears but round eyebrows.

As you can see, the eyebrow pointiness phenotype differs depending on which parent it is inherited from, whereas the ear pointiness phenotype appears to be dominant, much like stripeyness.

tl;dr: Spock is a liger and Lorian is a tigon.