Hereditary Change spell

[Anthony]

Originally Posted by Whitewings

That is the intent, yes.

I would probably make it a variant on the spell itself -- i.e. you cast a spell that adds an inheritable trait. I think biotech has spells like that already.

[Pursuivant]

Take a look at GURPS Biotech for some basic Biotech spells.

I wrote a Biotech spell that's similar to what you're proposing:

Spoiler:  

Dominant Gene Regular; Resisted by HT

Makes a single heterozygous or recessive trait dominant, so that the subject’s offspring will always exhibit that trait.

Each species, or group of closely-related, interfertile sub-species, requires a separate variant of this spell which must be learned separately.

Depending on the genetics of their other parents, children of the subject will either be homozygous dominant (meaning that their children will also always show the same trait) or heterozygous dominant (meaning that their offspring will have a 50% chance of having the trait). For example, the subject’s dominance pattern for eye color could be switched making a recessive trait for green eyes dominant, and ensuring that all his children have green eyes.

The downside of this spell is that selecting a gene for dominance could stifle natural mutation and evolution of that trait among a population. It could also be linked to unfavorable genes, which give disadvantages.

This spell can be used offensively to make a recessive lethal gene dominant, guaranteeing that all the subject’s offspring die!

Note that the spell has no effect unless the subject has the gene the caster wishes to manipulate. To determine which genes are dominant or recessive, the caster might need to do research into the subject’s genealogy, perform genetic tests, or cast a spell such as Analyze Genes. In such cases, one or more successful rolls vs. Bioengineering, Biology, Genetics, and/or Research might be required.

Duration: Permanent.
Cost: 1 to cast per ±10 character points in traits the “tweaked” gene conveys (minimum 1), 1 to bestow a Feature, Perk or Quirk, or 5 to make a lethal recessive gene dominant.
Time to cast: 30 seconds.
Prerequisites: Magery 2, Sequence DNA.
Item: (a) Staff, wand, clothing, jewelry, or similar item. Usable only by mages. Energy cost to create: 250. (b) Any item, typically clothing or jewelry. Casts this spell on the wearer/user. When worn or carried for more than a few minutes, the item “tweaks” a gene for a particular trait within a particular species, as decided by the enchanter when the item is created. Energy cost to create: 20 times the basic casting cost.

Edit: Taking a second look at this spell, I'd allow one variant to work for each general class of creatures (e.g., mammals), similar to how Animal Spells work. I'd also allow a particular trait to always be dominant, no matter what, for double the normal casting cost. For quadruple the normal cost, I'd allow traits which require multiple genes or which are normally gained due to individual experience (e.g., Combat Reflexes), to always be dominant.