As I was digging through my Midrea files I came across an essay on Mage Genetics I wrote and feeing inspired I wanted to show how some different system assumptions interact with the world.
Now GURPS does kind of have clerical magic system, during the design I decided the deal that out of the picture and concentrate on having all magic the domain of Mages.
The biggest changes this wrought was a deemphasis on religion, faith became a social force more than a temporal one. It played much the same at the player level but it left more room for doubt and religious wars. I liked it well enough but it didn't have anything to prefer it over a more D&D focused version.
About one person in 10 had second sight (they could take seer abilities) and 1 in 100 had Magery with each tier of power, 1/10 as common as the last. I allowed power ratings from 0 to unlimited a a house rule (its a standard rule now) back then topping at Magery 10 for the Master Magister (technically with a 1 in a trillion chance of having that level of power) that when combined with his 17 IQ (GURPS is a bell curve system BTW and a 17 is far more powerful than in D&D) and 3000 point magical fatigue pool gave him Archmage level with every spell and enough power to smash cities.
Another oddity of Mages only was the emphasis on bloodlines and genetic components to magic.
Below is the essay
My genetics in my game world (assuming GURPS rules BTW) work like this
Magery is a recessive trait carried by about one percent of the population. How it evolved is unknown however Its linked to high "intelligence" and good "health" thus all mages are smart and tend to be long lived. They also tend to low fertility for various reasons, lack of sociability, autistic autodidact personality types (mages tend to be abrasive jerks) and perhaps chromosomal error correction needed to handle the N dimensional energy flows .
Two mages will always bear a mage child but if both partners carry the gene it becomes very difficult to conceive.
A mage's child by a non carrier won't necessarily get the mage gift but has a higher chance of conception. Still the number of mages with children is smaller than might be expected for two reasons
#1 my world has cheap reliable birth control that works on both genders so there needs to be a conscious desire to conceive on both partners account.
#2 There are a lot of solitary Mages and while they may have magically expended lifespans a mages tend to have abrasive personalities and ASPIE like traits that reduce mating opportunities.
When a mage has a child by a non mage there is a 20% chance (or so) that the child may have a weak version of the mage gift called Second Sight. This will give them 6th sense related abilities and sometimes the ability to cast some hedge magic spells which work only through subtle means. Maybe 1/10 of the population has this ability though few train it to any degree.
Doing this allows me the option of "son/daughter of a powerful mage" without too many mage dynasties and keeps numbers very low. In summation, It is assumed that mages are rare because
Its an intelligence linked recessive carried by about 1.5% of the population
Mages also tend to have some social spectrum disorders that reduce social bonding
Two carriers of the mage gene will have diminished fertility.
These traits limit the number of mages to (even allowing for for greatly enhanced survival odds) about 2% of the population as persons with mage potential of some kind or another and 5x that number with second sight.
In retrospect don't know that I would do it again but it would be possible even in D20 to have Black and White Mages replace Arcane and Divine casters if it was desirable.