Well this is a tough nut to crack.
First, what you described is not completely genetically sound. Usually you'd get something like NN = doesn't have magic YY = has magic, YN = is passive carrier of trait for magic (can't use a magic but if they have children with YY, 1/2 of them will be YY and half YN, and if they have children with other YN, 1/4 will be YY, 1/2 YN, and NN). But let's make a work around and say that to have magic, you need to inherit more traits and it thus makes it more unlikely for people without magic to have children with magic, so let's just handwave that part away for a moment.
Now for that quarter of population with the magic, let's do some calculations:
Not all of them do survive till they have children and not all of them have children with partner who can use magic. So simplified equation will be:
M = number of adult Magic pop right now
A = aspect of how many adults survived to adulthood and decided to have children with other adult capable of magic (in %)
N = average number of children per such union of two adults with magic powers.
and number C of children in next generation will then be
C = M/2(because you need two adults to make children) * A * N
For sustained population C needs to be C =~ M
And A itself is problem here. It all depends on your setting. Do people with magic mostly marry other people with magic? Etc.
Let's now make an example for this equation.
Let's have pretty advanced society, where 95% of magic children survive to adulthood and have opportunity to have children(thanks to healing magic). Let's have 4% homosexuality presence in population, but due to not so advanced society, let's have around 1/2 of them in closet, marrying and having kids anyways, and other 1/2 in secret homosexual relationships or staying alone(not having kids). Next, let's have around 5% of magic population marry non-magical people, because your magic people are something like nobility, and they avoid such marriages, and marriage between magic and non-magic people are rare. Next, let's have around 5% of loners who fail to find a partner. Then, let's have around 5% infertile or nearly infertile people however, these people can find partner and prevent that partner from having children(though, sometimes, that partner leaves them, so we'll only have 8% of people not having children due to that, rather than 10).
In that case, A will be (0.95).(0.98).(0.95).(0.95).(0.92) [.100%] = 77.3%
With that:
C = M/2 . 0.77 . N = M . 0.39 . N
now, for C
if C < M, that means their population will decline. In this case, if on average two magic people who are in relationship have less than 2.5 children, their numbers will drop. Any war, or any natural disaster will cut into their numbers and will send them even faster towards extinction.
If C = M, that means a stable population... hypothetically. But then there are wars, plagues(if healing magic is not omnipotent) and natural catastorphes. Ideally to get stable number, you want C = 105-110% of M at least, more likely C=120-130% of M (it's likely magic users are needed in wars, and while they might have tradition like ransoms, you can still expect quite a few deaths in those wars).
That spikes this up to 2.7-3.3 children per an average family needed just to sustain the number over longer period of time.
2.7-3.3 children per average family might not seem like much, but you need to understand that quite a few people will just have 1 or 2 children. Women who have magic power won't want to be reduce to carriers for next generation, so you need to balance that out. And that's just to keep the population stable, and with just small amount of mages marrying out.
for C > 1.2-1.3 M, that's already quite unlikely. More importantly, if non-magic population isn't stable in numbers, but growing, fraction of people with magic power compared to all will grow smaller, even if their total number remains same. So if non-magic people are currently growing more numerous, the fraction will still go down. There's little hope for your magic power people to become dominant numerically, unless they literally genocide non-magic people.
This was an idealized example. More liberated society will not have homosexuals in straight relationships having kids, will have more people marrying non-magicians, etc. Thus, it is very likely that magic powers will eventually disappear from your world, at least with the genetic scheme you wanted, where only two parents with magic can have children with magic.
Edit: I shall add that these average numbers(more than 3) of children per family aren't impossible, but are usually a thing in developing countries, usually among people who struggle with poverty. While you can find examples of nobles having 8+ children, these are mostly somewhat rare exceptions[when we're talking about children per woman, not in case of a noble with multiple wives, because that's the number that matters,]. As I stated before, unlike with nobles, woman with magic will be less likely to be content to be child factory than a noblewoman whose only power is societal and dependant on her being a mother to her husbands children. I think I didn't make this problem clear previously, so I decided to add this in form of an edit.
TL;DR: In a traditional, authoritarian, magic-feudal society, they might break even, but in liberal society, they're likely to completely disappear, possibly even in matter of two to three hundred years.
