...so I just posted all of that for how carriers work.
Now, there is a situation in which the inheritance is higher than 25% such as:
when a diseased partner marries a carrier with the disease not attached to the sex chromosomes. With this setup, the offspring will have 50% chance of getting the disease and 50% of being carriers. This regardless of whether the disease is dominant or recessive.
In this scenario, if the disease is lethal then the offspring will be dead 50% of the time or live to be a carrier.
The second is the offspring of two diseased partners which will have a whooping 100% inheritance success rate in passing the disease. However, if the disease is lethal, then there's no way any of the child can survive, let alone mate.
Despite all this, when applied to a larger population, suddenly all of these high probabilities shrink. It's because then you'll have to put in population number into the equation to in which you divide them into the percentage. This will shrink the probability to meaningless level again and therefore, not enough justification.
And as it stands, it's almost unthinkable any genetic disease to spread uncontrollably unless everyone on Earth is holding the Idiot Ball for several generations. It's because certain or high-risk inheritance require both partners to have the diseased allele, which chance is higher when marrying people who are closer to them. So,unless we have generations of inbreeding, it's just impossible for the risk to get really high.
Even THEN... humanity has proven that even with generations of inbreeding with lots of potential carriers and diseased people, the disease STILL won't spread.
And so let's say welcome to the Jewish people here. Jews, even though they mix with other ethnicity, have mostly throughout history marry among themselves and because their starting population was low to begin with, they have come to have gained lots of genetic disorders in their genes that are RAMPANT.
This is a really good example on what happens when people with genetic disorders and carriers are let loose and being able to marry whoever they want. So the ultimate result? Most of the Jewish population are healthy even when the risk of their offspring getting a disease when doing the simplest thing of having a child with another Jew is comparatively higher than with any other partner.
Why is this true? Because people aren't utterly stupid of course. That and the beginning probability to work with is already low enough.