Germany had a population of about 70,000,000 in 1945, when the T-4 program ended. This program was officially responsible for about 70,000 deaths of those in psychiatric wards. Unnoficially, the tally is estimated around 200,000 by the end of the war.
These small numbers (0.1%-0.3% roughly, depending on official/unofficial body count) are simply not enough to affect large change in a population about two generations forward.
Without statistics from both before and after the war, there is no way to determine the answer to this question. I am relatively certain that comprehensive statistics do not exist, especially from before the war.
By way of speculation though, we can assume it probably had little statistical impact. First, you're talking about a total of about 70K out of a population of (at the time) about 70 million. That works out to about 1/10th of 1% of the total population. Second, we must remember that the program was stopped prematurely as a result of protest from the German people, so not all of those people the Nazi deemed "useless eaters" were actually murdered this way. Third, the categories the Nazis used to determine who was to be killed was not exclusive to hereditary illnesses. Non-hereditary conditions that might get one swept up int he T4 program include: syphilis, physical disability, dementia/senility, and encephalitis, to name but a few. Furthermore, habitual criminals and those who had been locked up for more than five years were also included. Fourth, the doctors who diagnosed these individuals were not told why they were being selected, making it possible that doctors fudged the diagnoses one way or the other. At least one scholar argues that doctors thought these patients would be saved from work, and so counted more patients as mentally ill than actually were (that turned out to be a really bad idea).
Short Answer: No.
There is a simple biological reason: The vast majority of hereditary diseases is inherited recessively, that means you only get it if you inherit both alleles that cause it from your parents, if you only carry 1, you are healthy.
Killing/sterilizing people that show symptoms is futile (from the perspective of the nazis), because most people who carry the allele for the disease are asymptomatic carriers, and can spread it further.
Take cystic fibrosis as an example: In Europe, about 1 out of 25 people carry the allele that causes it, but only about 1 out of 2500 newborn children have the disease, because in order to get it, 2 carriers have to be your parents and even then the chance is only 25% that you get it.
That means that only about 1% of the persons that have the allele are actually sick, killing them does not affect the prevalence of the allele in the population, because the other 99% are healthy and not distinguishable from others in that regard, and they can still hand it down to their children.
Tl,dr: Killing people that show symptoms affects the frequency of the allele only negligibly, nazis were not only insane but also bad at genetics.