Perhaps it was not clear from the post you answered, but the assumption which what you call assumption 1 was speaking of was of a completely new mutant duplicate gene or of a completely new mutated gene which of course was not to be found anywhere in the species. So for that assumption, due to mendellian genetics, the explanation given is correct.
Let me also add that neutral alleles remain at a constant frequency only in very large populations (due to chance events evening out the larger the sample) and in very stable populations (ie, no migrations, no splitting off of parts of the population, no sudden environmental changes).
Where your example went wrong is that your mutants were only reproducing as per assumption 1, but your population was expanding as per assumption 2.
I stated my assumption above, which perhaps was not clear from what you read. I do not see how your 2nd assumption fits the question I was trying to answer. Kindly explain.
Go back and reread both your post and my answer. I was using the situation which you set up. I will restate my assumption 1 about your hypothetical situation: That a population of 1000 has 500 breeding pairs, and each pair has 10 offspring (10 is your number, not mine. Your hypothetical placed one mutant in that population, with a 50% chance of any one offspring inheriting the mutation.
Let me also add that neutral alleles remain at a constant frequency only in very large populations
The rules are the same for any population size. For mathematical simplicity, we discount random events. In the real world, random events do occur, and they are powerful forces pushing evolution.
I stated my assumption above, which perhaps was not clear from what you read. I do not see how your 2nd assumption fits the question I was trying to answer. Kindly explain
In your example, your numbers indicated that the mutant, and only the mutant, was part of a breeding pair that produced 10 offspring, half of which inherited the mutation, as per my assumption 1 (sexual reproduction). However, your population size as a whole was increasing as if each individual was producing 10 offspring, as per my assumption 2 (asexual reproduction). Either every individual pairs up to produce offspring or every individual produces offspring asexually, in which case all the mutant's offspring are mutants. You cannot mix the two types of reproduction, as you did in your example.