You lost me on the premise. I don't even think its murder so how can it be immoral. And I get what you say about the chromosomes but under that logic pulling the plug on someone who is brain dead is murder and I don't think that is either. Humanity is self awareness and conscious not genes and chromosomes.
That's why the argument should be rather about personhood then it should be about humanity. You must first establish what it means to be a person before you can declare abortion to be moral or immoral.