And that's because both of these autosomes, both of these homologous chromosomes have a non functional gene that is given by lowercase D. And so they will not be able to produce that fully functional protein that is needed by the cell. So now that we know what we mean by an Autosole and recessive disease, let's quickly look at four individual examples. Now, we're not going to go into too much detail on each and every one of these diseases. We'll simply go into discuss briefly what they actually mean. Let's begin with phenyl ketonuria. So what exactly is PKU?