 Let's move on to. Hardy-Weinberg population gen X. This is a very important formula to know. I would recommend memorizing this. We're putting it on a cheat sheet for you to do when you go into the exam. First and foremost, the PQ equals 1. That is just stating that P, which is 1 allele and Q, which is a second allele, have to equal 1. They cannot equal 0.9. They cannot equal 1.1. They have to equal 1. And then secondly, P squared plus 2PQ plus Q squared equals 1. And if you'll look to this chart over here on the right, you'll see kind of an explanation of how this works out. So P and Q crossover in the Punnett squares here. And you'll be able to do these, draw these out on your exam. And you see the P from the horizontal axis and the P from the vertical axis come together in the top left box and give you P squared. Similarly, it goes down. You get PQ on the top right and bottom left. And then the Q goes over to the bottom right square. And you get Q squared. So if you add all of those up, multiplying the 2PQs together, then you will get 1. You'll see this often used on the exam as likely a three-part question where they will ask you if the chances of certain disease happening or the chance of a certain genotype or phenotype being present in an offspring. Moving on, imprinting disorders. This is where one gene copy is silenced due to methylation. And this leaves only one copy to be expressed. So this happens when one of the parents has a gene that is silenced and the other parent is no longer there. So there are two common ones of this is Prater-Willi syndrome and the Angelman syndrome. Prater-Willi starts with a P. And that is a paternal allele that is mutated. So the mutated or deleted paternal allele causes the problem with Prater-Willi. We see this manifested as hyperphagia, obesity, intellectual disability, hypogonadism, and hypotonia. Angelman syndrome, you see the M is highlighted here. That is a maternal mutation or deletion on chromosome 15 that causes the issue here. This is seen with seizures, ataxia, a severe intellectual disability, and inappropriate laughter. We call this the happy puppet syndrome.