The Genetics of Huntington's Disease



hello my name is Jean and I'm a strand of DNA and I'm here to explain the genetics behind Huntington's disease whoa whoa whoa why do I look like a pair of jeans seriously guys you had one job I'm meant to look like a genetic strand of DNA not a pair of denims I guess you want me to do the whole video looking like this don't you oh sorry Jean we can't change it now uh-huh hello my name is Cody and I'm a CAG repeat me and gene our co-presenters today gene hey why are you dressed like jeans for hi Cody thank this guy over here amateur I see you got Cody's outfit right though hey gene can you explain DNA to the viewers and give me a call when you are ready to teach them about CAG repeats that's my part sure Cody so HD is a genetic neurodegenerative condition which means it can be inherited through a family to understand the genetics of HD you need to first understand DNA genes and chromosomes DNA is something we inherit from our parents it's like a recipe on how to make each of us DNA is a genetic code which hello again everyone you call G are you done with DNA already that was real quick hi Cody no I was talking about the genetic code oh my bad I thought you called my name my hearing is not so good Oh how many years so where were we yes DNA is a genetic code which decides things like our hair color height and aspects of our personality DNA is made up of things called genes oh that's real funny you should be a comedian Thanks you know very well it's not those kind of genes it's a gene there are around 23,000 genes in the cells in our bodies each gene is like a set of instructions for cells in our body to help it work properly Gene's are packed into things called chromosomes we have 46 chromosomes in our DNA 23 for Mom and 23 from dad we know that HD is caused by a gene on the fourth chromosome that's not fair I don't have any control over any of the genes in me dude okay as I was saying HD is caused by a mutation on a gene in chromosome four oh here's the comedian again stop it stop drawing the x-men mutation does not have to mean mutants in this case one gene is chromosome four is longer than it should be and that causes HD this long gene is caused by something called CAG repeats maybe I should call Cody now as he likes to do this part Cody hello again everyone I'm at the CAG repeat part Cody well I'm Cody the CAG repeat so I got this buddy so you may or may not know that our DNA is made up of four bases which are known as adenine abbreviated a sistah sign C guanine G and thigh mine T these bases are the code behind all our DNA if we were to write a gene code down it would look like each letter stands for one of the four DNA bases the gene that causes HD has a CAG repeat on it this is normal everyone has a CAG repeat on this team usually about 16 or 17 repeats but people with HD have more repeats than usual thanks G hi Cody you're an expert in explaining this how many CAG repeats does it take to get HD thanks gene people with HD usually have over 40 CAG repeats but there are actually four ranges that CAG repeats may fall into as I said everything over 40 is going to cause HD for that person and there is a risk of passing HD on to their children something between 36 and 39 repeats means you may or may not get HD but there is still risk of passing HD on anything between 27 and 35 means that person won't get HD but there is a very small chance HD could be passed on finally anything below 27 means no HD for you and no risk for future generations amazing I can see why they hired you Cody I hope they are paying you for this job you know your CAG repeats ergy it's for charity I'm doing it for free oh wow how nice of us remind me to thank my Asia next time I see him all right moving on so how do you inherit HD from someone and what risks does that give people of getting HD great questions Cody I have a question for the audience how many copies of a gene do we have inside us the answer is two copies of each gene you might think it would be one but if you remember we said that you get one chromosome of genes from your mother and another from your father giving you two of everything so people actually have two copies of the gene that causes HD one from their mom and another from dad because HD is what we call autosomal dominant it only takes one gene that has HD to cause someone to get HD so in this example this person has one gene that is 40 plus CAG repeats and will cause HD they have another which is completely normal but because they have one gene with HD the person will unfortunately develop HD gene so what happens if someone who has HD wants to have a child is there a risk of inheriting HD yes there is Cody let's use this guy as an example he has the gene that will cause HD but he doesn't have any symptoms so he is healthy right now let's say he is a married man his wife comes from a family of know HD and has two normal versions of the gene that causes HD so she has no risk of getting HD or passing it on if they were to have a child together the child would inherit one gene from Dan and another from mom we don't know what gene they would inherit from mom or dad but they must have one from each this leads to four possible outcomes for the child one the child inherits dad's normal gene and mums first normal gene equaling know HD 2 the child inherits dads HD gene and mom's first normal gene equaling HD 3 the child inherits dads HD gene and mums second normal gene equaling HD or the child inherits dad's normal Jian mums second normal gene equaling no HD if you are good at math you will realize that out of those four possibilities to cause HD and to don't which means the child would have a 50% risk of inheriting HD from their parents nice work Jean what about if your grandparent has HD but you don't know if your parent does what kind of risk do people have in that situation thanks Cody well in this case we could say that the grandparent has HD so the risk is 100% and we don't know if their adult child the parent has HD so they are still at risk 50% of inheriting HD if they had a child then we would have the risk of their parent to give a probability that the child would get HD in this case 25% but this probability could change over time because this probability was done with no knowledge of the parents HD status so if the parents HD status becomes known at some point that would change the child's risk if the parent did have HD and the child's risk would increase to 50% but if the parent didn't have HD then the child's risk would disappear to 0% okay folks that's it we are at the end of this video hope you understand how the inheritance of HD works a bit better now please if you have any questions just ask us at hdyo support at hdyo dot org thank you everybody hopefully next time we meet and i'm dressed correctly i'm going off now to say hello to my dear agent

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