tay-sachs is a genetic disease and let's say capital t is normal and lowercase t is the disease now taysac's disease remembers a really nasty disease that is found in usually people of eastern european jewish descent it involves the loss of one lysosomal enzyme in the brain that breaks down fats and because it's not there these lipids accumulate and they cause blindness they cause dementia they cause seizures and generally death before the age of three so it's a really nasty nasty disease and so you have two genes governing each trait so you could be this in which case you'd be normal [Music] you can be this in which case you were a carrier or you could be this and in that case you would have the disease now with a disease like this they're never going to reach maturity so they're never going to be able to pass it on so a parent would either have to be this or this and so tay sachs in particular is generally going to come from two parents who are carriers it pretty much has to okay because if one parent isn't a carrier you can't have the disease you have to have both parents being carriers so what this is mom and this is dad and they had babies each child has a one in four chance of being normal a two and four chance of being a carrier and a 1 in 4 chance of having the disease so when you have two parents who are heterozygotes that means this heterozygote has two different copies two different alleles for that trait this is up here that's homozygous dominant down here is homozygous recessive that means you have the same gene from both parents but you have two heterozygotes and you have a one to two to one ratio in terms of the genotype okay in terms of the phenotype what you see when you have two heterozygotes it's three to one three normal to one abnormal because even though these are carriers they're normal okay so one to two to one would be the genotype for two heterozygotes three to one would be the phenotype for two heterozygotes so each child would have a one in four chance of having the disease a one in four chance of being normal and a two and four or fifty percent chance of being a carrier now if one of the parents was homozygous dominant in other words they were normal you would have a zero percent chance of having any child that has a disease you would have a 50 chance or two and four chance of uh each shot for each child being normal and a 50 chance of being a carrier but none of them would have the disease if one parent was normal and see if you have if if you consider this one to two to one you have a 50 chance of being a carrier so you have all these people who all carry that same mutation having children and their carriers and you know there's a 50 chance that each child will be a carrier and since you have so many people that have this genotype the disease persists but if you have somebody outside the community somebody like this who does not have that in their in their genetic makeup it stops you from um although you still have the same number of carriers actually yeah you still have the same number of carriers even if one person is from outside the community but you wouldn't have it be able to have any children with the disease but this is why there still is a problem with tay sachs because it's there and regardless of whether you are marrying within your community or outside of your community you still have a 50 chance of producing carriers with each child okay um but the more you have children with people outside the community the more people that do that um it does dilute the bad gene okay so let's look at another thing let's look at um let's look at dominant diseases okay so for instance we'll look at huntington's it's a dominant disease so capital h is the disease lowercase h is normal so in that case this person i'm not sure if having double disease genes is fatal in this condition or not a lot of times it is or it will cause them to experience these symptoms a lot earlier so this would be disease this would be disease and the only time you're normal would be this so if you have two individuals who have the disease or let's just make it one that has the disease if you have one individual one parent that has a disease each child has a 50 50 chance of having it that's a pretty high risk okay and that's with one parent if you have two parents and certainly it would go up um if you had two parents that had it then [Music] each child would have a 75 chance of having it okay and so that's a dominant disease now an x-linked disease take a look at that quick it's carried on the x chromosome and um it would include things like um i've got like kalin there we go it would include things like red green colored blindness duchenne's muscular dystrophy hemophilia um there are lots of different things that are carried on the x chromosome so this would be a normal woman and this is generally going to be a normal carrier and then this would be a normal man and then this put a little dash there would be disease so the only people that generally show the disease are males and if you have a woman who is a carrier who is with a guy who does not have the disease then you're going to have approximately 50 each daughter is going to have a 50 chance of being a carrier each son is going to have a 50 chance of having the disease okay so in x-linked diseases females carry guys guys express the disease um the last thing that we'll look at here is um is blood types with blood type a b a b and o when that has to do with the antigens on the outside and so type a can either be here we have a instance of something called codominance so i a is dominant ib is dominant and lower case i is recessive okay so we have codominance so a can be i a i a or i a i b can be ibid or ibi a b can be only i a ib and o can only be lower case i lower case i so if you have a parent who has type a blood um so we'll say parent with a blood parent with b blood child with o blood there's only one way it can happen and that is that both parents are heterozygous because they would both have to have one of those lowercase i's and in that case if both parents are heterozygous then they have the opportunity each child is going to have a one in four chance of being a b a one in four chance of being type a a 1 in 4 chance of being type b and a 1 in 4 chance of being o but the only way you can be type o is if both parents are carrying the recessive gene okay um to be a b one parent has to have an a and one parent has to have a b so the parents have to either be both be a b or one has to be a b one has to be a or b or one has to be a and one has to be b okay so this is codominance the final thing about genetics is multi-factorial and we looked at this when we talked about um skin color and height those things all are multifactorial there are a bunch of genes that contribute to how tall you are not just one and there's also the effect of your environment so if you're not getting good nutrition even if you have a whole bunch of genes that say you should be big and tall you're not gonna um but you are going to if you have a mother say who's really tall and a dad who's tall as well it doesn't necessarily mean you will be tall because there are so many different genes that are contributing to height okay you could both parents could have a bunch of short jeans and a bunch of tall jeans and you could get a bunch of short jeans from your parents or if you have one parent who's tall and one parent who's short you can see the whole spectrum of sizes in their children and i gave you the example of my family okay so um let me get out of this and we'll go back to the female reproductive system