hi everybody and welcome back today we're going to be looking at pedigree diagrams and how to interpret them and what to do when you have to answer questions based on them basically a pedigree diagram is a family tree that shows us how individuals are related to one another and how certain traits diseases um or any kind of genetic characteristic is passed on from one individual to the next so in these family trees they are often constructed with squares and circles these squares will represent males and these circles represent females any of the individuals that are colored in often have the disorder or they have the trait that we are following in the family you'll also notice that running alongside on roman numerals and these represent the generations now the most important and key factor to any pedigree diagram is to be able to tell what everybody's genotype is and we're going to look at that now over the course of the video so there are three major pedigree diagrams that you may be asked to interpret and they are as follows the first one is what we call a autosomal dominant disorder and so as you can see here in this diagram we have a short pedigree diagram and autosomal means that it is on your autosomes it means your chromosomes 1 to 22 and dominant means that you only need one capital letter to have this disorder perhaps the letter d and what that means is that all the individuals in this pedigree diagram who are colored in have at least one capital letter d the only person who doesn't have it is this person over here that's not colored in now how does that actually work out well if this is an autosomal dominant disorder it means that this individual here has at least one capital d because they have the disorder and so does this person also have a capital d now if we look at their offspring this person has the disorder but now this individual that's not colored in doesn't have the disorder the only way that that's possible is if they have two lower case letter d's now if we know anything about inheritance we know that you inherit one allele from each parent which means that the parents above can't just have two capital entities their other allele must be a lower case letter d and that's how we ultimately determine our offspring and our parental genomes now this remaining offspring must either be two capital d's because they would have inherited one the capital d from the one parent and one from the other or this person could also be a capital d with a lowercase d because they got one from each of their parents and their one parent has each of those available so sometimes what happens is the best way to work out what the parents are is you have to start from the children and you work your way back up again next let's look at autosomal recessive and that means that these individuals will have all lower case letters and they will be the ones that will have the disorder but we must remember that the rule for recessiveness means that you must have at least two recessive alleles to have it so in this family tree the only individual that has the disorder is the one that's colored in and so we put the two lowercase r's to represent the recessive trait they have but now what are the rest of the individuals well let's work backwards so first of all if each parent gives one allele to their offspring that means that this parent must have at least one small letter and this parent must have also one small letter but what about the other letters that they have well they have an individual here who doesn't have the disorder which means that they must have at least one capital letter and that is because a dominant disorder masks a recessive now if this is the case that means that this child must have a lower case r because they got this one over here from one of their parents but where did they get this other capital well they would have got it from one of their other parents which means that the only possible combinations that these two parents have is what we call heterozygous they have one capital letter and one lowercase letter yet again we worked from the offspring to the parents and then it revealed the rest of the members in the family the next one we're going to look at is what we call x-linked also known as sex-linked recessive disorders these are disorders that are carried on the x chromosome and often they look like a small letter that is attached to the top of the x now this is a little bit more complex because you get combinations based off of the sex chromosomes that an individual has so i'm going to write out all the combinations so that you can see and i'm going to use hemophilia as an example which is a sex-linked disorder now in a disorder like hemophilia it affects the x chromosomes and it is recessive and so there are five possible combinations that can be created the first one is where the x's both carry two capital h's and this would produce a normal female the second possible combination is one x has a capital h while the other has a lowercase h now this female is still normal however we can write in brackets carrier it's important to know that carrier is not a phenotype so in exams and tests please do not write carrier as your answer instead you can put the word in brackets just to help you remember that she is a carrier and she has a big letter and a small letter and finally we can also get a small letter and a small letter and this will produce a hemophiliac female now i've noted here in brackets that these individuals are not born and that is true you can calculate the chance of a female having hemophilia in a punnett square but hemophiliac uh girls are not actually born unfortunately the hemophiliac gene when it is present twice ultimately causes a miscarriage now in nails they only have one x chromosome so they either have hemophilia or they don't they can't be carriers so in this example we have a capital h on the x and this is produced a normal male whereas we only have one small lowercase h and therefore we have a hemophiliac male now this inheritance pattern that we see in males and females is really important and you need to be able to explain why males more frequently have diseases associated with their sex chromosomes than females do so let's take a look at how you would explain why males experience these kinds of inheritance patterns more frequently than females do so why do males have more sex-linked disorders than females well it comes down to the number of x chromosomes that these individuals carry females have two x chromosomes so if there is anything on the one x chromosome that might cause a disease it's not a problem for that female because often that female can have an extra copy of her x chromosome that is healthy for example in hemophilia the one x chromosome can carry a capital h which means it's a normal gene and the other can carry the recessive allele which means that this female doesn't experience any of the side effects of hemophilia in males on the other hand they only have one x chromosome and the issue is the y chromosome is actually not a homologous pair to x it carries its very own genes that are separate and it's much much smaller that means that for example in hemophilia if a male has just one of the small lower case h's meaning that they have the recessive allele their y chromosome can't mask those effects and that is why males are more often likely to have the disorder than not females have this backup copy which males lack and so if we were to explain it you could use the following points so how do we explain this well males only have one x chromosome therefore if it carries the recessive allele the male will have the disorder here's an example of what it would look like in females there are two x chromosomes where a dominant allele can mask the recessive allele meaning she will not have the disorder and that's what this can look like often we call these individuals carriers but how do you explain if a female does well in order for a female to have the disorder she would need a recessive allele on both her x chromosomes and that's what this would look like now let's do some examples this is an autosomal dominant pedigree diagram and we're going to work through it we're going to give each individual here a genotype and a phenotype and i'm going to make up my own phenotype key alongside so you can use it too so what i've done here is i've made my own key and let's assume that we are going to be doing this for a disease called huntington's if you have the capital letter h you have the disorder remember this is a dominant disorder that we are looking for which means you need at least one h capital if you don't have huntington's then you'll have a lower case now let's not forget that in order to have this disease you must have at least one capital letter so that means that we need to see a capital h at each one of the individuals who are colored in all of the individuals who aren't colored in will have two lowercase h's because this is a dominant disorder the only way you can avoid having it is if you have two lowercase h's i'm going to go in here and fill in all the lowercase and higher case h's so you can see how you would do it now because this is an autosome disease there's going to be no x's and y's here make sure you read the question really carefully when you do get it it'll say it on the paragraph about whether or not this is autosomal or sex-linked or it's dominant or recessive now what i've done here is i've gone in and i've filled in all of our individuals who don't have huntington's they're the ones that are not colored in so any of the circles or squares now now that i've done this i know these for certain because the only way to not have huntington's is to have two small lower case h's but what about the rest of these individuals well anybody who's colored in on this diagram has huntington's which means that they have at least one capital h each but what about their other letter the problem is is that we don't really know what their other letter is unless we take a look at the children they have produced and that is going to tell us how they have inherited their disorder and they're going to tell us also how they've passed it on now that i've put in all the capital h's to all these other individuals let's now see what could they possibly be the other allele well in our earlier part of our video i told you to work backwards in other words you're going to work from the children and go back a generation so you can work on the parents and if we look at this individual here three and four you will notice that they produced an individual that doesn't have the disease um and the only way that that's possible is that if this individual number three gave their lower case h we need another lower case h as well and the only ways that we can get that is from individual number four now they have one capital h but the other letter must be a small letter h because that's how person 8 or individual 8 inherited one small letter from each parent and now they are a homozygous recessive and now we've figured out what four is but let's figure out what one two five six and nine are as well if we look over at one and two the key here is yet again to look at their children and work backwards you'll notice they produce an individual who doesn't have the disorder now the only way that that's possible is if the two parents before them also carry two lower case h's the only way that you can get this kind of combination they've also produced individual five and six but they have the disorder now if they've inherited one capital h from each parent which is possible they actually have combinations that they could have received in other words let's look at individual five individual five could actually have two capital h's or they could have a capital h and a lowercase h why because it works with the law of segregation essentially individual five could have inherited one capital h from their one parent and another one from their other parent but they could have done the same with the higher case h from one parent and they lower case h from the other the same can be said for individual six we're not really certain if there are two capital h's or two small h's and so in a test or an exam you can actually write both it's not incorrect to have two options the next kind of problem we need to look at solving is a sex-linked recessive disorder now this one is remember focused on the x chromosome so unlike the previous problem that we worked with which was autosomal meaning you were going to use a capital letter d and a small case d sex link means that you're going to have to use x's and y's and you're going to have to superscript them it means you're going to write that little letter at the top now i'm going to use red green color blindness as an example to work out this pedigree diagram and i'm just going to put the key up so we know what we're using so i'm going to use the following key we're going to use a capital r for anybody who is not colorblind and we're going to use a lowercase r at the top of the x for anyone who is colorblind i want you to remember that in order for females to have this disorder they must have the allele which is the little letter r on both of their x chromosomes in order for them to be colorblind now in any sex linked disorder my suggestion is to my students to fill out all the x's and y's first and then to go back and fill in the allele so i'm quickly going to do that right now that i have filled in everybody's x's and y's remember circles are females and squares are males now what you should go and do is fill in all of the known alleles in other words everybody who is colored in here has the disorder that means then that if you're a male you'll have one lowercase r on your x and if you're a female you should have two so let's look at individual number one at the top this female has the disorder which means that her two superscripts are going to be two small r's on both of her x's you'll see on her partner um the male at number two he doesn't have the disorder which means that he will have a capital r on his x now what i suggest is that the easiest thing to do is actually to go to all the individuals that are colored in and put in their alleles so let's go down to number five this male has it which means he has a small r on his x and number seven also a small r on his x and female eight she also has color blindness so we're going to put two lowercase r's on her x's and i'm going to do the same for the rest now that we have completed all of the known individuals the ones that we know are for certain we now need to go through some of the individuals that we're not so sure about and perhaps fill in the rest of the blank spaces so if we go to all the males and let's look at male number three he hasn't have a colored in block however we know that males must have it on their x chromosome and if he doesn't have it then that definitely means he has a capital r likewise with male number 11 he doesn't have it therefore he has a capital r now what about all these other females that aren't colored in but there's a possibility that their allele combination is different so if we look at individual four and six they are sisters and they come from a family that has uh color blindness in it as well because their mother number one carries it now we must remember that when um we inherited alleles we get one from each parent now the father on this hand only has one capital rx to offer which means that all the daughters should have at least one capital r on each of their x chromosomes because that's where they're going to get one from their dad and the other one is going to come from their mother now their mother only has two small r's to offer which means that these are lower case r's now we call these individuals carriers because they carry the disease but they don't experience any symptoms if we look at individuals 10 and 12 they are also sisters and now we need to look at their parents so here is their parents it's an unaffected dad and a carrier mom now because their dad only has one x chromosome to offer which is the x with a capital r each of these girls will have a capital r on their x's but now here's the tricky part because their mother has both a capital r and a lowercase r we're not really sure which one they got because neither of them have the disorder so in this case and you can't do this in a test they can either be two capital rs or they can have x capital r and x lowercase r now something that i want to point out which is really important is when you're writing the phenotype for these kinds of crosses it's important to remember that um you are going to group carriers with people who also just don't carry or don't have the disease at all so what i mean by that is if we just look at the females only in this family tree three are colorblind and that is number one number eight and number fourteen they are colorblind however there are four females that are not colorblind and that's 4 6 10 and 12. now i know that they are carriers but remember a carrier is not a phenotype you cannot write the word carrier if you do just to help you remember what it is you must always put it in brackets so let's round up the rest of this lesson so we looked at lineages which remember is the way in which organisms acquire their characteristics from their parents remember that sometimes it's really difficult to tell what the parents were so you're going to need to look at your children first and you're going to work your way backwards your children reveal what parents possible genotypes could be we then also looked at autosomal dominant disorders and that's when you only need one letter to have it whereas autosomal recessive you're going to need two so that means that if you had our and a little r it means that you have this disorder however for autosomal recessive you would need two little r's in order to have the disorder we spoke about carriers these are mostly female individuals who carry the disorder and their phenotype cannot be classified as carrier you're just going to classify them as a normal or unaffected female if you're going to write out your genome and phenotypes then we looked at sex-linked or x-linked and essentially what that means is these diseases are carried on the gonozomes or the six chromosomes and they affect males and females differently and last but not least affected and not affected is the way in which we differentiate between the two when we write phenotypes thank you everybody and i hope this was helpful i'll see you all again soon bye