[Music] in this video we're going to look at family trees which are sometimes called family pedigrees the one you can see here is an example of a family tree for cystic fibrosis which is a recessive inherited disorder the fact that it's recessive means that to actually have the disease cystic fibrosis you'd have to have two of the cystic fibrosis alleles and because they're excessive we show these alleles with a lower case f in contrast the healthy version of the allele which doesn't cause cystic fibrosis is dominant and so we represent that with a capital f so this means that if you have the homozygous recessive genotype which is two or lowercase f's then you'll have the disease cystic fibrosis and if you have the homozygous dominant genotype of capital f capital f then you won't have the disease finally if you only had one cystic fibrosis allele and your other one was the healthy dominant allele so you had the genotype capital f lowercase f you'd be classed as heterozygous and whilst you wouldn't have cystic fibrosis yourself you'd be considered a carrier because you carry the allele for it and can pass that on to your children now whenever you're given a family pedigree like this in an exam you'll also be given a key which explains what all of the symbols mean so for this example circles represent females and squares represent males if the shape isn't shaded then it means the person doesn't have the condition whereas if it is shaded then they do have the condition and lastly these half shaded shapes indicate somebody who's a carrier remember though they'll always be given a key so you don't have to remember any of this you just need to understand how the key works you also need to be able to work out the genotype of each person using the diagram remember that this diagram is for cystic fibrosis which is a recessive inherited disorder so people with the disease will be homozygous recessive carriers will be heterozygous and people completely unaffected will be homozygous dominant for the tree itself each new row is a new generation so we could think of this top row as the parents the second row as their children and the bottom row as grandchildren if two people are connected directly with a horizontal line like these four pairs are then it just means that they're a couple so sam ryan and jack are all unrelated individuals from different families but they're shown on this family tree because they're partners of members of this main family so if we now take a look at what's actually happening in this family tree we can start off with jenny because she's represented by an empty circle she must be an unaffected female and so have the genotype capital f capital f or homozygous dominant meanwhile her husband paul who's represented by a half-shaded square must be a male who's a carrier of cystic fibrosis so he must have the heterozygous genotype capital f lowercase f together we can see that ginny and paul have four children ron anna harry and lucy anna and lucy are both carriers so have the heterozygous genotype just like that that whereas ron and harry are both unaffected so have the homozygous dominant genotype like their mum and finally sam one's wife is a female who has cystic fibrosis so she's the only one here who actually has the disease and so has the genotype lower case f lower kcf or homozygous recessive in the exam one of the things you could be asked to do is to use the diagram to work out the chance of a certain person having the disease for example let's say that lucy and jack have a baby what's the chance that the baby will have cystic fibrosis at this point you'd have to look at the family tree and think about the two parents genotypes and because both lucy and jack are carriers we know they both have the heterozygous genotype of capital f lowercase f next we need to draw out a punnett square and put lucy's alleles on the left and jacks on the top then all we have to do is fill out the punnett square which we cover in another video and we see that there's a one in four chance of the baby being completely unaffected with a capital f capital f a two in four chance of them being a carrier and a 1 in 4 chance of them actually having cystic fibrosis so the answer to our question is that there is a 1 in 4 or 25 chance of their baby having cystic fibrosis anyways that's everything for this video so if you found it useful then please do give us a like and subscribe and we'll see you again soon you