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 recessive, we show these alleles with a lowercase 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 lowercase f's, then you'll have the disease cystic fibrosis. And if you have the homozygous dominant genotype, of FF, 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 FF, 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 that you'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 FFF, 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 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 their dad, whereas Ron and Harry are both unaffected, so have the homozygous dominant genotype like their mum. And finally Sam, Ron'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 lowercase f, lowercase f, 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 Jack's 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 1 in 4 chance of the baby being completely unaffected, with a capital F capital F, a 2 in 4 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.
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