 Today, we are going to go through some more examples of pedigrees, and how we can use them to work out the pattern of inheritance of a particular allele. Now we have talked about how alleles can be dominant or recessive, but we have not discussed how these alleles can be found on autosomal chromosomes, or on X and Y chromosomes. In all of the cases where an allele is dominant, only one needs to be present to be expressed. Contrastingly, both alleles need to be their recessive allele in order to show a recessive trait, except for in biological males for X-linked traits, because they only have one X chromosome and thus only one X-linked allele. Let's now remind ourselves of how genotypes are represented, with lower case letters indicating a recessive allele and upper case letters representing a dominant allele. Remember that biological males only have one X chromosome and therefore cannot be heterozygous for X-linked traits. In this first worked example, we are also determined how we know that this trait displayed in this pedigree is recessive. Now we can work this out in many ways. For example, we could have a look at this individual here in the second generation, who was affected. If the trait in this pedigree was dominant, it would mean that this child has a dominant allele, meaning that at least one of his parents has passed this dominant allele onto him. But both of the parents do not display the trait and therefore do not carry this dominant allele. Consequently, we can say that this trait is not dominant and thus it is recessive. This is a nice summary of how to tell the difference between dominant and recessive traits when using pedigrees. If there is an unaffected child with two affected parents, then the trait is dominant. If there is an affected child with two unaffected parents, then the trait is recessive. In both cases, this is only possible when the parents are heterozygous. Another difference is that dominant traits cannot skip a generation, whereas recessive traits can. We can see generational skipping in the example of a recessive trait presented on this slide, where neither of the parents have the trait, but the child does. Thus the trait has skipped the parent's generation. For the next example, we are asked to prove that this same trait in the same pedigree is autosomal and not x-linked. But how do we do this? Well, to explore, let's imagine that this trait was x-linked and not autosomal. And let's look at this family in the bottom right with their sex chromosomes written below them. Now, as this trait is recessive, the daughter of this family must have two recessive alleles, so let's write those in. This daughter must have received one x-chromosome from her mother and one from her father, meaning that both parents must have an x-chromosome with a recessive allele on it. But the father only has one x-chromosome, meaning that he should also be recessive for the trait, as shown here. But if this was the case, the father would show the recessive trait and would be shaded in. So we can work out that this pedigree displays an autosomal recessive trait by denying the possibility that it could be an x-linked recessive one. Now this is a nice summary of how to tell that a trait is x-linked. For x-linked dominant traits, all affected males should have mothers and daughters that are affected. And for x-linked recessive traits, all affected females should have fathers and sons that are affected. If these trends are held, then the trait is said to possibly be x-linked. But they could still be autosomal. If these trends are not held, then the trait is autosomal. This is why we cannot confirm that a trait in a pedigree is x-linked. We can only deny it. So let's look at another example. And this question is asking us to determine whether this trait, shown in this pedigree, is autosomal dominant or x-linked dominant. First let's look at the second generation, individual two, who is an affected male. All of his daughters and his mothers are also affected. This holds true to dominant x-linked patterns in pedigrees. This pattern is the same for this affected male. And for this one. Consequently, this trait could be x-linked. But as stated before, this does not disprove that it is autosomal. So we say that this trait could be x-linked or autosomal. In summary, today we learn how pedigrees can display different types of traits. This slide nicely summarizes all of the trends that we can observe in pedigrees.