Looking at Kate’s pedigree here, we can see that the trait of lactose intolerance doesn’t appear in every generation. In fact, it looks like it skips generations.>>I’ve heard of genes skipping generations before, like male pattern baldness, or cystic fibrosis.>>Exactly. There are lots of examples of traits that do this, but it’s important to know there are also other traits that do appear in every generation, where many of the family members show the same phenotype.>>How do they do that, is there something different genetically?>>Well, we’ll get to that. But the first thing we need to do is better describe these inheritance patterns. And to do that, we’re going to pull on knowledge in all the lessons we’ve had up to this point. Particularly, the idea of an allele.>>So wait, let’s go over what an allele is again.>>An allele is just a version of a gene that can cause a version of a trait. For example, one allele may promote blue eyes and another allele may promote brown eyes.>>That’s right. And I have two, one from mom, one from dad.>>Exactly. It’s important for us to keep that in mind as we go through this lesson, as it’s going to be a very key concept. We have two copies of every gene. And we can have different alleles of a gene, but different alleles don’t always cause the same kind of trait.