So there are many different types of inherited traits. The first category that we're going to talk about is actually called continuous traits. And these are traits that could take a potentially infinite number of states within a range. So what do I mean by that? We're looking at human height, which is an example of a continuous trait. There's the shortest person in the world and the tallest person in the world, but any height in between there can be any number. Right? You can have someone who's 4 feet, 1.6298 inches. Right? You can have somebody who's 7 feet and 0.92346 inches. So anything that you can add these extended numbers of decimal points to is typically a continuous trait. And this differs from categorical traits, which are traits that have to be sorted into discrete categories. An example of this is a flower that is either purple or white. There's no mix. So either you're purple or white. Another example is spots on a Dalmatian, for instance. You can have one spot, you can have 2 spots, you can have 30 spots, but you can't have 30.67 spots. Right? It doesn't make sense. You can't have 0.67 spots on a Dalmatian, and therefore, that's going to be a categorical trait because it has to be divided into those categories.
Now within categorical traits, there are two types of classifications. The first is a little bit confusing to grasp, but it's called a threshold trait. These are traits that need a variety of different genetic and environmental factors to be expressed. Some people can have some of these genetic or environmental factors, but only people who reach a threshold number of those factors express the trait. An example of this is type 2 diabetes. It's a categorical trait because you either have type 2 diabetes or you don't. And type 2 diabetes is a combination of genetic and environmental factors, but you have to reach a threshold number of those to become a type 2 diabetic, which is why it's considered a trait. It's also a disease, but it's an interesting disease trait mixture because type 2 diabetes can be, at least in part, genetic. But not everyone who carries genetic markers for type 2 diabetes will be type 2 diabetic because they haven't reached that threshold of environmental and genetic factors required to become a type 2 diabetic.
A second type of categorical trait is much easier to wrap your mind around. That is a meristic trait or otherwise counting trait, and these are things that are divided into a range of discrete values. An example of this would be birds. They can lay 1, 2, 3, 4, 7 eggs, but they can't lay 1.26 eggs. Right? Or 2.58 eggs. It doesn't make any sense. They can lay 1 egg. They can lay 3 eggs. They can't lay 2.56. And so that is a categorical trait.
So, let's do an example and classify each one of these into categorical or continuous. The number of spots on a Dalmatian. Is it continuous or is it categorical? 50 spots, you can have 57 spots, you can't have 50.36 spots on a Dalmatian. Human weight. Continuous or categorical? Right. It's going to be continuous. And the reason it's continuous is because you can have somebody who weighs 130. You can have somebody who weighs 130.00001, as long as you have a way to detect it. So, human weight is a continuous trait. What about foot size? Continuous? Categorical? Right. That's continuous, because foot size can be a range of values between the smallest foot and the largest foot that exists in that organism. And then we have cat litter sizes. So, what do you think? Is that categorical or continuous? Right. Categorical because you can have a cat that will produce 1 kitten or 2 kittens or 3 kittens, but you can't have a cat that produces 2.58 kittens, and so that is categorical.
Now that we've divided these into continuous or categorical, let's talk about how these traits are inherited, and there are two main ways. The first is called complex inheritance, which involves multiple genes or environmental factors. We're looking at something, for instance, human height. Human height is controlled by multiple genes which means human height undergoes complex inheritance. This differs from simple inheritance, which involves one trait. Typically, you can figure out whether it's one trait or more by looking at Mendelian ratios. If you can do crosses of plants or flies and look for a 3 to 1 or a 9 to 3:3:1 ratio, that's going to be something that is simply inherited. Usually just involving one trait. So for complex inheritance, we've talked a ton about this.
Let's discuss polygenic inheritance or complex inheritance. What happens in polygenic inheritance is that there are multiple genes, but each gene behaves in a Mendelian fashion. If you were able to isolate every single gene and study it in a Mendelian fashion, what you would get is Mendelian numbers. But we're looking at phenotypes and all of these alleles and different genes come together and contribute to the phenotype. Not every allele in the phenotype is actually contributing to that phenotype. There are two types of alleles, an additive allele that contributes to the phenotype, meaning it's added to the phenotype when it's present, and a nonadditive allele that does not contribute to the phenotype. These almost seem similar to dominant and recessive, and we'll see that we talk about that a lot, but it's not exactly, because in a recessive gene, you only need two recessive alleles to show that phenotype. But in the case of additive and nonadditive because it's multiple genes, you can need a lot more non-additive genes to not see that dominant phenotype or additive phenotype. </>
