Genetics is very rarely clear cut. So far, we've been talking about very clear examples of dominant and recessive traits, but that's really very rarely the case because many traits that we have actually exist on a range. For instance, it's not just that somebody is tall or short; we have a huge range of height someone can be. The same is true for weight and even for hair color. These traits are not just dark or light, tall or short; it's usually a range. And so, when there is a range, of course, the genetics are going to be more complicated than just dominant and recessive.
This video is just an overview of a few different terms of ways that genes can be more complicated. The first one I want to talk about is polygenic. Polygenic means that a trait is controlled through multiple genes. If you have multiple genes controlling one trait, then you're going to get a continuous expression, like height. Let's look at an example of what that would look like in a plant. This plant is purple and white, and there are two genes that control this plant's color. We have p1 and p2.
If we do a cross where both parents are heterozygous for both genes, the possible outcomes include: getting a homozygous or heterozygous state for both genes, leading to at least two dominant alleles resulting in dark purple; getting homozygous or heterozygous for one gene, like p1, leading to one dominant allele resulting in light purple; and getting homozygous recessive for both, resulting in no dominant alleles and the color being white. The number of dominant alleles for every gene determines the dosage of how many genes you have, which, in turn, determines the exact color; the more dominant alleles present, the darker the color, and vice versa.
One way we get a range of traits is through multiple genes affecting doses of dominance. That's polygenic. Other ways that genes can interact include pleiotropy, where a single gene has multiple different effects on the phenotype of an organism. These effects can vary depending on several factors like age, weight, or environmental exposures.
Another term is variations on dominance, such as incomplete dominance or codominance, exemplified by blood types. Blood types show codominance, where having both A and B dominant alleles results in AB blood type. Another important concept is epistasis, which involves how two genes interact and how those interactions affect the phenotype. This topic can get quite complex and require significant time to fully understand.
The fourth concept is penetrance, which is about how many of the people with a certain allele actually express it. Expressivity then measures how well that allele is expressed, indicating that even if many people have the same allele, the strength with which it is expressed can vary, leading to different phenotypes. Each of these concepts will have its own dedicated video to further explore the complex ways genes interact to cause complex phenotypes. With that, let's move on.
