What do snapdragons, human height, and speckled chickens have in common? Well, they are easily identified as non-Mendelian traits. By non-Mendelian, we mean that, genetically, they are rule breakers. They don't follow the regular Mendelian rule that having a dominant allele means the dominant trait will show.
Remember in our guinea pig video, having a dominant allele represented by a capital H meant that the guinea pig would have hair? Only if there was no dominant present, a genotype of little h little h, would there be a hairless guinea pig? Well, that's a Mendelian trait. What we're going to cover now is when these traits are non-Mendelian and they don't follow these basic rules. So let's first start by snapdragons.
We confess that when we heard this word, we thought they were some really amazing kind of creature. Well, they are amazing, but they're flowers. So I don't know, that's not exactly what we envisioned. Well, in snapdragon genetics, there can be three phenotypes.
Red, white, or something in between, pink. It's called incomplete dominance. In incomplete dominance, the dominant allele is not completely expressed when the recessive allele is around.
There isn't really a clear dominant allele. If you cross a red flower, and we're going to write it here, big R, big R, and a white flower, little r, little r, you are going to get offspring that are big R, little r. But unlike a Mendelian trait, if this is incomplete, Incomplete dominance, that big R allele, is not completely expressed when the little r is around.
So big R, little r, in this case, is pink. If you cross two pink flowers, big R, little r, like shown in this Punnett square here, you can get offspring that are red, white, or pink. Incomplete dominance is different from co-dominance.
Co-dominance, like a co-worker, that prefix co-should make you think together. They work together. The alleles, that is. For that reason, we like to use different letters entirely to represent the alleles.
There is a codominance involving color in some types of chickens. Take a look at this Punnett square. If you cross a black chicken, represented here by BB, and a white chicken, represented here by WW, all the offspring here are BW. BW chickens are both black and white.
They're speckled. See, both traits show up. This is the essence of co-dominance. And you know, what's more awesome than a speckled chicken?
Well, except for those silky bantam chickens. By the way, you will find that many people use different formatting for incomplete dominance and co-dominance. Some people like to use different letters entirely for incomplete dominance, and some people don't. Some people also prefer to do exponents with co-dominance, and some people don't. But formatting aside, when you're solving these Punnett squares, since we find...
this really does vary from classroom to classroom. The real concept is that in incomplete dominance, one allele is not completely dominant over the other. So you see this almost in-between phenotype.
In co-dominance, both alleles are expressed. It's the application that matters here. Height is fascinating. In our immediate family, Pinky is taller than Petunia.
Our mom is also taller than Petunia. So how does that happen? There isn't just one height gene. There's a lot of genes that determine your height. What I mean by that is that you don't just have a pair of alleles, like big A, big A, that will code for your height.
It's more like someone having a genotype of big A, big A, big B, little b, big C, little c, and maybe more genes to ultimately determine your height. and you inherit one allele for each of the height genes from each parent. All of these genes work together to determine your height.
Your skin color is also determined by many genes, just like your height. These are called polygenic traits. Poly means many, so many genes coding for one trait is what polygenic means.
By the way, both height and skin color can be influenced by environmental factors as well. Nutrition growing up can affect your height. just as spending a lot of time in the sun can affect your skin color.
However, this doesn't change the genetics for this trait. And finally, one more thing that we want to talk about. Epistasis.
Epistasis is when one gene really depends on another gene for it to even be expressed. I mean, it really depends on this other gene. So let's pick an animal like, okay, a llama.
Let's say this llama has a dominant big b allele, which means its wool will be black. So, big B, big B, or big B, little b means that it will have black wool. And let's say that if a llama has a pair of recessive alleles, little b, little b, then it will have brown wool. Now, what if there is another gene that controls whether this pigment will even be expressed in the llama in the first place? In this hypothetical example, a llama could have this other gene coded for by big C, big C, big C, little c.
or little c little c in its genotype. However, if the llama has the genotype little c little c, it will not allow the other gene for wool color to even be expressed. Since we have two genes here, the gene for wool color and then this gene that controls whether the color is even expressed, this calls for our favorite 16-square dihybrid cross. If you notice in this dihybrid, crossing two heterozygote llamas, any llama with big B big B And Big B, Little B will typically give a black llama.
And Little B, Little B will typically give a brown llama. In all cases, unless they inherit a little c, little c. If the llama has a little c, little c in its genotype, then the gene for wool color is not expressed.
And the llama is albino. This means that no pigment is expressed at all. Pretty interesting. There are many other non-Mendelian traits. We have another video on multiple alleles and a video on sex link traits.
These are also non-Mendelian. It's kind of fascinating to see what can happen when you have these rule breakers. One last thing. Keep in mind that when problem-solving in genetics, you do not want to just assume it's non-Mendelian unless you are provided information or clues in the problem that it might be.