Erika Tan lists and describes the different complex inheritance patterns found other than Mendelian genetics.
*** If there are any pictures used in this video, they are NOT MINE and I will not take credit for them. ***
It’s important to at least know about Mendelian genetics before reviewing the complex inheritance patterns found in biology. So first of all is complete dominance: the simplest one. You probably already know what it is: when we cross two organisms, their offspring can have different phenotypes depending on if they get dominant or recessive alleles. This is when we see blue eyes and brown eyes: the blue eye allele is recessive and the brown eye allele is dominant. When we cross two heterozygotes, which means that both of the parents have both alleles, we get this ratio in the Punnett square: 1 to 2 to 1. However, that’s only the ratio for the genotype of the organism. When we talk about phenotype, we see that the ratio is 3 to 1 because these three will have brown eyes since they have at least one dominant allele, and this last one will have blue eyes since it has two recessive alleles.
Incomplete dominance is when you see an intermediate between two distinct phenotypes. If you breed one red snapdragon with one white snapdragon, you get a pink one. It’s not really a blending of the colors, since later on, a pure red or a pure white snapdragon can still come about. Here’s the genetics behind all of this: we’ll use this variable, C, to stand for color. This red snapdragon will have to alleles: Cr and Cr, while the white snapdragon will have two alleles called Cw and Cw. Notice that I’m not using the capital r and lowercase r like I would for a regular complete dominant cross, since both of the alleles in this situation would have an equal presence. Anyways, when we cross the snapdragon alleles in a Punnett square like this, we get this result. And when we cross two of THEIR offspring, we get THIS result: the ratio of 1 to 2 to 1. And this time, the phenotypic ratio is the same: 1 to 2 to 1, since this single snapdragon will be red, these two will be pink, and this last one will be white.
Now let’s move on to codominance. This one is kind of like incomplete dominance, except there isn’t an intermediate phenotype – instead, there’s a phenotype in which both characteristics are seen. A really common example of this would be blood type. You know, there’s blood type A, B, AB, and O. But we can determine this on a genetically molecular level: the fact that there’s a blood type AB shows that codominance is occurring. Immunoglobin A results in blood type A, Ib results in blood type B, and lowercase I represents blood type O. Both Ia and Ib are dominant, while O is recessive. That means if you’re blood type A, you can either be Ia Ia or Ia i. This is saying that you have type A molecules on your red blood cells. But, if you’re type AB, you can only be Ia Ib. Both phenotypes A and B will show on the red blood cell. Remember, it’s not an intermediate; it’s a circumstance in which both traits show.
Blood types are also a good example of multiple alleles, which is when there are more than just two forms of a gene. For example, the allele for the blood type gene can either be Ia, Ib, or i.
Alright, so pleiotropy! This is when a mutation in one gene impacts many different phenotypic traits. An example of this would be the disease called cystic fibrosis, where mucus buildup damages the lungs and other systems of the body. The fact that a single gene is able to affect multiple systems and characteristics of your body proves that cystic fibrosis is a common model of pleiotropy.
Next up is epistasis, which is when one gene can mask the expression of another. So let’s say that a mouse has a gene for fur color: it can either be brown or black. Black is the dominant allele, and brown is the recessive allele. But there’s also another gene that determines if the mouse has any color at all! So we’ll say that capital C stands for color, and lowercase c, which is the recessive allele, will stand for no color. So a mouse has this genotype (BbCc). The presence of the uppercase B determines that the mouse will be black, since it’s a dominant allele and it doesn’t matter if there’s a dominant or recessive allele afterwards, and the presence of the uppercase C verifies that the mouse will indeed be black because it will have color. But what if we changed that uppercase C into a lowercase one? Then the mouse would be white, because the genes for color would be no color no color. So, this gene masks this one, and it doesn’t matter if the mouse has brown or black alleles anymore.
Last up is the influence that the environment has on genes. What’s in the environment surrounding an organism will impact the phenotype of the organism. For instance, the pH of the soil influences the color of hydrangea flowers. That means color in hydrangeas isn’t necessarily genetic, it relies on the pH value of the soil.