Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!


Captioning is on! To turn off, click the CC button at bottom right. Follow the amoebas on Twitter (@amoebasisters) and Facebook! 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 the guinea pig had
hair? Only if there was no dominant present, a genotype of hh, would there be a hairless
guinea pig? That’s a Mendelian trait. But what we’re going to cover is when these
traits are non-Mendelian and don’t follow those basic rules. 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. In snapdragon genetics, there can be 3 phenotypes. Red.
White. Or something in between—PINK! It’s called incomplete dominance. In incomplete
dominance, the dominant allele is not completely expressed with the recessive allele is around.
So if you cross a red flower (written RR) and a white flower (written rr), you get babies
that are Rr. But unlike a Mendelian trait, if incomplete dominance, that R allele is
not completely expressed when the r is around. So Rr in this case is pink! If you cross two
pink flowers (Rr), like shown in this Punnett square here…you can get offspring that are
red, white, or pink.Incomplete dominance is different from codominance. Codominance—like
a coworker—that pre-fix “co” should make you think together. They work together.
The alleles, that is. For that reason, we like to use different letters entirely. In
some breeds of chickens, there is a codominance involving color. Take a look at this Punnett
square. If you cross a black chicken—represented by BB—-and a white chicken—-represented
by WW——all the offspring here are BW. BW chickens are both black and white. Speckled!
See, both traits show up—this is the essence of codominance. And you know, what’s more
awesome than a speckled chicken? Well except for those silkie bantam chickens….By the
way, you will find that many people use different formatting for incomplete dominance and codominance.
Some people like to use different letters entirely for incomplete dominance—some people
don’t. Some people also prefer to do exponents with codominance—-some people don’t. Formatting
aside when solving Punnett squares, since we find this can really vary classroom to
classroom, the real concept is that in incomplete dominance—one allele is not completely dominant
over the other so you see an almost “in between” phenotype. If codominance, neither
allele is dominant over the other, so both alleles are expressed.Height is fascinating.
In our immediate family, Pinky is taller than Petunia. Our mom is also taller than Petunia.
How does this happen? There isn’t just one height gene. There’s LOTS of genes that
determine your height. What I mean by that is that you don’t just have a pair of alleles,
like AA, Aa, or aa that code for your height. It’s more like someone having a genotype
of AABbCcDD etc to ultimately determine height. And you inherit one allele for each of the
height genes—from each parent. All of those 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 that we want to talk about, epistasis. Epistasis is when one gene really
depends on another gene for it to be expressed. I mean, it REALLY depends on this other gene.
So let’s pick an animal like a…. Ok…a….llama. Let’s say that this llama has a dominant
B allele which means its wool will be black. So BB or Bb means it will have black wool
and let’s say that if a llama has a pair of recessive alleles—bb—it will have brown
wool. Now what if there is another gene—an epistatic gene—that controls whether the
pigment will even be expressed in the llama wool in the first place? A llama can have
a genotype of CC, Cc, or cc for this epistatic gene. However, if a llama has the genotype
cc, 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 the epistatic gene—- this calls for our favorite
16 square dihybrid. If you notice in this dihybrid crossing two heterozygote llamas
(BbCc and BbCc), BB and Bb will typically give a black llama and bb will typically give
a brown llama in all cases UNLESS the epistatic gene inherited is cc. If the llama has a cc
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-linked
traits—which are also non-Mendelian. It’s kind of fascinating to see what can happen
with 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. Well that’s it for the amoeba sisters and
we remind you to stay curious!

100 thoughts on “Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

  1. I LOVE YOUR VIDEOS! I've been watching your videos for years now and it really helps me with my biology!

  2. Disliked because this video is required to be watched in class while we could’ve had free time

  3. i have a big test this week on topics i never learned but then i saw this video and now i know that am getting a+ !!! thankyou

  4. Quick question: If an offspring exhibits codominance, does that mean that the parents are always going to be homozygous dominant? Or can they be heterozygous?

  5. I love your videos and your way ofΒ 
    concepts is Excellent and unique!! Thank you very much for making videos!! Please keep up making more videos!!!

  6. 2:17 The Legend of Zelda reference is 100% accurate

    greetings from my homework, ur videos help so much with understanding my Freshman bio Honors class

  7. Stop! you violated the law. Pay the court a fine or serve your sentence, your stolen goods are now forefit

  8. Here I am, 4:30 am, on a dark, rainy Sunday trying to really get the hang of this topic and bawling endlessly. Then I decided to see what resources were available on youtube, originally scrolling and thinking I was wasting my time. Then the eye-catching, adorable, cute amoeba sisters, shed light on my life! This topic makes sense and I feel like I can always count on the amoeba sisters for anything. Truly changed my life for the better.

  9. my teacher assigned me this. It is rare to find Nintendo references let alone Zelda references in educational videos, especially when it is school work. This made my day.

  10. So let's say that I was born from a white family and then all of a sudden, another gene expresses my genetic makeup so then I become black. Will everybody then say i will become adopted ?

  11. Just wanted to let you guys know I watch your videos before every single bio test and I always ace them thanks to u!(:

  12. This is not helpful and I’m gonna fail my biology class, F you πŸ‘ŽπŸΌ

  13. How the incomplete dominance can have both capital letters?! You said at the beginning of the video that the big allele is not completely expressed when the little allele is around. There should be one capital and one small

  14. So even though the big allele is represented as dominant it can be a mixed in colour to form pink. Answer me please

  15. Really helping me out with college bio. An absolute lifesaver!

    And love the LotR and Skyrim references. LOL

  16. I’m in my senior year of biology and I still use these videos. I don’t know if I should be ashamed of myself.

  17. Your videos suck. Take some advice from Morgan Freeman and drop the Amoeba Sisters act. This puts me to sleep. Like literal definition of a coma.

  18. I am already became your huge fan. Thanks for the lovely videos. Love and support from India 😊😊

  19. when a 7 minute video explains your whole text book……sigh

  20. You don't how much you made my life so so easier😭😭😭 thank youuu!

  21. Can you make a video about quantitative traits and heritability please?! πŸ™‚

  22. I'm actually studying for my bachelor's! I'm 22. Informative video!

  23. Someone with accurate knowledge answer me only please.
    So incomp, comp and codom only apply in Non-Mendelian genetics?
    What are some of the hints in problems to let us know if something is Mendelian or Non-Mendelian?

  24. I'm studying high school biology final, and although these videos seem like they're aimed at 10 years olds, it never fails to make me laugh. (and learn!)

  25. This video is so informative and straight to the point!!! Helped me so much πŸ™†πŸ»πŸ™†πŸ»πŸ™†πŸ»yay thankssss!

  26. Both sisters are soo cute 😍 I bet they inherit it from there mother πŸ˜…

  27. Tyyyyy so much! I dont usually comment but damn these videos are really helpul for my nmat! Thank youuuuuu.

  28. you guy are so helpful, literally the only reason im getting higher grades in biotechnology

  29. I learned more in these 7 minutes than I did in the hour I spent taking notes on this in class.

  30. Amazing channel. Keep up the good work guys! You actually make this stuff interesting to me again πŸ™‚ thank you so much!

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