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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. There isn't really a clear dominant allele! 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!