Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!
Summary
TLDRThe video script from the Amoeba Sisters dives into the fascinating world of non-Mendelian genetics, where traits don't follow the traditional rules laid out by Gregor Mendel. It explains how snapdragons showcase incomplete dominance, leading to a spectrum of colors between the dominant red and recessive white. The concept of codominance is illustrated through speckled chickens, where both black and white traits are expressed simultaneously. The video also touches on polygenic traits like human height and skin color, which are influenced by multiple genes and can be affected by environmental factors. Lastly, epistasis is introduced with the example of a llama's wool color, where an epistatic gene can suppress the expression of another gene. The script encourages viewers to approach genetic problems with an open mind, considering non-Mendelian possibilities only when clues suggest it. The Amoeba Sisters end with an invitation to stay curious about the complexities of genetics.
Takeaways
- đ« Non-Mendelian traits are genetic rule breakers that don't follow the usual Mendelian inheritance patterns.
- đș In snapdragon genetics, incomplete dominance results in three phenotypes: red, white, or pink, where the dominant allele isn't fully expressed.
- đ When crossing two pink snapdragons (Rr), the offspring can be red, white, or pink due to incomplete dominance.
- đ Codominance is a genetic condition where both alleles are expressed in the phenotype, as seen in speckled chickens resulting from a cross between black (BB) and white (WW) chickens.
- 𧏠Polygenic traits like human height and skin color are determined by multiple genes, not just one pair of alleles.
- đ¶ Environmental factors can influence traits like height and skin color, but they don't change the underlying genetics.
- đŠ Epistasis is a phenomenon where one gene's expression is dependent on another gene, as in the case of llama wool color, which can result in black, brown, or white (albino) wool.
- đ In genetic problem-solving, non-Mendelian inheritance should not be assumed without specific information or clues indicating it.
- đ Non-Mendelian traits can be fascinating and complex, with examples including multiple alleles and sex-linked traits.
- đ The video script emphasizes the importance of understanding the nuances of genetic inheritance beyond basic Mendelian principles.
- đ Staying curious and asking questions is encouraged when exploring the intricacies of genetics.
Q & A
What are non-Mendelian traits?
-Non-Mendelian traits are genetic characteristics that do not follow the basic Mendelian inheritance patterns, where traits are determined by a single pair of alleles with clear dominance and recessiveness.
What is incomplete dominance in genetics?
-Incomplete dominance is a genetic phenomenon where the dominant allele is not fully expressed when paired with a recessive allele, resulting in an intermediate phenotype, such as a pink snapdragon flower when red (RR) is crossed with white (rr).
How is codominance different from incomplete dominance?
-Codominance is a genetic condition where both alleles are expressed in the offspring, resulting in a blend of both traits, like a speckled chicken when a black (BB) is crossed with a white (WW) chicken. In contrast, incomplete dominance results in a single intermediate phenotype.
What is a polygenic trait?
-A polygenic trait is determined by multiple genes, rather than a single gene. Examples include human height and skin color, where many genes collectively contribute to the final characteristic.
How can environmental factors influence polygenic traits?
-Environmental factors can affect the expression of polygenic traits without changing the underlying genetics. For instance, nutrition can impact height, and sun exposure can affect skin color.
What is epistasis in genetics?
-Epistasis is a phenomenon where the expression of one gene is dependent on the presence of another gene. The second gene can either enhance or suppress the expression of the first gene.
How does epistasis affect the wool color in llamas?
-In the case of llamas, the wool color gene is expressed as black (BB or Bb) or brown (bb) unless there is an epistatic gene with a cc genotype, which prevents the wool color gene from being expressed, resulting in an albino llama with no pigment.
Why is it important not to assume non-Mendelian inheritance when solving genetic problems?
-Assuming non-Mendelian inheritance without evidence can lead to incorrect conclusions. It is crucial to look for specific information or clues in the problem that suggest a deviation from Mendelian inheritance before considering non-Mendelian traits.
How do the Amoeba Sisters use social media to engage with their audience?
-The Amoeba Sisters engage with their audience through Twitter (@amoebasisters) and Facebook, where they likely share updates, additional information, and interact with their followers.
What is the significance of the Punnett square in understanding genetic inheritance?
-A Punnett square is a diagram used to predict the genotypes of offspring in a genetic cross. It helps visualize the possible combinations of alleles and is a fundamental tool for understanding both Mendelian and non-Mendelian inheritance patterns.
How do the Amoeba Sisters explain the concept of dominance in traits?
-The Amoeba Sisters explain dominance by using the example of a guinea pig's hair trait, where a dominant allele (H) results in hair, and only the absence of a dominant allele (hh genotype) results in a hairless guinea pig.
What is the role of the recessive allele in non-Mendelian traits?
-In non-Mendelian traits, the recessive allele does not simply disappear when a dominant allele is present. Instead, it can influence the phenotype in various ways, such as in incomplete dominance, where it results in an intermediate phenotype, or in codominance, where both alleles are expressed.
Outlines
đ Understanding Non-Mendelian Traits
This paragraph introduces the concept of non-Mendelian traits, which are genetic characteristics that do not follow the standard Mendelian inheritance rules. It uses the example of snapdragons to illustrate incomplete dominance, where the dominant allele does not fully express itself in the presence of a recessive allele, resulting in a blend of traits. The paragraph also differentiates between incomplete dominance and codominance, using the example of speckled chickens to show how both alleles are expressed. Finally, it touches on polygenic traits, such as human height and skin color, which are determined by multiple genes and can be influenced by environmental factors. The concept of epistasis is introduced with a hypothetical example involving a llama's wool color.
đŠ Non-Mendelian Inheritance: Epistasis and Beyond
This paragraph delves deeper into the concept of epistasis, where the expression of one gene is dependent on the presence of another gene. Using the example of a llama's wool color, it explains how an epistatic gene can prevent the expression of another gene entirely, leading to unexpected outcomes such as an albino llama. The paragraph also emphasizes the importance of not assuming non-Mendelian inheritance without evidence, as there are many variations and exceptions in genetic traits. It concludes with a reminder to stay curious about the complexities and wonders of genetic inheritance.
Mindmap
Keywords
đĄNon-Mendelian Traits
đĄIncomplete Dominance
đĄCodominance
đĄPolygenic Traits
đĄEpistasis
đĄPunnett Square
đĄAlleles
đĄPhenotype
đĄGenotype
đĄMendelian Inheritance
đĄEnvironmental Factors
Highlights
Snapdragons, human height, and speckled chickens are non-Mendelian traits that don't follow the basic Mendelian rules.
In snapdragon genetics, there can be 3 phenotypes - red, white, or pink - due to incomplete dominance.
Incomplete dominance means the dominant allele is not completely expressed when the recessive allele is present.
Codominance is when both alleles are expressed, as seen in speckled chickens that are both black and white.
Polygenic traits like height and skin color are determined by many genes working together.
Environmental factors like nutrition and sun exposure can influence traits like height and skin color, but don't change the underlying genetics.
Epistasis is when one gene depends on another gene to be expressed, as in the example of llama wool color.
If a llama has a cc genotype for the epistatic gene, the wool color gene is not expressed and the llama is albino.
Non-Mendelian inheritance can involve multiple alleles, as covered in a separate video.
Sex-linked traits are also examples of non-Mendelian inheritance.
When solving genetics problems, don't assume a trait is non-Mendelian unless the problem provides clues to suggest it.
The Amoeba Sisters provide engaging and informative videos on genetics concepts.
Follow the Amoeba Sisters on Twitter and Facebook for updates on their latest videos.
The video explains Mendelian vs. non-Mendelian traits using clear examples and Punnett squares.
Different formatting styles can be used to represent incomplete dominance and codominance in genetics problems.
The video uses relatable examples like guinea pigs, chickens, and llamas to illustrate key concepts.
The Amoeba Sisters emphasize the importance of staying curious and thinking critically when learning about genetics.
Transcripts
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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 donât 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? Thatâs a Mendelian trait. But
what weâre going to cover is when these traits are non-Mendelian and donât follow
those basic rules. Letâs 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 theyâre flowers. SoâŠI donât knowâŠthatâs not exactly what we envisioned. In snapdragon
genetics, there can be 3 phenotypes. Red. White. Or something in between---PINK! Itâs
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 hereâŠyou can get offspring that are red, white, or pink.Incomplete dominance is
different from codominance. Codominance---like a coworker---that pre-fix âcoâ 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, whatâs more awesome than a speckled chicken? Well except
for those silkie bantam chickensâŠ.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 donât. Some people also prefer to do exponents
with codominance----some people donât. 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 âin
betweenâ 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 isnât just one
height gene. Thereâs LOTS of genes that determine your height. What I mean by that
is that you donât just have a pair of alleles, like AA, Aa, or aa that code for your height.
Itâs 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 âmanyâ
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
doesnât 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 letâs pick an animal like aâŠ. OkâŠaâŠ.llama.
Letâs 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 letâs 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. Itâs
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 itâs
non-Mendelian unless you are provided information or clues in the problem that it might be.
Well thatâs it for the amoeba sisters and we remind you to stay curious!
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