Co-dominance and Incomplete Dominance | Biomolecules | MCAT | Khan Academy

Summary

This educational video script delves into the genetic concepts of dominance in traits, focusing on blood type as an example. It explains the terms homozygous and heterozygous, and clarifies the difference between dominant and recessive alleles, using 'A' being dominant over 'O'. The script then introduces three patterns of dominance: complete dominance, where one allele fully expresses the phenotype (e.g., blood type A from AO genotype); co-dominance, where both alleles influence the phenotype equally, as seen in a flower with both red and blue petals; and incomplete dominance, where a blend of alleles results in a new phenotype, such as a purple flower from red and blue petal alleles. The summary aims to clarify these genetic inheritance patterns for better understanding.

Takeaways

• 📚 Homozygous individuals have two identical alleles for a trait, while heterozygous individuals have two different alleles.
• 🔍 Dominant alleles mask the expression of recessive alleles in a heterozygous genotype.
• 🌟 The 'A' allele is an example of a dominant allele, overpowering the 'O' allele in determining blood type.
• 🌺 Complete dominance is when one allele completely masks the effect of the other allele in the phenotype.
• 🌷 Co-dominance occurs when both alleles in a heterozygous genotype are expressed in the phenotype simultaneously.
• 💐 Incomplete dominance results in a phenotype that is a blend of the two alleles' traits, rather than one being dominant.
• 🌹 The flower example illustrates three patterns of dominance: complete, co-, and incomplete dominance.
• 🔴 Under complete dominance, a heterozygous genotype (e.g., red R/blue R) would result in a red flower phenotype.
• 🌈 Co-dominance in the flower example would result in a flower with both red and blue petals.
• 🟪 In incomplete dominance, the flower's phenotype would be a mix, such as purple, from the combination of red and blue alleles.
• 🧬 Understanding these patterns of dominance is crucial for grasping how genetic traits are expressed in organisms.

Q & A

What is the definition of homozygous?

Homozygous refers to having two identical alleles for a particular gene.

What is the definition of heterozygous?

Heterozygous refers to having two different alleles for a particular gene.

What is an example of complete dominance mentioned in the script?

An example of complete dominance is the blood type AO, where the A allele is completely dominant over the O allele, resulting in a blood type A phenotype.

How does co-dominance differ from complete dominance?

In co-dominance, both alleles in the heterozygous genotype are fully expressed in the phenotype, whereas in complete dominance, only the dominant allele is expressed.

What example is used to explain incomplete dominance?

The example of a flower with red and blue alleles resulting in a purple flower is used to explain incomplete dominance.

How is the heterozygous phenotype expressed in co-dominance?

In co-dominance, the heterozygous phenotype shows a flower with some red petals and some blue petals.

What is the key characteristic of incomplete dominance?

The key characteristic of incomplete dominance is that the heterozygous phenotype shows a mixture of the two alleles.

In the flower example, what phenotype is expected from a genotype with two red alleles?

A genotype with two red alleles is expected to produce a flower with red petals in all dominance patterns.

What phenotype is expected from a genotype with two blue alleles in the flower example?

A genotype with two blue alleles is expected to produce a flower with blue petals in all dominance patterns.

How does the phenotype change in the heterozygous example under different dominance patterns?

In complete dominance, the heterozygous phenotype is a red flower. In co-dominance, the flower shows both red and blue petals. In incomplete dominance, the flower shows a mixture, resulting in a purple flower.

What do we learn from the examples of dominance patterns provided?

We learn that the expression of the heterozygous genotype can vary significantly depending on the dominance pattern: complete dominance shows only the dominant allele, co-dominance shows traits of both alleles, and incomplete dominance shows a mixture of the two alleles.

Outlines

00:00

🌟 Introduction to Dominance Patterns in Genetics

This paragraph introduces the concepts of Co-Dominance and Incomplete Dominance, starting with a review of blood type genetics as an example of Complete Dominance. It explains the terms homozygous and heterozygous, and the roles of dominant and recessive alleles, specifically using the A and O alleles to illustrate blood type A. The paragraph sets the stage for a deeper dive into different dominance patterns using the example of flower color genetics, where red and blue petal colors are determined by different alleles.

🌺 Exploring Complete, Co-Dominant, and Incomplete Dominance

The paragraph delves into three distinct patterns of genetic dominance: complete dominance, co-dominance, and incomplete dominance, using a flower as a metaphor. It describes how each pattern affects the phenotype based on genotype. In complete dominance, the dominant allele (e.g., red petals) overshadows the recessive one (e.g., blue petals). Co-dominance results in a phenotype that displays both traits (e.g., a flower with both red and blue petals). Incomplete dominance leads to a blended phenotype, such as a purple flower from a red and blue petal allele combination, where neither allele is fully dominant.

Keywords

💡Co-Dominance

Co-dominance refers to a genetic phenomenon where two different alleles for a trait are both expressed in the phenotype of an organism. In the context of the video, it is exemplified by a flower with both red and blue petals, indicating that neither the red 'R' allele nor the blue 'R' allele is completely dominant over the other. This concept is crucial for understanding the diversity of phenotypes that can arise from a single genotype.

💡Incomplete Dominance

Incomplete dominance is a pattern of inheritance where the phenotype of the heterozygote is a unique blend of the two alleles' traits, rather than displaying the trait of one allele exclusively. The video uses the example of a flower with red and blue petals that results in a purple phenotype, illustrating how the red 'R' and blue 'R' alleles combine to create a new characteristic that is not simply a dominance of one over the other.

💡Homozygous

A homozygous organism has two identical alleles for a particular gene. The video explains that a person with blood type A resulting from the genotype 'AA' is homozygous, meaning they have two copies of the dominant 'A' allele. This term is key to understanding genetic inheritance and how certain traits are consistently expressed in an organism.

💡Heterozygous

Heterozygous describes an organism that has two different alleles for a trait. In the script, a person with genotype 'AO' is heterozygous, possessing one dominant 'A' allele and one recessive 'O' allele. The concept is essential for discussing how traits can be expressed differently depending on the combination of alleles present.

💡Alleles

Alleles are different forms of a gene that determine the characteristics of an organism. The video script discusses how the 'A' and 'O' alleles determine blood type, and 'red R' and 'blue R' alleles determine flower color. Understanding alleles is fundamental to grasping the genetic basis of phenotypic variation.

💡Dominant Alleles

Dominant alleles are versions of a gene that will be expressed in the phenotype even when paired with a recessive allele. The video script uses the 'A' allele as an example of a dominant allele that masks the expression of the 'O' allele in determining blood type. This concept is central to understanding how certain traits are more likely to be visible in an organism.

💡Recessive Alleles

Recessive alleles are versions of a gene that are only expressed in the phenotype when there are two copies of the allele present. In the script, the 'O' allele is recessive to the 'A' allele in determining blood type, meaning it will only result in type O blood if both alleles are 'O'. This term helps explain why some traits skip generations or appear less frequently.

💡Phenotype

Phenotype refers to the observable characteristics of an organism, which result from the interaction of its genotype with the environment. The video discusses various phenotypes such as blood type and flower color, which are determined by the expression of different alleles. Understanding phenotypes is crucial for studying how genetic information is manifested in physical traits.

💡Genotype

Genotype is the genetic makeup of an organism, describing the specific set of genes inherited from its parents. The video script explains genotypes such as 'AA', 'AO', and 'OO' for blood type, and 'RR', 'Rr', and 'rr' for flower color. The concept of genotype is key to understanding the potential for certain traits to be expressed in an organism.

💡Blood Type

Blood type is a phenotypic trait determined by alleles at a specific gene locus. The video uses blood type as an example to explain the concepts of dominance and recessiveness, with 'A' being dominant over 'O'. Blood type serves as a practical example of how genetic inheritance can affect a visible characteristic.

💡Flower Color

Flower color is used in the video as an example of a phenotypic trait influenced by genetic inheritance. The red and blue flower colors are determined by the 'red R' and 'blue R' alleles, respectively. This serves to illustrate the concepts of co-dominance and incomplete dominance, showing how different genetic combinations can result in unique phenotypes.

Highlights

Introduction to the concepts of homozygous and heterozygous in relation to blood type alleles.

Explanation of dominant and recessive alleles with the example of blood type A being dominant over type O.

Illustration of complete dominance where the A allele is fully expressed over the O allele in blood type A.

Introduction of three patterns of dominance: complete dominance, co-dominance, and incomplete dominance.

Use of a flower example to differentiate between the three dominance patterns.

Description of the genotype with two red Rs resulting in red flower petals in all dominance patterns.

Similarity in the phenotype for a genotype with two blue Rs, expecting blue flower petals.

Difference in heterozygous phenotypes under the three dominance patterns.

Complete dominance results in a red flower for the heterozygous genotype under the assumption of red R being dominant.

Co-dominance is characterized by a flower with both red and blue petals, showing traits from both alleles.

Incomplete dominance leads to a purple flower when red and blue alleles mix, neither being completely dominant.

Summary of the three dominance patterns observed with a heterozygous genotype of red R and blue R.

In complete dominance, only the dominant allele is expressed in the phenotype.

Co-dominance shows both alleles in the phenotype, as seen in the flower with mixed red and blue petals.

Incomplete dominance results in a phenotype that is a mixture of the alleles' traits, exemplified by the purple flower.

Transcripts

- [Voiceover] So today we're gonna talk about

Co-Dominance and Incomplete Dominance,

but first let's review the example of a blood type

and how someone with the same two alleles

coding for the same trait

would be called homozygous

and someone with different alleles

would be called heterozygous.

Also remember, the concept of dominant

and recessive alleles

and how the A allele is dominant

over the O allele in this example.

This means that the same phenotype,

blood type A, can result from these

two different genotypes.

Now, the example that I just gave you

was an example of Complete Dominance.

So if a person had a genotype AO,

since our phenotype is just blood type A,

it means that the A allele

is completely dominant over the O allele

and only the A allele from the genotype

is expressed in the phenotype.

But there are actually three different patterns

of dominance that I want you to be familiar with

and to explain this I'm going to use

a different example.

Let's say we have this flower

and the red petal phenotype is coded for

by the red R allele

and the blue flower phenotype is coded for

by the blue R allele.

So I'm going to introduce

three different patterns of dominance

and they are complete dominance,

which you've already heard of,

co-dominance, and also incomplete dominance.

I'm going to explain what these two

new patterns are through this flower example.

Let's start by looking at three different genotypes

and the phenotypes that you would see

for each of them under each

different dominance pattern.

We'll start with the genotype, two red Rs,

which we could expect that in all cases

the flower petals will be red

since we only have red Rs in the genotype.

Similarly, if our genotype had two blue Rs

then we could expect that in all cases

the flower petals will be blue

since we only have blue Rs in the genotype.

Now these three different dominance patterns

change when we look at the heterozygous example.

That's what makes these three patterns different.

Now we're already familiar with the example

of complete dominance, so if we said

that the red R is dominant over the blue R

then this would make the heterozygous phenotype

a red flower for complete dominance.

Now what co-dominance is,

is when the heterozygous phenotype

shows a flower with some red petals

and some blue petals.

So it's when the two alleles are dominant together

they are co-dominant and traits

of both alleles show up in the phenotype.

Now what incomplete dominance is,

is when the heterozygous phenotype

shows a mixture of the two alleles.

So in this case the red and blue flower petals

may combine to form a purple flower.

Neither allele is completely dominant over the other

and instead the two, being incompletely

dominant, mix together.

So what did we learn?

Well, if we assume the heterozygous genotype,

red R, blue R, then there are three different

dominance patterns that we might see

for a specific trait.

In complete dominance,

only one allele in the genotype,

the dominant allele, is seen in the phenotype.

And this was the example with the red flower.

In co-dominance, both alleles in the genotype

are seen in the phenotype.

This was the example with the flower

with both red and blue petals.

Finally, in incomplete dominance,

a mixture of the alleles in the genotype

is seen in the phenotype

and this was the example with the purple flower.