Variations of Dominance - Online Tutor, Practice Problems & Exam Prep

Dominance in genetics can be categorized into complete dominance, incomplete dominance, and codominance. In complete dominance, the presence of a dominant allele results in the same phenotype, regardless of whether it is homozygous or heterozygous. In incomplete dominance, heterozygous individuals exhibit a distinct phenotype, such as pink flowers from red and white parents. Codominance occurs when both alleles are expressed equally, exemplified by AB blood type. Understanding these concepts is crucial for grasping genetic inheritance patterns and their implications in traits and phenotypes.

Variations on Dominance

So there are many different types of dominance, and the one that we've been referring to so far is called complete dominance. That is, any time a dominant allele is present, it's going to be expressed. So, even it doesn't matter if it's homozygous or if it's heterozygous, you're going to have the same phenotype. So, homozygous is not phenotypically different than heterozygous. This is what the Punnett square normally looks like. You have a red flower. Here you go. It's homozygous dominant. You have a white flower. It's homozygous recessive. If you mate them together, you get 4 offspring. They're all red. They look exactly like this parent, because they have one dominant allele, and then, it looks the same as the two dominant alleles. This is what we're familiar with. This is what we've been talking about. This is called complete dominance. But genetics is very rarely like this; it's very rarely complete. So let's talk about some types of dominance that aren't complete.

The first one is incomplete dominance, and this is when the number of dominant alleles affects the phenotype. So, homozygous dominant is going to appear different than heterozygous dominant. If you do the same mating, so you have a red flower that's homozygous dominant and a white flower homozygous recessive. When you mate them together, you get 4 offspring, each with one dominant allele and one recessive, but that doesn't look red. Instead, it looks pink, and that's because you're just getting one dose of the dominant allele. So, what's the phenotype of the offspring from a red flower mating and a white flower mating under incomplete dominance? It's going to be pink.

Then, the third type is codominance, and this is a really interesting example because there are usually two dominant alleles and both are expressed equally. A good example of this is human blood types. We say it's the ABO blood type because some people have A, some people have B, and some people have O. So, what this looks like is if you're actually looking at the genotype. To get blood type A, you can have IA and you can be homozygous dominant for this gene or you can be heterozygous dominant for A, and that will give you A. For B, it's the same, except you replace the superscript A with superscript B, and it can be homozygous or heterozygous, and you get B. Now the codominance comes in here. Codominance happens because what happens if you have one A dominant allele and one B? Well, you get both proteins and that makes you type blood type AB. And then if you're homozygous recessive, meaning that you have neither dominant allele, you're classified as blood type O. Right here is an example of codominance because there are two dominant alleles, IA and IB, and both are expressed giving you the AB blood type. So, if you were to mate a heterozygous blood type A with a heterozygous blood type B, you get one dominant allele and one recessive allele for each one. And so when you get 4 offspring, you get this offspring, this offspring, this offspring, and this offspring. And so that produces four different types of blood. You can have type AB, which is the codominance because there are two dominant alleles. You can have type B because you have your B allele. You can have type A with your A allele, or you can have type O because there's no dominant allele present. This is an example of codominance. It can be challenging to understand, but it's just that there are two dominant alleles and both are expressed equally. So, with that, let's now move on.

Which of the following is NOT a type of dominance?

• A. Complete dominance
• B. Incomplete dominance
• C. Codominance
• D. Leaky dominance

Which of the following parents could produce offspring with an AB blood type?

• A. A x A
• B. O x A
• C. A x AB
• D. AB x O

Blood types are an example of what type of dominance?

• A. Complete dominance
• B. Incomplete dominance
• C. Codominance
• D. Leaky dominance

Here’s what students ask on this topic:

What is the difference between complete dominance and incomplete dominance?

Complete dominance occurs when a dominant allele completely masks the effect of a recessive allele in a heterozygous individual. This means that the phenotype of a heterozygous individual is identical to that of a homozygous dominant individual. In contrast, incomplete dominance results in a phenotype that is intermediate between the two homozygous phenotypes. For example, if a red flower (homozygous dominant) is crossed with a white flower (homozygous recessive), the offspring will be pink (heterozygous) under incomplete dominance. This intermediate phenotype occurs because the single dominant allele does not completely mask the effect of the recessive allele.

How does codominance differ from incomplete dominance?

Codominance and incomplete dominance are both forms of non-Mendelian inheritance, but they differ in how the alleles are expressed. In codominance, both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows both traits equally. A classic example is the AB blood type in humans, where both A and B alleles are expressed, resulting in the AB blood type. In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes. For instance, crossing a red flower with a white flower results in pink offspring. Thus, codominance shows both traits simultaneously, while incomplete dominance shows an intermediate trait.

Can you explain the concept of codominance using human blood types?

Codominance in human blood types is exemplified by the ABO blood group system. The A and B alleles are both dominant and are represented as I^A and I^B. When an individual inherits both I^A and I^B alleles, both alleles are expressed equally, resulting in the AB blood type. This means that both A and B antigens are present on the surface of red blood cells. If an individual inherits two I^O alleles (recessive), they will have type O blood, which lacks A and B antigens. Thus, codominance allows both dominant alleles to be expressed simultaneously in the phenotype.

What is an example of incomplete dominance in plants?

An example of incomplete dominance in plants is seen in the snapdragon flower. When a red-flowered snapdragon (homozygous dominant, RR) is crossed with a white-flowered snapdragon (homozygous recessive, rr), the resulting offspring (heterozygous, Rr) have pink flowers. This pink color is an intermediate phenotype, demonstrating incomplete dominance. The single dominant allele (R) does not completely mask the effect of the recessive allele (r), resulting in a blend of red and white, which appears as pink in the heterozygous plants.

How does the phenotype of a heterozygous individual differ in complete dominance versus incomplete dominance?

In complete dominance, the phenotype of a heterozygous individual is identical to that of a homozygous dominant individual. This means that the dominant allele completely masks the effect of the recessive allele. For example, in pea plants, a heterozygous plant with one allele for tallness (T) and one for shortness (t) will be tall, just like a homozygous tall plant (TT). In incomplete dominance, the phenotype of a heterozygous individual is intermediate between the two homozygous phenotypes. For instance, in snapdragons, a heterozygous plant with one allele for red flowers (R) and one for white flowers (r) will have pink flowers, an intermediate phenotype between red and white.