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Multiple alleles, incomplete dominance, and codominance

In the real world, genes often come in many versions (alleles). Alleles aren't always fully dominant or recessive to one another, but may instead display codominance or incomplete dominance.

Introduction

Gregor Mendel knew how to keep things simple. In Mendel's work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear-cut dominance relationship (with the dominant allele fully overriding the recessive allele to determine the plant's appearance).
Today, we know that not all alleles behave quite as straightforwardly as in Mendel’s experiments. For example, in real life:
  • Allele pairs may have a variety of dominance relationships (that is, one allele of the pair may not completely “hide” the other in the heterozygote).
  • There are often many different alleles of a gene in a population.
In these cases, an organism's genotype, or set of alleles, still determines its phenotype, or observable features. However, a variety of alleles may interact with one another in different ways to specify phenotype.
As a side note, we're probably lucky that Mendel's pea genes didn't show these complexities. If they had, it’s possible that Mendel would not have understood his results, and wouldn't have figured out the core principles of inheritance—which are key in helping us understand the special cases!

Incomplete dominance

Mendel’s results were groundbreaking partly because they contradicted the (then-popular) idea that parents' traits were permanently blended in their offspring. In some cases, however, the phenotype of a heterozygous organism can actually be a blend between the phenotypes of its homozygous parents.
For example, in the snapdragon, Antirrhinum majus, a cross between a homozygous white-flowered plant (C, start superscript, W, end superscript, C, start superscript, W, end superscript) and a homozygous red-flowered plant (C, start superscript, R, end superscript, C, start superscript, R, end superscript) will produce offspring with pink flowers (C, start superscript, R, end superscript, C, start superscript, W, end superscript). This type of relationship between alleles, with a heterozygote phenotype intermediate between the two homozygote phenotypes, is called incomplete dominance.
Diagram of a cross between C, start superscript, W, end superscript, C, start superscript, W, end superscript (white) and C, start superscript, R, end superscript, C, start superscript, R, end superscript (red) snapdragon plants. The F1 plants are pink and of genotype C, start superscript, R, end superscript, C, start superscript, W, end superscript.
We can still use Mendel's model to predict the results of crosses for alleles that show incomplete dominance. For example, self-fertilization of a pink plant would produce a genotype ratio of 1 C, start superscript, R, end superscript, C, start superscript, R, end superscript colon 2 C, start superscript, R, end superscript, C, start superscript, W, end superscript colon 1 C, start superscript, W, end superscript, C, start superscript, W, end superscript and a phenotype ratio of 1, colon, 2, colon, 1 red:pink:white. Alleles are still inherited according to Mendel's basic rules, even when they show incomplete dominance.
Self-fertilization of pink C, start superscript, R, end superscript, C, start superscript, W, end superscript plants produce red, pink, and white offspring in a ratio of 1:2:1.

Codominance

Closely related to incomplete dominance is codominance, in which both alleles are simultaneously expressed in the heterozygote.
We can see an example of codominance in the MN blood groups of humans (less famous than the ABO blood groups, but still important!). A person's MN blood type is determined by his or her alleles of a certain gene. An L, start superscript, M, end superscript allele specifies production of an M marker displayed on the surface of red blood cells, while an L, start superscript, N, end superscript allele specifies production of a slighly different N marker.
Homozygotes (L, start superscript, M, end superscript, L, start superscript, M, end superscript and L, start superscript, N, end superscript, L, start superscript, N, end superscript) have only M or an N markers, respectively, on the surface of their red blood cells. However, heterozygotes (L, start superscript, M, end superscript, L, start superscript, N, end superscript) have both types of markers in equal numbers on the cell surface.
As for incomplete dominance, we can still use Mendel's rules to predict inheritance of codominant alleles. For example, if two people with L, start superscript, M, end superscript, L, start superscript, N, end superscript genotypes had children, we would expect to see M, MN, and N blood types and L, start superscript, M, end superscript, L, start superscript, M, end superscript, L, start superscript, M, end superscript, L, start superscript, N, end superscript, and L, start superscript, N, end superscript, L, start superscript, N, end superscript genotypes in their children in a 1, colon, 2, colon, 1 ratio (if they had enough children for us to determine ratios accurately!)

Multiple alleles

Mendel's work suggested that just two alleles existed for each gene. Today, we know that's not always, or even usually, the case! Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist in a population level, and different individuals in the population may have different pairs of these alleles.
As an example, let’s consider a gene that specifies coat color in rabbits, called the C gene. The C gene comes in four common alleles: C, c, start superscript, c, h, end superscript, c, start superscript, h, end superscript, and c:
  • A C, C rabbit has black or brown fur
  • A c, start superscript, c, h, end superscriptc, start superscript, c, h, end superscript rabbit has chinchilla coloration (grayish fur).
  • A c, start superscript, h, end superscript, c, start superscript, h, end superscript rabbit has Himalayan (color-point) patterning, with a white body and dark ears, face, feet, and tail
  • A c, c rabbit is albino, with a pure white coat.
Allelic series of the color gene C in rabbits.
  • A C, C rabbit has black fur.
  • A c, start superscript, c, h, end superscriptc, start superscript, c, h, end superscript rabbit has chinchilla coloration (grayish fur).
  • A c, start superscript, h, end superscript, c, start superscript, h, end superscript rabbit has Himalayan (color-point) patterning, with a white body and dark extremities.
  • A c, c rabbit is albino, with a pure white coat.
Image credit: "Characteristics and traits: Figure 5," by OpenStax College, Biology (CC BY 3.0).
Multiple alleles makes for many possible dominance relationships. In this case, the black C allele is completely dominant to all the others; the chinchilla c, start superscript, c, h, end superscript allele is incompletely dominant to the Himalayan c, start superscript, h, end superscript and albino c alleles; and the Himalayan c, start superscript, h, end superscript allele is completely dominant to the albino c allele.
Rabbit breeders figured out these relationships by crossing different rabbits of different genotypes and observing the phenotypes of the heterozygous kits (baby bunnies).

Want to join the conversation?

  • duskpin ultimate style avatar for user Kashish
    What is multiple allele is one sentence that is easy to understand?
    (13 votes)
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  • leaf green style avatar for user Zaeen Iqbal
    isnt codominance the same as incomplete dominance in that case if both alleles can be expressed?
    (8 votes)
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    • leaf blue style avatar for user Esther Dickey
      Not really.
      In codominance, both alleles are completely expressed. If you crossed a red flower with a white one and the alleles were codominant, you might get flowers that are red and white in patches. If the alleles were incompletely dominant, the flowers would be pink because the traits blend.
      (40 votes)
  • aqualine ultimate style avatar for user Noela Seung
    I'm confused on why there are some "exponents" on the alleles. For ex, c^ch c^ch
    what does that exactly mean?
    (8 votes)
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    • orange juice squid orange style avatar for user Hector Alfaro
      It's to distinguish the alleles in an easier way, once you have to deal with different traits it will be useful since the base could specify the trait while the exponent specify the allele for that trait.

      For example, if you were talking about the traits color and height with the alleles red and blue, and tall and short, respectively, you could express genotype as:
      C -> Color
      H -> Height
      R -> Red
      B -> Blue
      T -> Tall
      S -> Short

      * C^R C^B H^T H^S (This would be heterozygous in both traits)
      * C^R C^R H^T H^T (This would be homozygous for red color and tall height)

      You can also use lower case to denote that an allele is recessive (although this is arbitrary).
      (10 votes)
  • blobby green style avatar for user Priyanka
    Is multiple allelism the same as polymorphism?
    (5 votes)
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    • female robot grace style avatar for user tyersome
      Not exactly — what is true is that genetic polymorphisms are responsible for the existence of (most) alleles.

      Polymorphism (literally "many forms") means different things in different contexts, but in a genetic context it really just means that there are differences in the sequences.

      For example SNPs (pronounced "snips" — stands for single nucleotide polymorphisms) are a very common type of sequence polymorphism.

      Having multiple alleles is (usually§) a consequence of multiple different sequence variants for a gene (i.e. genetic polymorphisms) being present in a population. However, any two alleles are likely to have multiple polymorphisms (i.e. sequence differences) that separate them. Furthermore, two alleles that appear to the same at a phenotypic level may have different sequences. A good example of this is the ABO blood groups — traditionally we have identified three alleles Iᴬ, Iᴮ, and i, but it turns out that are multiple sequences that correspond to each of those alleles!
      For more on this see:
      https://en.wikipedia.org/wiki/ABO_blood_group_system#Subgroups

      §Note: Sometimes alleles result from epigenetic changes (heritable changes that don't alter the sequence) — these can be referred to as epialleles and appear to be less common than alleles based on sequence polymorphisms.
      (12 votes)
  • starky seedling style avatar for user Gayathri sudarsan
    in case of pea plant tall is the dominant gene and dwarf is the recessive gene. does that hold good for other organisms as well? is dwarf always recessive gene and cant it be dominant in the presence of tall gene?
    (6 votes)
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  • blobby green style avatar for user Al-Hamzah  Omar
    I think there is a mistake in multiple alleles inheritance (in the rabbit example).
    The allele for the chinchilla coat is completely (not incompletely as mentioned in the text above) dominant to the allele for Himalayan coat and the allele for albino coat.
    (4 votes)
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  • blobby green style avatar for user Prachi Sharma
    Is the rabbit example also an example of epistasis?
    (1 vote)
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  • piceratops ultimate style avatar for user Nuo Chen
    How does co-dominant inheritance patterns differ from dominant/recessive inheritance patterns?
    (2 votes)
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  • blobby green style avatar for user 371008801
    My grandparents had a mental illness and my dad has it to, he is bipolar, he got it genetically, is it possible that i can get the bipolar disorder too?
    (1 vote)
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    • female robot grace style avatar for user tyersome
      If possible, I recommend that you consult with an expert in medical genetics.

      My understanding is that bipolar disorder is a multigenic trait (many genes involved) with a strong environmental component.
      Consequently, it is difficult (and maybe impossible) to have a good idea how much more likely you are to develop bipolar than most people.
      This also means that if you take good care of yourself (don't smoke, eat a healthy diet, get lots of exercise, avoid activities likely to result in concussions, get sufficient sleep, etc.) you can reduce your odds of developing bipolar or any other serious health condition!

      To learn more, you could start with this:
      https://ghr.nlm.nih.gov/condition/bipolar-disorder#
      (3 votes)
  • aqualine ultimate style avatar for user Raja, Kaushik
    So if I was asked the pattern of inheritence and the genotype was RW for red dominant of white which would it be: incomplete or co-dominant
    (2 votes)
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