2.2: Multiple alleles, incomplete dominance, and codominance

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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 ($C^WC^W$) and a homozygous red-flowered plant ($C^RC^R$) will produce offspring with pink flowers ($C^RC^W$). 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 $C^WC^W$ (white) and $C^RC^R$ (red) snapdragon plants. The F1 plants are pink and of genotype $C^RC^W$.

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 $1\, C^RC^R : 2\, C^RC^W : 1\, C^WC^W$ and a phenotype ratio of $1:2:1$ red:pink:white. Alleles are still inherited according to Mendel's basic rules, even when they show incomplete dominance.

Self-fertilization of pink $C^RC^W$ 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 $L^M$ allele specifies production of an M marker displayed on the surface of red blood cells, while an $L^N$ allele specifies production of a slightly different N marker.

Homozygotes ($L^ML^M$ and $L^NL^N$) have only M or N markers, respectively, on the surface of their red blood cells. However, heterozygotes ($L^ML^N$) 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 $L^ML^N$ genotypes had children, we would expect to see M, MN, and N blood types and $L^ML^M$, $L^ML^N$, and $L^NL^N$ genotypes in their children in a $1:2:1$ 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: $C$, $c^{ch}$, $c^h$, and $c$:

A $CC$ rabbit has black or brown fur.
A $c^{ch}c^{ch}$ rabbit has chinchilla coloration (grayish fur).
A $c^hc^h$ rabbit has Himalayan (color-point) patterning, with a white body and dark ears, face, feet, and tail.
A $cc$ rabbit is albino, with a pure white coat.

Allelic series of the color gene C in rabbits.* A $CC$ rabbit has black fur.* A $c^{ch}$$c^{ch}$ rabbit has chinchilla coloration (grayish fur).* A $c^hc^h$ rabbit has Himalayan (color-point) patterning, with a white body and dark extremities.* A $cc$ 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 $c^{ch}$ allele is incompletely dominant to the Himalayan $c^h$ and albino $c$ alleles; and the Himalayan $c^h$ 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).

[How do these alleles change the rabbit's color?]

The $C$ gene in rabbits encodes an enzyme that’s needed to make a type of pigment called melanin in hairs^1,2.

The $C$ allele of this gene encodes a fully functional enzyme that makes lots of pigment and results in black fur.
The $c^{ch}$ allele encodes an enzyme that is less effective at making pigment, resulting in lighter, more grayish fur.
The $c^h$ allele encodes a defective enzyme, where the defect makes the enzyme very sensitive to temperature: it works fine at low temperatures, but doesn’t work at all at higher temperatures. The rabbit's extremities (paws, ears, etc.) are cooler, so the enzyme functions there and makes pigment. The rabbit’s main body is warmer, so the enzyme does not function and no pigment is made.
The $c$ allele encodes a completely nonfunctional enzyme, leading to an albino rabbit (one that does not produce any pigment in its hairs).

Note to rabbit fanciers: Rabbit coat color is determined by a number of genes, not just by the $C$ gene. The allelic series described here for $C$ assumes a certain genetic background for the other genes, one in which a $CC$ genotype results in a black rabbit.

Contributors and Attributions

Khan Academy (CC BY-NC-SA 3.0; All Khan Academy content is available for free at www.khanacademy.org)

[Attribution and references]

Attribution:

This article is a modified derivative of “Characteristics and traits”,” by OpenStax College, Biology (CC BY 3.0). Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85.

The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited:

Coat color in the Himalayan rabbit. (n.d.). In Cengage learning. Retrieved from http://www.cengage.com/biology/discipline_content/animations/himalayan_rabbit_m.html.
Hinkle, Amy. (n.d.). Rabbit coat color biochemistry. In Amy's rabbit ranch. Retrieved from http://www.amysrabbitranch.com/Color%26Genetics/Biochemistry-CoatColor.pdf.

Additional references:

Blumberg, R. B. (1997). Mendel's paper in English. In MendelWeb. Retrieved from http://www.mendelweb.org/Mendel.plain.html.

Coat color photo matrix. (2013, September 29).In Green barn farm. Retrieved from http://www.gbfarm.org/rabbit/holland-colors-matrix.shtml.

Color genetics: The C series. (n.d.) In The nature trail. Retrieved from http://www.thenaturetrail.com/rabbit-genetics/color-c-series-chinchilla-sable-himalayan-rew/.

Cystic fibrosis transmembrane conductance regulator. (2015, November 12). Retrieved November 22, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Cystic_fibrosis_transmembrane_conductance_regulator.

Dominance (genetics). (2015, November 3). Retrieved November 21, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Dominance_%28genetics%29.

Fox, Richard R. (1974). Taxonomy and genetics. In S. H. Weisbroth, R. E. Flatt, and A. L. Kraus (Eds.), The biology of the laboratory rabbit (6-22). New York, NY: Academic Press.

Orfano, Finn. (2010, December 15). Codominance in genetics: An overview. In Bright hub. Retrieved from http://www.brighthub.com/science/genetics/articles/99400.aspx.

Purves, W. K., Sadava, D., Orians, G. H., and Heller, H. C. (2003). Alleles and their interactions. In Life: The science of biology (7th ed., pp. 197-199). Sunderland, MA: Sinauer Associates.

Rabbit color genotypes chart. (n.d.). In The nature trail. http://www.thenaturetrail.com/rabbit-genetics/rabbit-color-genotypes-chart/.

Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., and Singer, S. R. (2014). Patterns of inheritance. In Biology (10th ed., AP ed., pp. 221-238). New York, NY: McGraw-Hill.

Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B. (2011). Mendel and the gene idea. In Campbell Biology (10th ed., pp. 267-291). San Francisco, CA: Pearson.

Sweat test. (2015, Septebmer 24). Retrieved November 22, 2015 from Wikipedia: https://en.wikipedia.org/wiki/Sweat_test.