Mendelian Genetics in Avatar:  The Last Airbender and Avatar: The Legend of Korra

By Humphrey Yu, BS

Author’s note:  This article is meant for entertainment and educational purposes.  All characters, images, storylines, and merchandizing of the Avatar series belong to Nickelodeon television.  This article is written with the express purpose of demonstration one hypothetical inheritance model for bending abilities based on Gregor Mendel’s studies of genetics.  Punnett squares and pedigrees have been added at the end of the article to clarify the examples made within the paragraphs.

Dominant vs. Recessive

            Debate abounds on internet forums about whether bending is a dominant or recessive gene trait.  Both explanations have been made examples and argued by fans of the series on tv.com, avatar.wikia.com, avatarspirit.net, and other sites.  But as with the complexity of eye color phenotypic expression in people, the most logical answer is the interplay of multiple alleles to activate the bending gene.  Or as the opening title to Avatar:  The Last Airbender series states, there must be “balance to the world”.

On the one hand, Tenzin, introduced as Avatar Aang’s (Aa) and Katara’s (Ww) son¹, would be the truly last air bender (Aw), if air bending were a recessive trait.  His offspring should all be nonbenders like his wife, Pema (rr)¹.  Instead three of his children, Jinora, Ikki, and Meelo, all carry active air bender genes (Ar)¹.  This characterization of an air bender and nonbender cross can only fuel the argument that bending is a dominant trait.

But the argument for a dominant trait (AA, Aa) for bending does not hold true for all cases.  If bending is a dominant trait, then all offspring of benders should be benders.  Again this is disproven by another offspring of Avatar Aang and Katara, Bumi (aw), who lacks any ability to bend¹.  Furthermore, if bending were a dominant gene, two nonbenders could not mate to create a bender offspring.

Bending must contain a recessive component (ee).  Otherwise, characters Toph and Katara could not become benders.  Katara is identified as the last water bender of the Southern Water tribe².  She is stated to be the last water bender in a long time in a tribe that considers bending to be tantamount to freakish magic due to the rarity of the trait².  The former last water bender, Hama, was a blood bender during her grandmother’s generation³.  If water bending had become extinct for an entire generation, a recessive component (ee) is needed to allow for reemergence of the trait in Katara (Wwee).

Toph’s own experience is similar to Katara’s.  Toph is introduced as the only daughter of earth kingdom nobility, the Beifong family⁴.  Her introduction also characterizes Toph as the earth bending daughter of Poppy and Lao, two nonbenders.  For Toph (RRee, Rree) to be able to bend after a generational absence in earth bending, she is similar to Katara in her need for a recessive component to exist in Avatar genetics.

The author of this blog postulates that there must be a minimum of two active genes for bending to be heritable.  One gene must be a dominant gene for bending a specific type of element:  categorized for simplicity as earth (RR, Rr), fire (FF, Ff), Air (AA, Aa), and water (WW, Ww) rather than a uniform bending gene with superscript denoting bending element and separating dominant and recessive (B^A, B^a, B^R, B^r, B^F, B^f, B^W, B^w).  The other gene must be active only when recessive similar to phenotypic expression of albino animals⁵:  this gene possibly being an older gene stipulated by the Lion Turtle as the ability practiced “in the era before the Avatar, [when they] bent not the elements, but the energy within [themselves]”⁶ and characterized as the energy bending gene (ee).  An active gene confers the ability to allow element bending as demonstrated by Avatar Korra as she activated Linn Beifong’s earth bending⁷.  An inactive gene (EE, Ee) makes a bender into a nonbender as shown by Avatar Aang’s removal of Fire Lord Ozai’s fire bending (FFee into FFEe)⁶, and Avatar Aang’s removal of blood bender (WWee or Wwee into WWEe or WwEe) Yakone’s water bending⁸.  The benefit of such a gene which limits element bending to only those capable of bending the energy within, allows for a recessive, heritable trait that can reside within everyone while only allowing maximum utilization, the ability to “bend another’s energy” to those whose “own spirit must be unbendable” otherwise they will be “corrupted and destroyed”⁶.

The logical result is that there are at least two genes for bending.  One gene that determines element being bent and one that allows for bending to be performed in the first place.  Alternately, there are many combinations of genes that will inhibit a bender from being born (WwEe; wwEe; WwEE; wwEE; WWEE; WWEe), thus making bending a rare trait in the population unless heavily selected such as in the Fire Lord family⁹.  Furthermore, to prevent bending from being expressed in the general population, capture or destruction of all benders of a specific nation like the capture of all water benders in the Southern Water Tribe^2,3 or the destruction of the Air Temples^2,10 can prevent reemergence of a dominant trait as was practiced by the Fire Nation during the hundred year war.

 

Hybridization of Bending Genes

            Thus far, only the Avatar has been shown to master all four elements.  The Avatar possesses a spiritual component that allows him to reincarnate and master the elements “a thousand times in a thousand lifetimes”¹¹.  As the Avatar is the only character that reincarnates according to Sokka¹⁰ and the only character that can bend multiple elements, there must be a factor that inhibits other benders from acquiring more than one element.

            One potential explanation for the absence of bending hybrids is embryonic death.  The most common example of homozygous dominant allele underrepresentation in the general population is the study of normal gray or agouti mouse coloration¹².  Agouti mice (yy) are homozygous recessive mice that occasionally produce a dominant color yellow offspring (Yy)¹².  But the offspring of two yellow mice will have 2:1 ratio production of yellow to agouti mice when bred that only carry heterozygous (Yy) or homozygous recessive (yy) alleles¹².  No homozygous dominant alleles (YY) for the color gene have been found except in dissection of fetuses¹².  This is because of embryonic death.  Similar to fur color in mice, benders of different elements may either not conceive double benders or their double bender offspring may die in utero.

            Although bending hybridization is impossible, two benders of different nations can create offspring of their respective nations.  Avatar:  The Legend of Korra introduces the characters of fire bender, Mako, and earth bender, Bolin¹³, brothers that provide examples of how a fire nation citizen (FfEe, Ffee) and an earth nation citizen (Rree, RrEe) can cross (FfEe x Rree; FfEe x RrEe; Ffee x RrEe; Ffee x Rree) to make offspring that are fire benders (Free) and earth benders (fRee)¹⁴.   Both parents must have a heterozygous bending dominant gene (Ff, Rr) similar to the heterozygous yellow gene in mice (Yy).  The resulting crosses of the earth and fire nation citizens would produce a hybrid dominant (FR) that has never been conceived in the series, a fire bending dominant (Fr), an earth bending dominant (fR), and a nonbender (fr). 

A better example of Mendel’s genetics to create the ideal ratios would be the offspring of Avatar Aang and Katara described by Katara in Avatar:  The Legend of Korra¹.  Again heterozygous parents for respective bender parents for Air (Aang; Aaee) and Water (Katara; Wwee) result in a cross (Aaee x Wwee) that produces an air bender (Tenzin; Awee), a water bender (Kya; aWee), and a nonbender (awee)¹.  The hybrid bender offspring (AWee) is not shown because it does not exist in the avatar world, potentially because it died before birth or was impossible to conceive.

 

Twins

Much of the debate about the heritability of Avatar genetics revolves around the twins Poi and Ping from Makupu Village¹⁵.  Both brothers, identified as twins by fortune teller Aunt Wu, announce that one can bend earth and the other cannot¹⁵.  Both twins appear similar from a distance, but on closer inspection, Poi is shown with freckles as he bends an earthen channel to the river¹⁵. 

The primary issue of identical twins lies in the premise that two cloned eggs from the same spermatozoa and egg should have identical bending abilities.  If bending is heritable and based on Mendel’s studies, then both should have an active dominant earth bending gene (RR, Rr) and the active recessive energy expression gene (ee). 

This limitation is only possible if both brothers are identical.  Aunt Wu states only in her prediction that it will be a good year for twins¹⁵.  She does not specify that it is a good year for identical twins.  Twins also exist in a fraternal form as two individual fertilized eggs are dropped and deposited into the uterus at the same time.  Even fraternal twins can appear near identical without having the same genetic makeup as categorized by Gina Bellefante’s “…genetically fraternal, but identical-looking” description of the Olsen twin¹⁶.  This provides for a plausible reason for the absence of bending in one twin brother: the brothers do not carry the same genotype or phenotype for earth bending.

 

Specialized bending

            At this time, there is not enough data to compile an explanation for blood bending, lightning bending, or steel bending.  All three traits are heritable, and can be followed as base traits for water, fire, and earth in a punnett square, but whether they are refined abilities, new genotypes or spiritual remains to be seen.

General assumptions for genetics in Avatar: The Last Airbender television series.

1. There must be at least one active dominant [bending genotype (AA, Aa, WW, Ww, FF, Ff, RR, Rr)] and one active recessive allele [energy bending (ee)].  All bender offspring would have to be benders with a dominant allele which is shown incorrect with Katara and Aang’s children Bumi, Kya, and Tenzin.  Interbreeding between nonbenders and benders could not result in bender children if bending is a recessive trait as shown by air benders Jinora, Ikki, and Meelo .  An explanation for two combined genotypes producing one phenotype is Mendel’s study of tall purple and small white pea plants. 
2. There must be at least one active dominant [bending genotype (AA, Aa, WW, Ww, FF, Ff, RR, Rr)] and one active recessive allele [energy bending (ee)].  All bender offspring would have to be benders with a dominant allele which is shown incorrect with Katara and Aang’s children Bumi, Kya, and Tenzin.  Interbreeding between nonbenders and benders could not result in bender children if bending is a recessive trait as shown by air benders Jinora, Ikki, and Meelo .  An explanation for two combined genotypes producing one phenotype is Mendel’s study of tall purple and small white pea plants.
3. The only way for two bender offspring of different phenotypes [Mako (Free) and Bolin (fRee)] to exist is if both parents possess dominant bending genes.  Both parents can still be nonbenders if they do not have an active recessive gene (Ee).
4. All bending genes are equal and equivalent in terms of chromosomes.  Rather than place all genes on superscript Bending^A vs. Bending^W, they are simplified as individual genotypes for punnett square formation.
5.  Identical twins would result in both benders or both nonbenders.  If two similar appearing siblings of the same age do not bend in the same way, it can be assumed they are fraternal (two separate eggs and two separate sperm). 
 

Avatar Aang and Katara Pedigree

  Hakoda (WwEe) x Kya (WwEe)

	WE	We	wE	we
WE	WWEE	WWEe	WwEE	WwEe
We	WWEe	WWee (water bender)	WwEe	Wwee (water bender)
wE	WwEE	WwEe	wwEE	wwEe
we	WwEe	Wwee (water bender)	wwEe	wwee

Sokka can carry many nonbender allele combinations (WWEE, WWEe, WwEE, WwEe, wwEe, wwee), but Katara must be heterozygous for the water gene (Wwee) otherwise her children would never include an air bender.

Aang (Aaee) x Katara (Wwee)

	Ae	ae
We	AWee- not represented in general population (lethal homozygous)	aWee 33% (Water bender)
we	Awee 33% (air bender)	awee 33% (Nonbender, energy recessive)

Both Aang and Katara must be heterozygous for their respective bending types for either a water bender (Kya) or an air bender (Tenzin) to appear.  While double bending alleles do not have to be lethal, the lack of a bender with multiple types aside from the avatar could suggest embryonic death.

 

Tenzin (Awee) x Pema (_r_e)

 

	re	_ _
Ae	Aree	A_ _ e
we	rwee	_w_e

Pema is assumed to be an earth bender native due to the presence of Republic city in Earth kingdom territory.  If she is an emigrant from one of the other nations, the punnett squares still apply as long as she is recessive for bending allele.

 

     Tenzin (Awee) x Pema (rree)

 

	re
Ae	Aree (Air bender offspring 50%)
we	rwee (nonbender offspring 50%)

Due to the high number of bender children (3) seen thus far, the combination of a bender with a complete recessive is far more likely to produce bender offspring than if Pema was an energy bending dominant (phenotypic inhibitor of the bending gene).

 

Tenzin (Awee) x Pema (rrEe) combination 2

 

	re	rE
Ae	Aree (Air bender 25%)	ArEe (nonbender, carries air/energy genotype 25%)
we	rwee (nonbender, carries energy genotype 25%)	rwEe (nonbender, carries energy genotype 25%)

Fire Lord Pedigree

Fire Lord Sozin (F_ee) x Unnamed Wife (_ _ _e)

	_ e
Fe	F_ ee
_e	_ _ ee

Due to the high number of males with fire bending skills in the Firelord family, it is reasonable to assume that bending is not a sex linked trait; otherwise more female benders would be present. 

 

Fire Lord Sozin (Ffee) x Unnamed wife (FfEe)

	FE	Fe	fE	fe
Fe	FFEe (nonbender, carries gene for bending)	FFee (fire bender)	FfEe (nonbender, carries gene for bending)	Ffee (fire bender)
fe	FfEe (nonbender, carries gene for bending)	Ffee (fire bender)	ffEe (nonbender)	ffee (nonbender, energy recessive)

This combination only allows for 37% of the offspring to be firebenders.  With such a high heredity of the trait in this family, it is an unlikely combination

Fire Lord Sozin (Ffee) x Unnamed wife (ffEe) combination 2

	fE	fe
Fe	FfEe (nonbender, carries gene for bending)	Ffee 25% (fire bender)
fe	ffEe (nonbender)	ffee (nonbender, energy recessive)

This combination only allows for 37% of the offspring to be firebenders.  With such a high heredity of the trait in this family, it is an unlikely combination

 

       Fire Lord Sozin (Ffee) x Unnamed wife (ffee) combination 3

	fe
Fe	Ffee 50% (fire bender)
fe	ffee (nonbender, energy recessive)

This combination allows for 50% heredity of the firebender phenotype and is a marked improvement in combined genotypes.

 

Fire Lord Sozin (FFee) x Unnamed wife (ffee) combination 4

	fe
Fe	Ffee 100% (fire bender)

This combination allows for 100% heredity of the fire bender phenotype .  With the high number of fire benders in the Fire Lord family, it is highly likely that this combination exists.  For this combination to exist, homozygous fire bending alleles must be nonlethal.  It is possible that one active fire bending allele can suppress the other firebending allele while both alleles of mixed types (AW, AF, AR, FW, FR, RW) would attempt to synthesize contradictory and lethal proteins.

 

The combinations for Fire Lord Sozin and his unnamed wife can apply to all of the Fire Lords Azulan, Ozai, and Zuko, as well as Avatar Roku and Uncle Iroh.  If the Fire Lord family has a high level of dominance in bending, there is a higher probability that some if not all of the above are homozygous dominant fire benders.  This probability improves if Ilah, Sozin’s wife, or Iroh’s wife are benders.

Fire Lord Sozin (FFee) x Unnamed wife (FFee) combination 5

	Fe
Fe	FFee 100% (fire bender)

Fire Lord Sozin (FFee) x Unnamed wife (Ffee) combination 6

	Fe	Fe
Fe	FFee 50% (fire bender	Ffee 50% (fire bender)

 

Lao Beifong (RrEe) x Poppy Beifong (RrEe)

	RE	Re	rE	re
RE	RREE	RREe	RrEE	RrEe
Re	RREe	RRee (Earth bender)	RrEe	Rree (Earth bender)
rE	RrEE	RrEe	rrEE	rrEe
re	RrEe	Rree (Earth bender)	rrEe	rree

Similar to the recessive nature of Katara’s water bending, Toph’s earth bending could have emerged phenotypically after acquiring the genotypes needed to energy bend.  Once the phenotype for earth bending is expressed, it can readily be passed on to a subsequent generation if both parents are earth benders.  Examples in nature of a buried phenotype with an active recessive allele include white lions and albino alligators. 

Mako and Bolin provide one of the more difficult combinations to calculate via punnett square due to absence of history.  However several combinations remain possible.  Due to the nature of each brother expressing a different type of bending, their parents must carry heterozygous alleles for bending.  Neither parent is required to be a bender.  Either can be benders or neither can be benders.

Unnamed Father (Ffee) x Unnamed Mother (Rree)

	Fe	fe
Rr	FRee (lethal dominant)	fRee 33% (Earth bender)
re	Free 33% (Fire bender)	free 33% (nonbender, energy recessive)

Unnamed Father (FfEe) x Unnamed Mother (Rree) combo 2

	FE	Fe	fE	fe
Re	FREe	FRee	fREe	fRee (Earth bender)
re	FrEe	Free (Fire bender)	frEe	free

 

Unnamed Father (Ffee) x Unnamed Mother (RrEe) combo 3

	RE	Re	rE	re
Fe	FREe	FRee	FrEe	Free (fire bender)
fe	FrEe	fRee (Earth bender)	frEe	free

 

Unnamed Father (FfEe) x Unnamed Mother (RrEe) combo 4

	RE	Re	rE	re
FE	FREE	FREe	FrEE	FrEe
Fe	FREe	FRee	FrEe	Free (fire bender)
fE	fREE	fREe	frEE	frEe
fe	FrEe	fRee (Earth bender)	frEe	free

References
1.  "Welcome to Republic City".   Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos, Ki Hyun Ryu (directors).  Avatar:   The Legend of Korra. Nickelodeon.  April 14, 2012.  No. 1, Book One:  Air.
2.  "The Boy in the Iceberg".  Michael Dante DiMartino, Bryan Konietzko (writers) & Dave Filoni (director).  Avatar: The Last Airbender. Nickelodeon.  February 21, 2005.  No. 1, Book One:  Water.
3.  "The Puppetmaster".  Tim Hedrick (writer) & Joaquim Dos Santos (director).  Avatar: The Last Airbender.  Nickelodeon.  November 9, 2007.  No. 8, Book Three:  Fire.
4.  "The Blind Bandit".  Michael Dante DiMartino (writer) & Ethan Spaulding (director).  Avatar: The Last Airbender.  Nickelodeon.  May 5, 2006.  No. 6, Book Two:  Earth.
5.  Jay Phelan.  What is Life?: A Guide to Biology.  W. H. Freeman.  2010.  262-3.
6.  "Sozin’s Comet Part 4:Avatar Aang".  Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos (director).  Avatar: The Last Airbender.  Nickelodeon.  July 19, 2008.  No. 21, Book Three:  Fire.
7.  "Endgame".  Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos, Ki Hyun Ryu (directors). Avatar: The Legend of Korra.   Nickelodeon.   June 23, 2012. No. 12, Book One: Air.
8.  "Skeletons in the Closet". Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos, Ki Hyun Ryu (directors). Avatar: The Legend of Korra. Nickelodeon.  June 23, 2012. No. 11, Book One: Air.
9.  "Zuko Alone".  Elizabeth Welch Ehasz (writer) & Lauren MacMillan (director).  Avatar: The Last Airbender.  Nickelodeon.  May 12, 2006.  No. 7, Book Two:  Earth.
10.  "The Southern Air Temple".  Michael Dante DiMartino (writer) & Lauren MacMullan (director).  Avatar: The Last Airbender.  Nickelodeon.  February 25, 2005.  No. 3,  Book One: Water.
11.  "The Deserter".  Tim Hedrick (writer) & Lauren MacMillen (director).  Avatar: The Last Airbender.  Nickelodeon.  October 21, 2005.  No. 16, Book One:  Water.
12.  Michael Kent.  Advanced Biology.  Oxford University Press. 2000. p 421.
13.  "A Leaf in the Wind".  Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos, Ki Hyun Ryu (directors).   Avatar:   The Legend of Korra.   Nickelodeon.  April 14, 2012.  No. 2, Book One: Air.
14.  "The Revelation".  Michael Dante DiMartino, Bryan Konietzko (writers) & Joaquim Dos Santos, Ki Hyun Ryu (directors).   Avatar:  The Legend of Korra.  Nickelodeon.  April 21, 2012.  No. 3, Book One:  Air.
15.  "The Fortuneteller".  Aaron Ehasz, John O'Bryan (writers) & Dave Filoni (director). Avatar: The Last Airbender.  Nickelodeon.  September 23, 2005.  No. 14, Book One:  Water.
16.  "No Labels for the Twins but Their Own".  Gina Bellefante. New York Times.  October 24, 2008.