The Valyrian blood chromosome



The strong bond between dragonlords of Valyrian blood and their tamed dragons was a long-lasting scientific enigma, which was recently revealed. It was found that people of Valyrian blood possess a 24th chromosomal pair, which contains multiple genes, responsible for pheromone signaling between the two species, as well as the distinct features of Valyrians over other people. Moreover, it was found that this chromosome, called the Valyrian chromosome is also shared by the dragons themselves and undergoes horizontal transfer and homologous recombination during a process, known as “blood-hatching”.


From the greatness of the Valyrian Freehold, to Daenerys Targaryen, the intimate bond between particular individuals of Valyrian descent and dragons [Wes] Magnalacerta nobilis Barth, 94 AC was a scientific enigma. While many thought dragonriders are just warriors, who ride tamed and trained beasts, it soon became obvious, that the “Valyrian blood” is essential for a person to be able to control a dragon. This ability is best observed in the House Targaryen, whose members, both male and female are proficient dragonriders and practiced incestuous marriages to preserve their Valyrian blood and the ability to bond with dragons. However, progenies (often called dragonseeds) of mixed relations, such as Addam Velaryon and Jon Snow were also able to establish a bond with a dragon and become dragonriders.

Apparently, people of Valyrian descent also share some other distinct phenotypic features like silver hair, pale skin, and purple eyes, along with a certain degree of resistance to fire. All of this suggested that there is some genetic basis behind the expression “Valyrian blood”.

The 24th chromosome pair

Like the ordinary humans, inhabiting Westeros, Valyrians do have 23 chromosome pairs, of which 22 pairs are autosomes and the 23rd pair are the sex chromosomes, or XY. Each of them contains a different number of genes, varying from approximately 200 (in the Y chromosome) to over 3000 (in chromosome 1). However, people of pure Valyrian descent possess another chromosome pair, the 24th pair, also known as the “Dragon chromosome”, or “Valyrian chromosome” for a total of 48 chromosomes. It is a fairly small chromosome, comparable to chromosome 18 with approximately 70 million base pairs in length, but containing significantly less – about 250 protein-coding genes. While all pure-blooded Valyrians possess a pair, descendants of mixed marriages possess only a single Valyrian chromosome (a total of 47) and the next generations may possess one or none with a 50% chance.

This is easily understandable as while the normal cells of each individual contain 48 chromosomes (24 pairs, or 2n), the reproductive cells contain 24 chromosomes (or n). The fusion between a Valyrian reproductive cell (either sperm or egg) with 24 chromosomes and a non-Valyrian with 23 chromosomes will give rise to an embryo with 23 pairs and one unpaired (the Valyrian) chromosome. Although fully viable, such progeny will gradually lose the unpaired chromosome as 50% of the children will not possess it.

Gene interactions

Extensive sequencing studies established, that most of the genes on the Valyrian chromosome are just doubling the function of genes on other chromosomes or are interacting with them via non-allelic (epistatic) gene interactions. For example, there are two genes, called MIP1 and MIP2, encoding the melanin-inhibiting proteins 1 and 2, respectively. These proteins are potent inhibitors of melanin accumulation in both hair (MIP1) and skin (MIP2), contributing to the extremely white-colored phenotype of Valyrians by fully inhibiting the over 20 genes, responsible for color determination. Apparently, Jon Snow is defective in MIP1, while house Velaryon may have an inherited defective MIP2.

Figure 1. Schematic representation of the Valyrian chromosome with the position of some of the identified genes (including two copies of Hox 2).

Such deficiency in melanin in the skin, however, would be highly harmful because of high sensitivity to UV radiation and would cause various adverse effects from skin burns to skin cancers. Therefore, a number of genes are also responsible for the synthesis and constant secretion of various aromatic esters on the skin surface. They also contribute to the increased fire resistance of Valyrians. Some scholars also claimed that genes, responsible for aggressive behavior and psychiatric disorders are also situated on the 24th chromosome, but this is not proven.

The most important genes, however, are those, responsible for the bond with dragons. These include approximately 35 genes involved in a complex synthesis and release of communication pheromones, that are readily perceived by the dragons (and possibly other reptilians) and are responsible for a complex interspecies chemical communication.

Horizontal chromosomal transfer

As clearly seen, most of the dragons, known in history, have a very distinct individual appearance and various phenotypic differences, such as the size and number of horns, the shape of the tale, the length of the neck, the existence and size of flying membranes on the back limbs, etc. It was first thought, that there are several species within the genus Magnalacerta, but later it was found that it is almost impossible to distinguish them based on phenotypic features.

Simultaneously, it was found that almost 100 genes on the 24th chromosome appeared to be functional, but no particular function was successfully assigned to them. More interestingly, a large part of them was found to be homologous to known genes in reptiles, and also some are encoding transcription factors, involved in body shape determination, including at least four Hox genes.

This intriguing fact remained unexplained until the full sequencing of the dragon genome. It was found, that the 13th chromosome pair (out of 21) in dragons is highly homologous to the Valyrian chromosome and is involved in the embryo development in dragons’ eggs. According to old scholars, a dragon egg needs “fire and blood” to hatch, and could remain dormant for hundreds of years before this process.

The mechanism involves Valyrian blood, which is dropped on the top of the egg. A special subpopulation of macrophages then protrudes into the egg and travels to the vicinity of the zygote, where extracellular vesicles, containing the Valyrian chromosome are released and merge with the membrane, inserting the chromosome into the zygote. Generally, a single vesicle from a single macrophage is able to do this, after which the specific receptors on the cell membrane disintegrate and the rest of the macrophages respond to specific signals and serve as defensive cells during the embryo development. Then the single chromosome of human origin undergoes homologous recombination with both dragon chromosomes simultaneously, thus exchanging genetic material with them. During mitosis, as there are spindle fibers for only two of the 13th chromosomes pair, the Valyrian chromosome remains undivided into one of the daughter cells and is later digested by specific nucleases. The resulting recombinant chromosomes in the developing dragon embryo contain some of the alleles, bored by the donor Valyrian. In such a way, the hatching dragon shares some genetic information with the donor human, which contributes to the strong bond, established between a dragon and a particular dragonrider. Moreover, the newly acquired alleles define phenotypic differences between dragons.

Figure 2. Molecular mechanism of blood-hatching. A: Release of extracellular vesicles (2) by macrophage (1) in close proximity to the zygote (3). Extracellular vesicles transport the Valyrian chromosome between the two nuclei (4); B: Recognition of the extracellular vesicle (2) by specific dimeric receptor (5) on the zygote cell membrane; C: Three-way homologous recombination between the 24th human (6) and the 13th dragon chromosomes. Exchanged genetic material is shown in different filling.

Importantly, this process is not absolutely necessary for embryo development, and dragons, hatched from eggs, induced solely by fire (e.g., wild-type dragons) are equally viable, but not so strongly attached to a particular person. Wild-type dragons, however, could be tamed, as well as blood-hatched dragons could switch to another rider, mostly because they already share some genetic similarities with Valyrians, established during multiple generations of blood-hatching.


George R. R. Martin (1996-2011) A Song of Ice and Fire. Bantam Books

David Benioff and D.B. Weiss (2011-2019) Game of Thrones. HBO Entertainment.

Ryan Condal and George R. R. Martin (2022-ongoing) House of the Dragon. HBO Entertainment.

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