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Unlike human blood, which is iron-based, Vulcan blood is copper-based, making it green. Red blood cells in humans store oxygen by means of hemoglobin, which makes them bright red; that is why human arterial blood, on its way from the cardiopulmonary system, is brighter red than venous blood, which has carried its oxygen throughout the body's tissues and is on its return trip for re-oxygenating. Copper molecules, when oxidized are green--that's why copper pipes turn green eventually. Hence, green Vulcan blood. Vulcan venous blood, on its way back to the heart, is more copper- or rust-colored, containing less oxygen. Vulcan blood cells are also shaped differently, more like lentils, whereas human blood cells are concave.

If Spock is half human half Vulcan, why is his blood makeup that of a full Vulcan?
Did they perform gene therapy prior to his conception or are Vulcans and humans compatible by default with the Vulcan traits being dominant?

  • 2
    T he whole idea of an extraterrestrial species mating with humans is ridiculous from the get go. A s Cark Sagan said "we have more in common genetically with a petunia".
    – user42437
    Commented Mar 1, 2015 at 6:44
  • @jim Okay. Let's breed a Petunia flower.
    – user931
    Commented Mar 16, 2015 at 19:48
  • 1
    @jimgerard: No, not really, given the established relationship between humanoid species in Star Trek. Commented Mar 17, 2015 at 14:23
  • @LightnessRacesinOrbit is exactly right.. It's nonsensical SciFi biology, as we first see it, but that's missing the revelation from the end of the episode 'The Chase' which totally retcons a lot of what we thought we knew. (it's also a rather transparent metaphor, but that's not unusual.)
    – K-H-W
    Commented Jan 17, 2018 at 0:37

7 Answers 7


First off, there are a few invertebrates with copper-based blood. It's not necessarily green, I believe it's actually blue for some. Second, it only has to be a dominant trait. It may mean that he does have some Terran hemoglobin in his system, but the corresponding Vulcan gene hijacks the cellular machinery and doesn't let the other recessive gene to execute (much). Epigenetics is weird, and no one completely understands how it works. Similar things happen with hybrids in the real world.

I know of no canon answers.

  • The blood is green... Its shown in TOS.
    – user931
    Commented Aug 14, 2012 at 19:14
  • 8
    @SachinShekhar Vulcans' blood is green, real copper-based blood is blue.
    – Kevin
    Commented Aug 14, 2012 at 22:51
  • 5
    A good real-world example of copper-based blood being blue is the horseshoe crab
    – eidylon
    Commented Sep 5, 2012 at 2:31

If Spock's blood is green, it's probably vanadium-based rather than copper-based, though there is some controversy as to whether vanadium is a viable oxygen carrier and those few creatures (on Earth) that have vanadium in their blood also tend to have a larger proportion of haemocyanin (copper based) blood.


Yes - In "Spock's World" (1989 novel by Diane Duane) they go into explicit detail describing how different alien species can not mate naturally, and specifically note how Spock is effectively a genetically manipulated "test tube baby", in other words, Amanda and Sareks's DNA were gene spliced in a lab, and Amanda was artificially inseminated. I realize most novels are non Canon, so take it or leave it, as per your own inclinations.


To reply to this thread the years after the best answer, I would like to make clear, Star Trek: Enterprise did not make this clear, as there was a defect in the process of combining Human and Vulcan DNA in a lab. The only episode that deals directly with human/vulcan mating, is when the Enterprise NX-01 meets itself from the future, and the captain is the son of two crewmen. But it never makes it clear that he was naturally born.


Just as in human beings and the different races, when two people of different races have a child one of the parents will end up being the more dominent in the appearence and blood. For example will their blood be A+, B+, O- and so on. Then the same with appearance. Color of hair, skin, physical traits. Spock has the appearance of being a vulcan which more or less indicates Vulcan is the dominant trait and therefore his blood looks more like a vulcan's blood.


In terms of dominant vs recessive alleles, it is not that one gene "highjacks"the cellular machinery, so much that both genes can express, but one gives a phenotype that the other does not. For example in human blood, the alleles for type A and B both produce a sugar molecule that coats the surface of the blood cell. Whereas type O produces no such molecule. Therefore when a person inherits both A and O alleles, they produce the sugar molecule and are phenotypically type A. But when a person inherits both A and B alleles, they produce both types of sugar molecules on the surface of their blood cells, and are type AB.

Epigenetics is a different instance, epigenetic changes are not inherited, but can mean that only one allele from a pair is expressed at a time, while the other is silenced. We see something like this in tortoiseshell cats, which are all female. In this instance the cat has an allele for orange coat colour on the X chromosome. Because there are two X chromosomes, it can be that one carries the allele for orange colour, while the other does not. Because only one X chromosome is functional at a time, each cell with "turn off" one copy of its X chromosome. The mottled colour of the tortoiseshell cat is because during embryonic development different cells have "turned off" different copies of their X chromosome. Some patches of skin cells are descendants of cells that turned the orange allele chromosome "off", while others are descendants of cells that turned the orange allele chromosome "on". So epigenetics is very different to traditional inheritance of traits.

I believe that in Star Trek: Enterprise it's made clear that humans and vulcans could never successfully reproduce together, they are just too different. So we must assume that any offspring of a human-vulcan mating must have been genetically altered to make it viable.


Spock's blood is actually not completely Vulcan in properties. From "Journey To Babel":

SAREK: My blood type is T-negative. Somewhat rare, even for a Vulcan.

MCCOY: Yes, I'd say that's rare.

SPOCK: My blood is T-negative, Doctor.

CHAPEL: We've run a number of blood tests on Mister Spock. It isn't true Vulcan blood either. It has human blood elements in it.

SPOCK: It should be possible to filter out the human factors.

As to why it is "mostly Vulcan", who knows? There's no canon on that.

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