A simple genetics problem I could never put on an exam

My genetics students are learning a bit about sex linkage and modes of inheritance, so I’ve got them thinking about an entirely hypothetic problem outside of class. I’ll be interested to hear their explanations, but this is the kind of problem that would drive me nuts to try and grade. Let’s see how you all do with it.

In Guardians of the Galaxy, the assassin Gamora is bright green. Her sister Nebula is blue. 

a. Invent a genetics of skin color in her species, the Zehoberei, that explains their differences in color, using an autosomal gene and modeling it after human patterns of inheritance, and use it to predict the skin color of their parents. (Note: Thanos is purple, but he’s a different species and is their adoptive father.)

b. Now assume the trait is X-linked. Does it change your prediction for their parents? What color(s) would a hypothetical brother be?

c. A new twist: in Zehobereians, females are the heterogametic sex. What does this do to your model?

The fun part is (a), which could have all kinds of possible explanations (that’s why I couldn’t put it on an exam, the range of possibilities is huge), with the one constraint being that it has to be autosomal. I want to see some dueling discussions about how it would work, brought down to earth by the need to make a prediction about the parents.

Then (b) changes the assumptions, and forces their model to change, and (c) flips it around some more. I don’t want to grade this kind of question, I just want to see how their brains logic their way around it. It should be fun. I hope.

I also had some ideas about the inheritance of color in Minecraft sheep, but decided that was just too weird. Blending inheritance + acquired characters? Heretical. Not very instructive about Earthly rules of genetics.

Of course, it may all be moot because attendance in all of my classes is declining this semester — only half the students showed up today! I don’t know if it’s pandemic fears (no diagnosed cases in Morris), the imminence of Spring Break, or that I’m just horribly boring.


  1. wzrd1 says

    Given that her father is utterly unrelated to her, I can find no reason to assume that she and her sister are even the same species with the information provided.
    If a boy grows up with wolves, did he stop being homo sapiens or are the wolves now no longer members of canis and now part of homo?

  2. says

    Also some clown (like me) will point out that Gamora and Nebula are sisters by virtue of both being adopted by the same person. They are not genetically related and, in fact, are different species. Nebula is a luphomoid, while Gamora is a zehoberei.

  3. says

    If you really want some weird alien reproduction try the asari from the Mass Effect* games.

    Then there is Star Trek where it’s apparently easier to procreate with humanoid aliens who evolved light years away than it is with our closely related primate cousins here on Earth. Yeah, TNG did an episode to explain this, that one species way in the past seeded the galaxy so there would be countless humanoid species, but we’d still be more closely related to a gorilla than a Vulcan or a Klingon.

    Fun fact: Back when Bioware still had message boards I was involved in and the most ridiculous arguments I got roped into were doorknobs arguing that a Commander Shepard in a relationship with an asari wasn’t feeling same-sex attraction because the asari are a single sex species, despite them looking every bit like human women except with green/blue/purple skin and tentacle-like appendages instead of hair at the top of the head.

  4. neptis says

    My last biology class was way back in 2002, but I need to procrastinate some more important stuff, so here we go:

    a) The parents each have a pair of the chromosome this autosomal gene is on. From each parent, 1 chromosome gets picked for each kid, so they have 2 chromosomes.
    I’m assuming that “blue” as a basic color is one of the variants, let’s call it B.
    An individual with the BB gene variant will be blue.
    But there also has to be a variant for green. Since blue+yellow is green, I’ll call the yellow variant Y.
    So we have Nebula blue and with BB gene variants, and Gamora is green with BY variant.
    The parents now could be the following colors (doesn’t matter which is mother or which is father):
    BB + BY blue and green
    BY + BY both green
    BY + B? .. or even green and another color if there are more genetic variants than just B and Y.
    (There’s probably a lot more fancy stuff possible with recombination and mutation and whatnot, but I wouldn’t know)

    b) If the trait was X-linked, it means that there’s only one genetic variant coming from the father and 2 from the mother.
    So the parent color prediction would change to:
    B + BY blue father and green mother
    A male sibling would inherit a single variant from the mother and could thus be blue or yellow.
    (again without any fancy extra options, sorry)

    c) If the sisters can only have one variant of the color gene, then unfortunately I would have to change my model completely. The two possible variants would be blue B, and green G, since they are now only able to have one variant, like the males in b).
    So Nebula has B and is blue, Gamora has G and is green.
    The parents would now have:
    ? + BG: The mother can have any color really, blue green or yellow. And the father would be cyan or something, if we assume the colors just mix.

  5. microraptor says

    @3: I think that the “asari don’t count as women for the purposes of attraction” stupidity was why there was a line in Mass Effect Andromeda about Cora, who’s a heterosexual woman that trained in an asari military unit, considers them beautiful but is not sexually attracted to them.

  6. ANB says

    As a teacher (every grade, K-12, and at UCLA), I have a couple of ideas. 1) To increase turnout to classes, give out random “bonus” quizzes or questions (to be turned in the next class meeting) which could be easy, but relevant (however you choose to define that). You’d guarantee attendance at the subsequent class because that would be the only time they could turn it in (one shot opportunity). You’re rewarding attendance and thinking, and there’s no “punishment” for the other students. 2) You assign this “problem” to students (perhaps as bonus, perhaps not), but you grade it according to a rubric, which rewards thinking, knowledge of the facts you’re teaching in genetics, and creativity, but not necessarily “correct” answers. And you could use students answers to be a discussion point in class (as you’d anonymously present chosen answers for their discussion value).

  7. chrislawson says

    I love the idea of this kind of question but as a marker, who has time to critically analyse dozens of answers and come up with a fair, replicable marking rubric?

  8. jasonfailes says

    Yes, Star Trek is the very worst for this. They could have left Spock as a one-off hybrid, thrown out some bio-babble about advanced genetic engineering techniques only available to high-ranking ambassador types, and tried to ignore their flub, but no, the “torn between two worlds” themes proved so effective dramatically that they just leaned into the biological ridiculousness, not only featuring hybrid characters ever since, but having several of these characters* result from goddamn unplanned pregnancies!

    *Worf’s son Alexander, Naomi Wildman, and at least two children Gul Dukat had with Bajoran women

  9. methuseus says

    You could post it as a fun exercise for nominal extra credit, say, 3 or 5 percentage points on the midterm or something, with the caveat that if you don’t think they put any thought into it, they get no extra points. Then use the best one or two in class due discussion. Anonymously, of course.

  10. methuseus says

    To clarify, they would be extra credit points on something. Forgot that detail.

  11. Silent Service says

    1: Gamora and Nebula are only related by the fact of adoption. They are no biologically related, and even hint at this in the movies. So their parents can be of whatever genetic variations for skin tone that are appropriate to their species.

    2: Star Trek assumes that all humanoid life across the Alpha Quadrant are related, and relatively closely at that. Why? Because aliens. This also lets religion people enjoy Trek as if we postulate that humans are distantly related to all these aliens then we are not related to apes. We all know how Christians (and just plain a lot of egotistical people) hate the idea that we are evolved from a progenitor species of apes. They’d much rather imagine that we’re evolved from a progenitor species of aliens.

  12. lpetrich says

    The available information in this problem is very limited — too limited to get very far in finding a solution.
    Are Gamora (green) and Nebula (blue) biological sisters? Biological half-sisters? Neither?
    How are their skin colors determined by their genes? Are they also caused by something environmental?

    So I’ll impose some assumptions and work out the solution for them. The two are biological sisters, and their colors are determined by alleles of a color gene, G making green and B making blue. I will also make G dominant over B. One can then get B dominant over G by switching the two sisters and the two genes.

    I will consider the autosomal case first. Gamora and Nebula are either GG, BB or GB, BB. In the first case, their parents are GB, GB, and in the second case, GB, BB or GB, GB.

    Then the female XX male XY system. The sisters’ biological father must be G or B, and the only way to get his daughters’ two colors is B, from it being recessive. Their biological mother must be GB, and their daughters are thus GB, BB.

    Then the male ZZ female ZW system. The sisters must be G, B. That makes their biological father GB, and their biological mother can be either G or B.

  13. lpetrich says

    So in the autosomal case, G and N’s parents are green-green or green-blue. In the XX-XY case, their mother must be green and their father blue. In the ZZ-ZW case, their mother can be either color and their father must be green.

    Turning to the blended case with GG making green, BB making blue, GB makes cyan (let’s say). The two sisters are thus GG and BB, and in the autosomal case, their parents are thus GB – cyan. This case is not possible for the XX-XY case, because both sisters must share an allele in that case. But in the ZZ-ZW case, their mother can be green or blue, and their father must be cyan.

    The half-sister and not-biological-sister cases are less constrained.

    For the half-sister G > B autosomal case, the shared parent must be GB or BB. Likewise for a shared XX mother. For a shared XY father, one gets B. For a shared ZZ father, one gets GB, while a shared ZW mother can be either G or B.

    For the half-sister G = B autosomal case, the shared parent must be GB. Likewise for a shared XX mother. A shared XY father is not possible. A shared ZZ father must be GB, while a shared ZW mother can be either G or B.

    For the nonbiological-sister case, I will consider each sister separately.

    For Gamora and G > B autosomal, her parents can be GG GG, GG GB, GG BB, GB GB, or GB BB. For the XX-XY case, her parents can be GG G, GG B, GB G, GB B, BB G. For the ZZ-ZW case, her parents can be GG ?, GB ?, where ? is G or B.

    For Gamora and G = B autosomal, her parents can be GG GG, GG GB, GB GB. For the XX-XY case, her parents can be GG G, GB G. For the ZZ-ZW case, her parents can be GG ?, GB ? where ? is G or B.

    For Nebula and G > B autosomal, her parents can be GB GB, GB BB, or BB BB. For the XX-XY case, her parents can be GB B, BB B. For the ZZ-ZW case, her parents can be GB ?, BB ?, where ? is G or B.

    For Nebula and G = B autosomal, her parents can be GB GB, GB BB, BB BB. For the XX-XY case, her parents can be GB B, BB B. For the ZZ-ZW case, her parents can be GB ?, BB ? where ? is G or B.

  14. says

    I was told if I mated a red sheep with a yellow sheep I’d get an orange baby sheep. This did not happen. So I just dyed the sheep whatever color I wanted.

    Also, my assumption was always that Gamora and Nebula were not biological sisters. The only scene I remember directly referencing their adoption is from Infinity War where Thanos is shown “comforting” a young Gamora before slaughtering her people. I don’t remember baby Nebula in that scene. But I’m not exactly ingrained in the lore of the MCU.

  15. malleefowl says

    You do realize don’t you that virtually all blue colours in animals are due to physical light refraction and not to pigments (There are only two known exceptions in vertebrates (fish) and a very few invertebrates). So the blue colour is likely to be due to a lack of pigment. Green could be produced by green pigment overwhelming the blue refractive colour or yellow pigment mixing with the blue reflection to produce green.
    Your problem is easy to solve.
    Of course yellow pigment is most likely the end result of a biochemical pathway with a red pigment as an intermediate metabolite. A mutation blocking yellow pigment production could result in a build up of the red giving a purple colour when combined with the blue reflected light to give purple.
    It is fairly easy to come up with a two autosomal gene model in which a purple father has a green and a blue daughter. Might be a problem with two sex linked genes though.

  16. cag says

    Obviously, Nebula is a disciple of Jim Bakker, and has consumed the miracle silver cure.

  17. dianne says

    I also had some ideas about the inheritance of color in Minecraft sheep, but decided that was just too weird. Blending inheritance + acquired characters? Heretical. Not very instructive about Earthly rules of genetics

    Minecraft “sheep” are actually viruses? Or someone’s been sneaking lentiviral vectors containing color change genes into the population?