Riddle me this: how did these twin sisters come about?

116 thoughts on “Riddle me this: how did these twin sisters come about?”

1. Couldn’t they just be fraternal twins?

   1. I think it is definitely a possibility.

      The dark complected girl certainly looks like her mother and older sister. There is no doubt of relationship there. And I would think it possible that both parents may have more or less recessive genes for the fair complexion and red hair of the other twin.

      It might be quite an outlier, but I don’t see it being impossible.

   2. I think this is the case too. I’d add that the mom might be of mixed race herself (white/black). Since red hair (like Lucy’s)is a fairly rare and recessive trait in humans, both mom and dad would have had to contribute a “red hair” allele to Lucy. The red hair allele would be masked in a heterozygous mom. IIRC, red hair alleles might have been introduced into modern Homo sapiens via Neanderthals.

      1. she is mixed race (half jamaican).

         1. jamican (sic) is not a race.

   3. That is my guess. I blame my fstjer’s stupid white people genetics for why I ended up so white. My mom is very white & my dad is dark.

      1. Here comes the sun….da da da da…

         Hands Diana a bottle of Coppertone SPF 50

         1. And my skin is very sensitive. Copper tone gives me a rash!

            1. [Hands Diana an umbrella].

            2. Coppertone Water Babies?

            3. My daughter makes her own suntan lotion with zinc oxide which is not irritating. Indeed, it is used in diaper rash ointment. It makes the UV rays come to a dead stop and take a U-turn.

            4. Yes, I usually get the “natural” stuff for kids which is usually mostly zinc oxide.

2. I have a Jehovah Witness that comes over about once a month in the summer for a cup of tea and a chat. I asked him about how the descendants of Noah managed to produce so many different looking humans and split themselves into different geographic areas. The answer he came back with is that parents can give birth to mixed race twins and they split up when the Tower of Babel thing happened. Luckily for him DNA is something we can’t see and therefore doesn’t exist, like skeletons and electricity.

   1. Ha! That’s a Cunkism!

3. Fraternal twins can look very different, but the difference between these two is so striking that it suggests to me that they are dizygotic twins with different fathers. But I do find it remarkable that the fairer twin looks so different from her mother as well.

4. I guees they are dizygotic twins. If the mother is of mixed race then this is definitely possible (i.e. one of their grandparents on their mother’s side is white, the other black).

5. There are several genes involved in racial features. Dark pigmentation is dominant. Red hair is a recessive “mutation.” The mom has both European & African heritage. The dad’s DNA is a combination of dominant and recessive European DNA types. Lucy inherited the European coloration that was recessive in both the mom & dad. Maria inherited mom’s dominant African darker genes and a combination of recessive & dominant DNA from dad, mostly recessive. Mom’s white ancestor probably had red hair.

6. Sub

7. I really don’t think they look all that different except for coloring. The whiter one obviously straightens her hair now. In the younger pics she has very curly hair, and I think they’d look more alike if hers was unstraightened.

   My youngest brother has almost Brillo wiry, formerly almost black hair. My other two brothers and I have totally baby-fine, formerly blond and pin-straight hair. I’m the only one with blue eyes. All from same two parents.

   1. My daughter’s hair was quite curly in her early years and now it is as straight as the hair of the “whiter” daughter in the photo. No artificial anything involved. It just gradually changed.

      1. Or maybe her parents let her get her hair curled.

         1. all the curly-haired versions look natural, and the straight ones “straightened”.

      2. Yeah, my hair was a bit like that when I was really young. Granted, I never wore it long like that, but it had some curliness to it that it no longer has. It was also a much more vibrant shade of red (these days it’s almost blonde).

   2. I agree that they have much in common, facially and bodily as well.

      1. I agree with you and merilee. I think that we all tend to get hung up on colour: for obvious reasons. Take away the colour difference and they look a lot more alike than my twin sisters do. My sisters have very different height, build and hair colour, and people were frequently surprised to find that they were twins.

8. and sub

9. Ok, digging deeper, and accepting that the fellow in the photo is indeed the father of both, I’m wondering if 1) perhaps the fairer twin, just happens to have inherited multiple recessive lighter pigmentation/blue eye alleles from her father and mixed race mother, and 2) – and this is pure conjecture – perhaps even has multiple variants in HERC2, a gene associated with pigmentation variation in Europeans that maps to the imprinted CHr15q13.1 locus of the genome (so perhaps only Dad’s allele is expressed?).

   1. I’ve been wondering…if the couple split up “shortly after” the twins were born, would he really have come back for a family picture like that? Or is that perhaps the next husband (if there was one).

      1. All the kids have the dad’s eyes (full outer part of the eyelid making it slant, common in Scots/Irish like most of my relatives), and the boys (at least) have his ears. I see no reason to assume there’s any funny business here, but on certain other websites I’m sure multiple people would be claiming “Photoshop!”

         1. You are observant! Those traits do stand out, now that you mention them.

            Well, it must have been an amicable split.

            Photoshop was mentioned here somewhere, but I had the feeling only in reference to the twins’ teen age portraits, where the usual airbrushing and what-all to make women look unnaturally perfect have probably been employed.

10. Ok, this is probably completely wrong, but my recollection is that freckles/red hair in combination requires two copies (recessive) of a mutated gene that controls the relative balance of a couple of pigments. Two copies of the recessive gene give you freckled redheads. I think one copy gives you freckles only.

    Here’s where my recollection seems wrong because clearly mom lacks evidence of even one of the recessive genes.

    So, for this twinning to make sense, there would either have to be a new mutation in the redheaded girls genome (seems unlikely) or she and her sister would have to have been monozygous twins with one recessive copy from dad and one regular from Mom. Then when the division occurred, one kid would be homozygous recessive (redhaired) and one homozygous dominant (dark complexion and hair).

    But that sounds a bit off, based on what was presented.

    1. There probably wasn’t a division of an egg. Rather, likely two eggs were released at once and both fertilized by different sperm. In this scenario the fair twin would have inherited two recessive genes, one from each parent. The darker twin would have inherited her mother’s genes for darker complexion.

       1. With a simple recessive trait, the chance of an RR combination would be 0.25, no? And I see 5 kids, so…

          (I.e., I agree with you.)

       2. That seems completely obvious to me. However, in my experience, what seems obvious is often completely wrong.

11. It has been over 45 years since I took a genetics course, so I will have to defer to those with actual up to date knowledge. But, I do have an observation and a question. Some folks are speculating regarding the mother as being mixed race, but what if the father is also of mixed race from his mother’s side of the family, from whence he passed on an X chromosome to the kids.

    1. Or would that make no difference?

    2. He could certainly be mixed race, but if so it would likely be far back in his ancestry (based on his appearance) and if he were that would only make it less likely that the twins would look so different. Right? If you follow the CNN link the other sibs are shown now and they look more like the darker complexioned twin.

12. Without following the links or reading the other comments I’d say the mother is mixed so she was heterozygous for several genes involved in skin and hair color. The mother probably had a parent who was very light and another who was very dark. Lucy got most of the mothers ‘white’ alleles whist Maria got mostly the ‘darker’ alleles. The other kids seemed to be in the middle

    1. …I forgot to mention that the sisters are fraternal twins of course.

13. Have no idea but the mother could be of various mixes. It’s too many kids no matter how it turns out.

    1. They were trying to get p<0.05

14. They are the same height. Also, Wow — there’s a pair that could beat a straight-flush. (I will comment no further on this thread out of a foreboding that the dumb, shallow, atavistic sexist I’ve tried so hard to banish, but fear may still lurk, burning like a tiny pilot flame in some remote sanctum within, will burst forth and say something to embarrass me but good.)

    1. I hear ya, bro.

       1. Thing is, it has dawned on me, these two resemble younger versions of the red-haired woman (played by the exquisite Connie Nielsen) and the woman of mixed-race (played by the also exquisite Tamara Tunie) that Al Pacino employs to tempt Keanu Reeves to infidelity in The Devil’s Advocate.

15. I’m white like Lucy but without the red hair. So because I black the red hair I get grief about not tanning. My hair was strawberry blonde as a child and now it is auburn. In winter it is darker.

16. I’m going with fraternal twins, mother is of mixed race heritage and is heterozygous for pigmentation with Darker pigmentation being dominant trait. If it’s pure mendelian inheritance rules, which more than likely it’s not.

    So the lighter skinned daughter is recessive for skin pigmentation and because of that her X chromosome from the father is dominant over her mothers leading to strong paternal resemblance rather than strong maternal resemblance.

    Option B

    If they were identical twins and the ginger girl only her fathers X chromosome is expressed in her phenotype then that might work too?

17. Two eggs, two sperm. If Lucy’s hair or eye colouring is recessive and the mother a carrier it all seems quite possible.

18. I recently read Inside the Human Genome by Harold Avise, and in one of the lists of mutations, red hair is a mutation. So even someone who doesn’t have red haired ancestors could have red-headed children.

    (amazon link: http://www.amazon.com/Inside-Human-Genome-Non- Intelligent-Design/dp/0195393430)

    1. John Avise, not Harold, is a well-known and well respected population geneticist.

       1. whoops! Don’t know how I did that.

          The book is an excellent read and the list of known mutations is amazing.

19. Fraternal twins. But I see that is already said up there.

20. First, of course they’re fraternal twins.

    I don’t know the genetic details, but I’m pretty sure there are a lot of genes that influence pigment deposition, and that a lot of that variation is additive. If both parents are heterozygous at quite a few of those genes, you could get a very broad distribution of skin/hair colors in the offspring. If the parents are highly heterozygous, that would suggest that they’re both of “mixed-race” ancestry. (Scare quotes because “race” is a word of dubious utility.)

    Based on the fact that only one of the kids is that pale, I’m guessing that at least one of those genes comes in a recessive/dominant pair for which both parents were heterozygous. But that wouldn’t be strictly necessary. The light twin could just be in one of the tails of the distribution.

21. I read (years ago) of a similar fraternal twin pair, where the father was a black man from the West Indies and the mother was white and British. The one girl was blonde with blue eyes, while her sister had the phenotype that typically results when blacks and whites interbreed. Genetic testing confirmed both had the same father.

    I you look at the history of the West Indies, you’ll see there was a lot of interbreeding between early settlers of various ethnic origins, so in all likelihood, the dad in this case had some European ancestry and his genetics combined with those of his wife in just the right way so that one of the twin girls they had, expressed a typically European phenotype.

    The mother in this case is clearly not of purely African descent and as such I suspect the same thing happened here.

    There was a case here in South Africa during the height of the Apartheid era that should also be of interest to guys.

    Two parents who were, phenotypically, as
    white as the day is long, produced a baby girl who was, phenotypically, as black as the day is long and classed as such by the racist laws in effect at the time. Later genetic testing confirmed both the man and woman as the true parents.

    There was also a great deal of interbreeding among the early settlers of various ethnic origins in South Africa, so the genetic endowments of the parents in this case also combined in the exact right way to produce a daughter presenting a completely unexpected phenotype.

    1. That must have created huge problems for all of them!

    2. While I take your point entirely, using the term “interbreeding ” is a little… well, I would try to avoid it with populations of people, as it reminds one of the Eugenics movement, & that might get some folk riled.

       1. I don’t care about the reactions of knee-jerk, know-nothing cretins, it is the correct term to use when individuals from phenotypically and/or genetically distinct sub-groups produce offspring.

       2. Miscegenation was originally not a dirty word, though it sounds like it now (it’s not a ‘mis’-prefix pejorative, but from miscere = to mix). It’s precisely the word that should be required here, but has moved along the euphemism treadmill.

    3. I’m being intentionally pedantic here, but aren’t all people of purely African descent?

       1. If you do go back 150,000 years or so you are correct.

          However there has been quite a bit of genetic divergence since the various groups of exodites from Africa went their separate ways.

          There was also interbreeding with the other human species, the Neanderthals & the Denisovans, encountered during the diaspora, introducing further variance to the genetics of non-African Homo sapiens.

          In my original post, I was referring to the mother’s much more recent ancestry.

       2. +1

22. I see nothing unusual about this. I have nephews who are fraternal twins and are as different as can be. One is red-haired and freckled with pale skin that does not tan; the other is dark-haired, unfreckled and does tan.

    It is a big more striking with these girls, because the mother is mixed race. But is it anymore striking than a pair of boy-girl fraternal twins, of whom I have know several?

    Two eggs, two fertilizing sperm supplying a lottery of genetics. I’ve seen people ask, “How can I be short? Both my parents and all my siblings are tall.” They don’t understand that it isn’t your parents phenotype that determines your phenotype, it is their genotype. And their genotypes includes (or can include), genes from their parents (t=4), their grandparents (t=8), their great grandparents (t=16), etc.

    1. It is more striking than boy/girl twins, because, unlike sex, numerous genetic loci influence skin color. Because sex is chromosomally inherited (equivalent to a single genetic locus), there is a 50% chance that fraternal twins will be of the same sex. With many genetic loci, the probability of all inheriting one variant or the other go down. As a simple case, consider that four genetic loci affect skin color, with two alleles (dark and light) at each locus. Let’s further assume that the father has all light alleles, so that all his offspring inherit 4 light alleles from him. Suppose the biracial mother is heterozygous at all loci– one dark allele and one light allele at each locus. Whether the child inherits a dark or light allele from the mother is a 50-50 chance, and independent at each locus. So the probability of the mother giving 4 light alleles to a child is .5^4=.0625. So, there’s about a 6 percent chance of her child (with the all light father) being all light. Similarly, there’s about a 6% chance the child will inherit all dark alleles, and thus (with the all light father) have the same heterozygosity at all loci as does the biracial mother. So the probability of both things happening is 6^2, or less than .4%. More than 4 loci affect skin color, so the probability is, in fact, even lower. There are probably other genetic phenomena going on here, as noted by many readers above (e.g. recessivity of red hair, so both parents would be heterozygous for the red hair allele). So, it is a low probability event, but it’s (obviously) not impossible.

       1. If some maniac exterminated all of us red heads tomorrow (goodbye cruel world!), there would still be red headed children born because of the gene lying unexpressed in other people.

          1. Like blue eyes. My brown-eyed parents had 4 brown-eyed and one blue-eyed children, and my brown-eyed wife and I had two blue and one brown. Perfect Mendelian 1:3 ratio in the combined sample.

23. Haven’t read the previous comments so …. probably repeating some. Fun to put my ‘guess’ out there without other’s influence.
    I would guess that they are fraternal twins. The inheritance of the genes for skin color are multiple and under the control of independent assortment. The multiple genes located on the same chromosome may undergo independent assortment since x-over may separate the linkage of the genes. The combinations then would result in the more than just a couple/few recessive genes coding for lighter skin.
    Anyway it’s a thought. How’d I do???

24. alright, it seems pretty obvious they’re dizygotic. this story has been repeated over and over in britain over the years, can’t be double paternity every time. i posted on this: http://www.unz.com/gnxp/on-the-latest-blackwhite-twins/

    also, dark pigmentation is NOT dominant. the genetics of human pigmentation are very well eludicated, and when you look at the effects of light vs. dark alleles with melanin index scores the light tend to be more dominant

    http://blogs.discovermagazine.com/gnxp/2008/05/fear-of-a-white-planet/

25. So what is the answer?
    (all I am sure of is that it has nothing to do with
    Arnold Schwarzenegger and Danny DeVito)
    :0
    🙂

26. Stunningly beautiful young women. Period.

    1. lookist!
       😉

    2. Yep

27. I know skin color and hair color, and probably hair texture (?) are polygenic traits. If there are 6
    gene loci governing skin color, for example, maybe Dad has 5 white alleles and 1 dark allele. Mom might
    have 3 dark alleles and 3 white alleles. Luci got 3 white alleles from Dad and 3 from mom. Maria
    got 3 white alleles from Dad and 1 white white allele + 2 dark alleles from Mom. Something like that
    could have happened with the hair, as well. The phaeomelanin (red hair) trait might be governed at
    yet another locus – not sure. That’s my guess. : )

28. Me: 6’4″ tall (98th %ile), thick dark wavy hair, blue eyes, no freckles, tans dark – with short, red hair father with blue eyes; tall, dark wavy hair mother with water-blue eyes.
    Wife: 5′-4″ tall (slightly shorter than average) bang-straight thin mousy hair, green eyes, freckles, burns then tans – with average height blond father with water blue eyes, tall mother with bang-straight mousy hair and green eyes (both parents of Irish descent, all have freckles, but no known gingers in family)

    Offspring (all girls):
    Tall (99th %ile) bang-straight thin red hair, hazel eyes, freckles, does not tan;
    Tall (98th %ile) wavy thick dirty blond hair, water blue eyes, no freckles, tans; and
    Tall (90th %ile) bang-straight thin dark hair, blue eyes, no freckles, tans.

    The tanning is hard to gauge accurately, since we are liberal applies of zinc-containing sunblock and rigid enforcers of brimmed hats and rash guards for swimming – but the two younger girls do darken up anyway, and the dark-haired one tans more than the blond under the same conditions.

    Absent a genetic test, I suppose it’s hard to completely establish that the fraternal twins in the photos have the same father, but if they do, it would seem the fairer girl is just a very rare combination of traits, and the darker girl is an expected combination of traits that only seems unlikely because of her twin (I doubt we’d be talking about this if they were both dark-skinned).

    Granted I lack an understanding of genetics – but given the genetic salads that my wife and I each are, and the variety of results we got among the fruit of our loins (assuming fidelity), I’m willing to believe anything is possible when those lovely recessive genes find a match.

    1. I was once complaining that I was ripped off for having dark skin. My friend replied, “Diana, I’m part black”. She is and she is as pale as me so she got even more ripped off!

    2. 6’4″?

       You couldn’t come & paint my ceiling could you?!

       1. And dust off the refrigerator – also find anything I might have lobbed up there.

          1. Ditto for me and my fridge. I’m always grossed out when I have to get up on a chair to get to the cupboards above my fridge and see how bad the top of my fridge is. And then there’s behind the fridge, where the cat toys accumulate…

29. Can you clarify are they monozygotic or dizygotic?

    1. Dizygotic. Monozygotic = identical = but for a few bases genetically identical

30. I already knew the media reported answer. However I would have suggested it was a case of heterzygote genetics in, like that high school genetics example, two brown haired brown eyed parents could have a blonde haired blue eyed child.

31. How did this two come about? The usual way, most likely. A couple of glasses of wine, candlelight…

    As to the red hair, is their last name Farkel?

32. As to the original question, everyone seems about on the same page, and I agree with that. But as to their common ancestry, in the dark I don’t think the TSA would be able to tell their legs apart.

33. Hold on a sec . . . Is Prof Ceiling Cat pulling a fast one here? This is like that dress color thing that went viral last week, isn’t it? The one on the left is blue and the other is green.

    1. 🙂

34. Ohhh goody, science!!
    Because I’ve been fascinated with cat genetics, I wanted to see if the orange X-linked gene is homologous to anything in humans.
    I found this great 2009 paper about mapping that gene: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2666509/
    According to this and another paper about high conservation of sex chromosome organization between cats and humans, it seems there might be a similar gene in humans. (nothing definite in later papers citing this one though)

    In mammals (maybe others?)the MC1R gene (on human chromosome 16–on another in cats [homologous chromo or section I think])controls which type of melanin is produced–pheomelanin and eumelanin, the former results in lighter skin and hair and no UV protection.

    Now, here’s what I want to know more about. From the paper above: “Orange controls an unknown molecular mechanism that causes the suppression of black-brownish pigmentation (eumelanin) in favor of orange-yellowish coloration (pheomelanin).” So that orange gene acts as a switch that turns on (or off) production of those pigments.

    Could it be that this gene makes the red and light colors more prominent in humans? And maybe it only takes one of these alleles to do it? So that would mean the mother had to contribute the allele or the father would show it.

    Now there’s the problem that females are stripey (which makes cats with the orange gene calico) because one of the X chromosomes becomes a Barr body and deactivates. So I wonder if something else could have happened to this gene in humans–maybe it is on the part of the X chromo that doesn’t deactivate?

    So I started this trail, but can’t end it. I’ve got to do some more digging. Can anyone help? Has the orange gene been definitely identified–this research got it down to an area about 10,000 bases long. And does that gene have a homolog in humans?

    1. Skewed x-inactivation does occur in humans but it does not affect pigmentation as far as I know. I think Greg Mayer’s explanation above is the most likely scenario…The fairer twin just got a highly improbable set of recessive pigmentation alleles that were present in both parents.

       1. Of course, but the thing is that the genetics behind this is way cool. There are so many genes that control or interfere with that pathway to pigmentation–fascinating.

          I wonder how many genes will be found to regulate pigment. The paper I linked to below has a lot of information on all that goes into it.

    2. And this paper shows just how many genes* can regulate the MC1R gene (or its products) through different mechanisms.
       http://onlinelibrary.wiley.com.prox.lib.ncsu.edu/doi/10.1111/j.1755-148X.2010.00755.x/full
       Sorry, it’s behind a pay wall. The journal is Pigment Cell and Melanoma Research.

       *not inclusive!

35. They’re non-identical twins, but on top of that, could Lucy have albinism?

    1. I doubt she’d have been wearing blue contacts all her life…

       1. Yep, Lucy’s just a white girl. 🙂

    2. I think that is a good question, according to this paper, http://onlinelibrary.wiley.com.prox.lib.ncsu.edu/doi/10.1111/j.1755-148X.2010.00755.x/full there are more than 15 of albinism.

       For fun, I was looking for gene on the X chromosome that might correlate with that Orange gene. The only one I found in that paper causes ocular albinism, which, when I looked it up doesn’t mean there is no pigment in the eye, just sometimes less (along with other problems with structure and vision).

       According to this site, http://www.albinism.org/publications/what_is_albinism.html one cannot judge albinism by any pigmentation.

       1. That is most interesting, Linda, thanks for the links!

          Always glad to be corrected when I’m wrong!

          1. I was surprised about that too. But I did know that albinism in cats is much more complicated and eye color doesn’t always do what we expect.

             1. Just like cats.

       2. “one cannot judge albinism by any pigmentation” – that’s so postmodern and politically correct!

       3. Thanks, Lynn, for the interesting info. (I forgot to reply to your post. :))

    3. I must add that it’s pretty common for “West Indians’ to be of the Heinz 57 variety, and the mother is most likely of mixed heritage. (The father looks like he has had some sun, and is probably paler than in the photo.) In our extended family, it’s like that, and we never know what we’re gonna get, when the grandnieces and nephews are born…. there’s one slightly scandalous story there too.

36. One father, Two Sperm. One mother, Two eggs. Fraternal twins. But Mom is a chimera. Mom is the product of one sperm and two fused eggs.

    One child is the DNA daughter of mom and one is the DNA daughter of the “chimera.”

    I think mom’s mom is of mixed race.

    This American Life has the story of a mom who needed a bone marrow transplant. When they tested her two sons for best match, the result was, “You are not the mother of either of your sons,” even though she gave birth to them. Further testing showed she was a chimera, and both were the sons of her “chimera.”

37. I know a similar pair of fraternal twins. Their differences are not remotely as striking. One is short and blonde and the other is taller and darker. They like to call themselves “mirror twins”.

    1. Funhouse mirror twins?

       1. 😀

38. The mother probably has mixed ancestry. The father contributed x chromosome sperm to both eggs, but one of her eggs was x with a gene for darker skin tone, one was x with a gene that expressed very little skin tone.

39. Ok, my guess, without looking at the other comments:

    It seems to me the mother is herself of mixed origin. So one of the twins (Lucy) inherited the X chromosome of her white maternal grandparent in addition to her father’s, the other (Maria) that of her coloured maternal grandparent.

40. And don’t forget about: Makeup, hair styling, and, especially: Photoshop.

41. If I see them in Gloucester ( pronounced Gloss-ster btw ) I will give them a wave from WEIT ..

    I live just down the road !

    1. You could get the scoop for us!

       1. I think I might get four succinct words if I suggested that they become subjects for WEIT investigations !

          1. I can guess what the four words would be. But this is for science, no? Better you than Philomena as an interviewer/inquirer, in just this one case. 🙂

             1. WWPD?

42. Incredulous. I would think it would be awesome to have a couple of kids (boys or girls) who looked that dissimilar. Nevertheless, I am surprised these girls do not collectively melt platinum when they walk into a room together.

43. I dunno!
    Two eggs fertilized by two sperm?

44. As a breeder of alpacas, I am constantly aware of the fact that colour is controlled by multiple genes. In alpacas, there are at least 2 pigments which give black and brown, which have one gene each. Then there are four or five modifiers – again one gene for each – which give variations in how the pigments are expressed, such as roan and appaloosa. I have had a case where a grey sire and black dam have produced a very pale fawn cria. Another animal I have seen was dark brown in respect to her pigment genetics, but had so many of the modifiers that she was white – the white spots had completely covered her body! A mating between her and a recessive white male gave a brown and white cria.
    The result of this is that while the normal Mendelian ratios apply for each gene, getting the right combination of genes is not that easy! I know this from my breeding, because I try to get grey fleece colour…
    I also understand that the same or very similar genetics also apply to other mammals – horses and d*gs at least.
    So it also seems more than likely that there will be multiple genes applying to humans too. Then the extreme cases such as this can be explained as a full set of recessive alleles in the pigmentation, and in any modifiers. Getting a full set would be a quite rare, because they would have to be present (though not necessarily expressed) in each parent, and then every one would need to be passed down.
    So two sperm, two ova, one gets the full set of recessive traits, one does not; this seems the most plausible explanation.
    Chris.

    1. Hi Chris,
       You are right about the similarities among mammals. In my little foray into cat and human comparisons, I saw that they can all be very similar.
       More evidence, of course, that we all share a common ancestor–even alpacas!

45. This has a lot of assumptions, but: I guess that if a lot of “complexion”, eye colour, hair colour genes are in the X chromosome, then the X that got inactivated most often in the “fairer” twin was the mother’s X…

    1. Did I miss something? Hair and skin-color are sex-linked???

46. Sorry, didn’t mean to confuse the issue. The only thing I was wondering about is if there is a gene on the human x-chromosome that is homologous to the orange gene (on the x-chromosome) in cats. That gene somehow controls how the MC1R gene (on another, analogous chromosome in both humans and cats). So not all sex-linked because there are many other genes on quite a few other chromosomes that also control color.

    I just want to know if that gene on the x-chromosome does anything in humans.

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